One of the most amazing things about three phase electricity is that in a perfectly balanced system a neutral wire isn't needed. In this video, Joe Robinson shows how the neutral current sums to zero when an equal load is connected to each phase.
This helps to explain why the neutral conductor in a three phase cable is the same size as the other phases and why some industrial sockets don't have a neutral connection.
This electricians Q&A follows from our review of the Variabox, from CEENORM, which is a flexible power distribution system for industrial electrical installations.
👉 • This job could have be...
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00:00 Industrial Electrical Installations
00:42 Does current flow in the neutral conductor - yes & no?
00:50 Neutral current in a ingle phase circuit
02:00 Neutral currents in a Three Phase circuit
04:09 Lower than expected current?
05:08 No neutral current?
05:48 Why does this happen?
06:30 Electrical installations with no neutral
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Actually, technically No. Current doesn't flow in a conductor, it flows around the conductor, or so i was taught at trade school. Current in the neutral must equal the current in the active, how else does an ELCB work!
Try a star connected 3 ph motor
Don't know how UK electric works other than that you lot use 50 Hz instead of 60 Hz, but in the US you can't have 3 different devices share 1 neutral with 3 phase power. It used to be allowed, but the NEC was changed in 2014 I think. It's a lot of fun trying to fish 2 new neutrals in the old conduit to upgrade old buildings. (not) For fear of undiscovered nicks/shorts when the power is turned back on, oftentimes we just hook new wire with additional neutrals to the old, and use the old for the pull line to install the new and then recycle the old.
A couple of gotchas with 3 phase neutral: 1) If there is a fault the absence of a neutral connection can allow overvoltage on the phase conductors without tripping breakers and the risk of shock or fire in any connected loads and 2) someone might connect a 3 phase + neutral appliance like an industrial heater to single phase power, joining all 3 phases together and tripleing the current in the neutral. Best case the neutral wire burns out, worst case the building burns to the ground.
I just discovered this channel and I love it. I´m a Electrical and Computer Engineering student and love all things related to power grids and house installations, your explanations are more on a "practical" rather than theoretical approach and I just find it much easier to comprehend. Amazing work!
Cheers Hugo
An outstanding presentation. I am a retired electrician and most of my work was done on 480 VAC three phase control circuits. I worked with maintaining large 50+ horsepower 2-speed motors and things like that. I also worked with DC traction elevator motors. I found it to be very enjoyable work. Your presentation is spot on!! I just subscribed so I will be seeing more of you. Cheers!
Yeah, I did learn this back when doing my C&G part 1, 2, 3. It was really nice to see it all again in video format. I think we did it all in formulas back then.
It's good to b reminded of stuff like this, especially after not having to think about it in a long time.
This is exactly what I was wondering for a while now so thank you for explaining. I knew three phase motors didn't need the neutral because it was a balanced load but for some reason it never occurred to me that heaters would also work the same for some odd reason. Clever stuff.
That's so clear now ! 😀 Also, explains why the neutral doesn't have a bigger cross section cable!
This video just blew my mind. Great information and really clearly presented.
I knew this BUT you actually found a way to test and teach the theory. GREAT JR
Now that I found very interesting and informative thank you Joe excellent explanation. Fantastic video as always Joe 👍❤️
You are absolutely amazing! I wish ( and I sincerely mean this) I had this tutorial video when I was doing my A levels!
Your video on your “Joe Robinson” channel on this same subject (a few years ago) was the moment the penny dropped for me! Very well explained Joe & @efixx👏
Brilliantly explained as usual thanks 👍👍
This could also be drawn up as vectors. The phases are 120 degrees out of phase, a balanced load drawn as vectors would also show 0amps, and an unbalanced load would result in a vector length equal to the amps drawn in the neutral. That's how I learned it and understood it best in school, but this was also a great explanation! Nice and simple, keep it up! :)
Great video and practical example of some basic electrodynamics, much appreciated
I was trying to explain this to a cocky technician, who was 15 years senior to me. He was having none of it. I designed a control panel with a 3phase 2.5mmsq supply. There was a 16A single phase breaker on each live. His argument was that this arrangement would put 3x 16A=48A of load on the Neutral (2.5mmsq) wire. To prove he was right he even rang his mate, who was a lead in Intel, and they both were in agreement on this. Good guy though.
The neutral wire is the return path for unbalanced current in an AC circuit. It is not additive, it is subtractive. Put an amp clamp on the neutral wire of a nominal load, and you’ll even see that the current on the neutral will be equal to the current on the line wire.
Doesn't wire size limit current flow ??
@@PORTEnSious yes wire size affects current flow as smaller wires can introduce greater resistant. But this is effectively in series so affects current through the whole of that circuit. Same way an appliance would affect current. V=I/R But that doesn't really affect the topic in the video. As what ever current passes still follows the rules they described. Hope that helps
To be fair if it was the same phase they'd have been right. I think some brains just don't like coping with multiple phases. You'd hope better from an old boy though. 😁 They've had AC since Tesla lol
@@PORTEnSious Wire size doesn’t limit current flow, but depending on the size of the load vs. the undersized wire, what will happen is the wire will not be able to dissipate the heat generated quickly enough, and will cause rapid deterioration of the conductor insulation. Also what can happen is the the wire itself can melt, most likely at the point of termination closest to the load. A real life example would be a water heater that pulls 23 amps and is being fed from a 30A breaker @ 240V, but instead of using #10 AWG copper wire (which is rated for 30A using the 60°C in the NEC ampacity table 310.16 for residential applications). For the sake of this example, the installer uses #14 AWG copper for this installation which using the 60°C table of NEC Art.310.16, is rated for 15A maximum. In this situation, you have 23A running through wire that is rated for 15A maximum. In this install, you would definitely find overheating, burnt insulation, damage at the breaker and at the water heater disconnect switch, or directly at the water heater itself. One thing I forgot to mention is the ampacities mentioned are maximums for periods of time 3 hours or less. There are specific rules in the NEC I don’t have time to mention, but generally if a load is intended to be ran for 3 hours or more at any given time, it is not supposed to exceed 80% of the breaker capacity. For example, if a shop heater is rated to draw 24A, the breaker MUST be rated for 30A (30X.80=24A).
Great demonstration. Thanks. I know it but it is nice to hear/see it again.
Mind blown, but it all made total sense when you showed the wave form graph 👍
Wow, I’ve been looking for this. Thank you
Which is why years ago we used to install 31/2 core cables as supply cable with a reduced neutral when the single phase loads were relatively small
Very interesting video as always. Love this channel 🙌
Hats off to you sir ... really thank you for the simplicity ..
Fun to watch, when you already know why and he explains it well :)
Great explanation Joe.
Cheers Sergio!
Gray video and great explanation. Big thank you 👍🏼
Great Video, very helpful. Thank you guys!
For anyone having trouble envisioning this, it's a bit easier if you start with something like American (two phase) house wiring, which comes in from the pole transformer as 240V, but with a center-tapped neutral wire. That means that neutral to either live conductor is 120V, so you basically have two 120V circuits, but each one is 180 degrees out of phase with the other (and typically half of the house circuits will be wired to one side and the other half to the other side). Because of this, if you need more than 120V for some appliances (such as many stoves, water heaters, EV chargers, etc), you can just connect them across the two different "live" wires to get total of 240V instead. In that case, you don't need the neutral wire, because all of the current going in on one side is balanced by the (exactly opposite) phase on the other wire going out. But it also means that on the neutral line going back to the pole, there may or may not be a lot of current, depending on how well the loads on the different house circuits are balanced. If you've got the same amount of stuff going on one half as on the other half, then all of the current will go "in" from live A to neutral, then from neutral to live B and back out again, and there will be no net current on the neutral line. However, if there's more load on the A side than the B side (or vice-versa), then all of that excess needs to go back out the neutral instead.
Thanks for this explanation on the US voltage supply.
FFS! If you have trouble envisioning it then go back and re do your entire electrical training! It's that basic a question. And if you don't know the answer to it, you shouldn't be anywhere near mains electrical installations, even if you are "qualified". Because for you not to know the answer means that: 1. You have forgotten your basic electrical education, and 2. That you don't really understand electricity.
180deg across the phases. You sure about that?
@@greenpedal370 Yes, he is quite correct about their being 180 degree phase shift between the two lines in the US split phase system. If you know enough about sine waves and electricity, you will realise that 180 degrees is the only way that is possible. If you think he is wrong because you think it should be 120 degrees, then that is certainly incorrect. 120 degrees phase shift between phase conductors only occurs in 3 phase systems. And that is because they physically position coils around in the generator at 120 degrees around the rotor. In the US split phase system, the phase signals that are distributed to consumers do not originate from the power station and generator, they are produced by the down stream distribution former and is a result of how the secondary windings are connected. It's very different to 3 phase.
wow that explanation made my brain hurt
🤯 Wow, that was explained very well. 👍🏽
Nice video, very informative
Very good explanation with examples. 👍👍👍👍👍
Another great video Joe 👍
Always remember a site we took over from a previous company. They told the customer that all the 3 phase circuits had no neutrals and they all needed rewiring… just showed lack of knowledge on the subject. Maybe more should be taught about things like this before people are let out on the wide world 😅 Neutral in a 3phase is generally found if the item has other elements eg control panels and other single phase components with in the 3phase item. Great video guys!
Thanks very much for your time and effort
That was excellent work.
Subscribed! Very nice and informative! thanks!
Wonderful explanation . Thanks.
Very good video and information. Thanks👍🏻
Very good explanation. Thanks
My father taught me 3 phase theory 42 years ago. I never needed it but I see I still remember it correctly.
great explanations. great channel
Great Explanation 😍
This is spot on . I think most people missed the fact that , even though you don’t need neutral , and depending on what you are running , and in this case 3 units 1 phase heater: it is safer to run a neutral at 1/3rd the max amp capacity , in this case and if in the event if one of the 1st phase heater encounters problems .
It would appear from the video that you need the neutral to be the same size as the phase conductors, as if one heater fails you need to be able to carry the full rated current of the phase conductor.
Great explanation Joe
Thanks Brian
I challenge you to actually disconnect the neutrals with running equipment 🙂 Actually that would be a *really* bad idea. Happened at work once in the distribution board (faulty installation, took a couple of years to fail after the building was constructed). The insurance claim was I believe in seven figures and we were super lucky the building was not burnt down. Helped that the fire station was just across the road and they responded in under 90 seconds.
Used to do this on a similar experiment for my learners...
@@efixx For those that don't realize no neutral and all the lives float to 415V AC. The fire was started when the 400V rated capacitor in the power supply of a PC decided that being at 580V (415*sqrt(2)) was not good and flames came out the back of the PC (the slowmo guys have some nice footage of capacitors giving way), and set alight to the noticeboard behind (this is why the mains smoothing capacitor in a SMPS should be rated for 600V). The reason for the really high insurance claim was this was a biosciences lab and at lot of the very expensive equipment didn't take kindly to the 415V AC feed to them.
If it was unbalanced, the current would be present at the end of the disconnected Neutral. The equipment would look dead, right?
If you had just one heater and nothing else there would be no current flowing. If you had two heaters each 80 ohm (720 W) you would have 415 V over 160 ohm. That is 2.6 A and 1080 W (540 W each). That is if you have three identical resistors on a star connection with the neutral is connecting one reduces power by a third and two by 2/3. If you do not have a neutral then disconnecting one reduces power by 50% and two by 100%. The real problem is that if you have some other loads in parallel like an incandescent bulb with 960 ohm. If it if gets in series with the 80 ohm heater at 415 V it gets 383 volts, i.e. it gets 2.5 x the power and will not last long. If you disconnect the neutral upstream from the main panel then the voltage in it gets to the earth wire.
Had the neutral at the substation vanish with thieves, and the phases started to diverge on all the houses. Had the UPS start to warn about overvoltage, so measured mains at 270VAC. Immediately turned off all loads, and turned off the mains to the entire home as well, then went to the meter room and checked phase voltages. One was at 270VAC, one was around 230VAC, and the third was around 170VAC. Turned off the entire building, and got on the phone to the metro about a loss of neutral at the substation. Took them most of a day to fix it up, and they came back the next night to do it again, so they put up the steel doors instead of the wood, and welded all the cable access covers on, that they used to strip the SWA feeder cables leading to the street.
excellent video, congrats!
Interesting stuff...I was expecting "it depends on if it is fed from a Star or Delta transformer..." great explaination.
Great explanation thank you
Great video. Thank you
great explanation!
Very good explanation
Interesting demo. Will be valuable for all those domestic sparks that never get involved with three phase systems. Came across a large lighting circuit like this some time ago, gave me a bit of a head scratch till I realised that the neutral current will never exceed the current of one phase
Those domestic sparks that have only undertaken the domestic installation courses are not allowed to do 3 phase work (in that they are not allowed to do industrial installations). And if people are qualified to do industrial electrical installations then they have been trained in 3 phase and should not need this video.
You woke up on the wrong side of the bed Dean? The video could be helpful for people who are in the process of qualifying for carrying out industrial installation work? 🤷
@@efixx quite agree there are plenty of time served industrial engineers that I know that would have to head scratch about this one
@@deang5622 in many parts of the world its normal to have 3 phase power in your home, even in apartments...
Do you mean "domestic spark from UK" only. All other domestic sparks around the UK (and commonwealth and U.S.A. ) use three phase connection to every house/garage/shed/fabric hall since Siemens & Halske around 1880 use 3phase AC and build the generators around the world!
very nice demonstration of Kirchhoff's current Law
Great job! 👏 👍
Excellent presentation
Thanks
Brilliant explanation
Thanks
Thanks for the topic, effort & way explained.. Q: Why sometimes we find current flowing even thu breakers are off mode..
Great video 👍
Legend! Keep it up!
A good demonstration for non Sparky’s to understand 👍
Important info ... Thanks efixx
Very well explained from average person point of view. THANK YOU GUYS
Very interesting and well explained. However, if the incoming neutral becomes disconnected as I have experienced at a heritage railway, where the unbalanced loads caused serious problems with some devices burning out and other loads suffering low voltage. There seems to be no protection against such a problem and the supply company sometimes won't admit there is a fault.
I've come to the conclusion that I am 'Electro-blind' !!! I love videos like this one and it is extremely well presented but I just can't get to grips with the physics of electricity lol. Keep up the good work and I'll just be happy I can change a 3 pin plug!
I’m not a sparky yet I understood it. Thank you.
Loved it 😻
That's the reason the 400V continuous flow water heaters with 22 or 24kW, that are popular here in Germany for electric water heating for showers, don't require a neutral at all. In most cases we still install a five core cable. The five core NYM-J is only 1€ per meter more expensive than the four core one. Neutral isn't connected of course, but still available if it would be required in the future.
It’s a really good idea to pull the neutral always for future use, here in Finland it used to be acceptable to wire 3-phase sockets with no neutral, if they were intended for a large machine, it’s really fun when you rock up with a portable distribution board to only find that they left out the neutral. And it’s no fun either to have to install transformers for 400/230 just to get some new-fangled boiler with a control board that needs a neutral working. :)
@@sstorholm Or you install another motor, some require a neutral just to start, and while running it's disconnected.
@@Marcel_Germann What sort of abomination of a 3-phase motor requires a neutral to start?
@@sstorholm Not the motor itself, but eventually the electronics that regulate the speed of the motor. And maybe contactors if you want to reverse the direction of rotation. And also possible, the future replacement requires for other reasons 230V in addition to 400V. Unlikely in industrial applications, but not unthinkable, and in domestic applications this can always happen. I remember my grandparents had a Perilex socket outlet (three-phase socket outlet )in their bathroom for the washing machine. The plug always "wandered" from the old to the new washing machine because it was the old rare 25A version.
Good stuff very. Interesting
amazing video
That's an excellent explanation .being a growatt service engineer, I was wondering why the ongrid inverter works fine without neutral.
Nice, that was cool
pretty much exactly. A new way of saying things :)
So when you had two heaters connected and measured the current on the neutral, it wasn't the same as L1 + L2 because it was partially mitigated by the opposing phases, but not completely as it was when you activated the third heater? - fascinating.
Great video, actually really enjoyed that one !
Good video
Thanks for the current measurement on the neutral with the two loads on L1 and L2. Both because it's not intuitive, and will force me to interpret the KCL and the desmos graph on my own as an "exercise for the reader". Where in the circuit are the neutrals of each of the individual phases physically connected? Within each of the outlets on that piece of plywood? The diagram at 5:36 shows the conceptual drawing; but where is are the physical connections of that neutral in the Y-config?
@tti that packs a punch..good explanation
I received my new TIS voltage indicator today thank you eFIXX. It’s a great bit of kit , very much appreciated.
Great to hear!
Hello, I just looked up this information and found your channel. My dad is an (old time, retired) electrician and has taught me to do basic electric work. I replaced a ceiling outlet (lighting) and instead of turning off the circuit, I just turned off the outlet as it's and old house and it's supposed to only have 2 connections (phase + neutral, no earthing) in most places. Turning off the phase should do the trick, and even if there is some leakage, it should only tingle. I verified everything with a basic "glow lamp" style seeker. No phases. And then verified with a multimeter - No current flow from L1 to Neutral. However, I noticed the neutral had 2 wires attached. That was peculiar,but I thought it might just be sharing a neutral with some other light or WTF knows? It shouldn't be an issue. When I disconnected the L1 from the ceiling outlet = no issue. When I disconnected the neutrals, small spark and the lighting on the walls in that area of the living room (supplied by wall outlets) turned off. Interesting. I shat my pants, stepped down from the ladder, took a moment to check myself, wondered why the RCD didn't trip (obivously it only sparked, didn't touch me in hindsight) and then turned off the entire house main feed. I'm not messing around anymore when neutrals and phases are mixed around everywhere. Anyway, I told my dad and he refused the possibility of electricity in a neutral. I told him electricity is defined as a difference in charge, and there should be current flowing return in the neutral. Now I understand it better, and I'll show him this video. (Also, I'm the EU with a proper RCD and I use VDE tools with the 1-hand-behind-the-back principle when doing semi-stupid shit like this. Worst case is going to be a zap until the 30ma RCD cuts out)
I would clarify slightly in your eater example, if you disconnected the 3 individual neutrals all 3 heaters would switch off but disconnecting the common point they would run in a star configuration. I know most viewers will know thats whats happening but just for clarities sake.
it would be good to see a video on that 👍
@@brianhewitt8618 star points are an interesting thing as is generator earthing which is a related subject. Not sure if efixx will cover them but maybe sparkyninja?
I just done an energy study on a school and had upto 60A running through the neutral which I thought could be the difference between phases and all the different loads over the school. Drawing around 250A per phase
Absolutely. I've seen many a live neutral. Never touch without testing.
Finaly an answer to a question
Many moons ago I was working in a computer factory (not as electrician :) ) when a cloud of acrid smoke tripped the fire alarms. Turns out the busbar chamber had a reduced size neutral, as the load was considered pretty well balanced. Ad Hoc alterations had seriously unbalanced the system and the neutral was glowing red hot with all it's PVC insulation burnt off! Turned off and fixed, with half a day's production lost, but could have had much more serious consequences.
The neutral over heated due to harmonics. Most times it is the 3 rd harmonic. It is a result of having loads that are non linear to current. That is variable frequency drives, computers, discharge lighting etc.
Thanks for the vid. So if a 3 phase motor needs no Neutral how does the current travel back to the source?
I learned all this when I was an apprentice 60 years ago.
Couple years ago I used 3 phase for powering a few dosens of single phase crypto mining RIG's... At some point I check the cable and it was so hot. Then I did same as you mentioned to balance the phases and voila cable isn't hot anymore :)
Yo, this is magic stuff 😍 Iam just rolling into big buildings with 3 phase ac and hi power pumps and motors I'll stick to this channel Fwy, I imagined that it has to do with the 3 phase, if you would add up the 3 loads trough the neutral it should be way bigger than 2.5 2 waves can cancel each other out. Dad was a guitarist 😅
It's simple. The 3 phasers are vector quantities and therefore add up to a resultant vector in the neutral depending on the magnitude and phase displacement. Electricians were taught this as part of their apprenticeship in my day...
If your circuits are not balanced in the breaker box, you can get current flowing on the neutral line. Also, if the protective covering on the wires feeding your breaker box crystalize, you can also generate current flowing on the neutral line. You can argue with me all you want, but I learned this through experience when we installed a minicomputer in our newly renovated building. The computer was blowing boards every month, so we put a line analyzer on the system and discovered current on the neutral line. Apparently, computers do not like current on that line. Upon diagnosing the problem, we discovered what I mentioned above. Once we fixed those two problems, the current was no longer found on the neutral line.
1:23 hey I have that same multi meter in Green! Although that looks like it may have an extra button.
Great video. Well done. But it is clouding up the subject a bit. Of course, using three phase for single phase devices happens all the time, but single phase devices use one two hot lines. 120V and 277V devices use one hot line and neutral. 208V and 240V could be either one hot line and neutral or two hot lines and 480V is always two hot lines (all examples are single phase). I would like to see this using a single-phase supply, L1 of the 240V single phase supply wired to all three heaters in parallel, then check the neutral amps as each heater gets energized. A previous comment said the standard residential supply is two phase because of the two hot lines. That is wrong. The standard residential supply is 240 Volt, Single Phase. Best Wishes. Hope you keep up the good work.
Perfect
Great video here Joe, can I ask if you have a 3 phase motor which draws 60 amps for example, does 60 amps flow in each phase or does it get divided into the 3 phases i.e. 20 amps down each phase?
Motors are rated w/ s/ phase. So its Amp per phase
@@haidarbadavi9460 Just to confirm it's 60amps per phase.
@@ia4387 If it is being called 60 amps three phase, you will get three 60 amp readings, not three 20 amp readings. Hope this helps. Kevin
3 phase Motors are usually shown as Amps per Phase. 'Rule of thumb' 2 amps per KW. Takes account of power factor and efficiency.
One way of thinking about this is that the neutral current is the total phase imbalance. One phase loaded(= 1kw imbalance). 2 phases loaded (= 1 phase = 1kw imbalance). 3 phases equally loaded - no imbalance.
Thanks that was excellent. I've had this "discussion" many times with " professionals" who did not want to put a neutral line in at all. But you have raised a doubt now! If you remove the neutral from your star winding drawing doesn't it became 415 instead of 240? Sorry but that's a stupid question I hung the screwdrivers up a long time ago!
The way to get to 415 V given three-phase 240V, is to connect between the phases in a delta configuration, not a Y configuration.
I worked on a 3 phase oven a big big boy oven. It needed a neutral. It was 3 phase but worked as single phase. As each pair of elements. Was only on one pair at a time.
What was the voltage applied to the heating elements? 416 or 240 volts? Sometimes the elements are 416 single phase and multiples of three, and the controls are rated at 240 volts. It could be either way, just how they are connected in the equipment. Best wishes, Kevin
@@KevinCoop1 Don't recall now just the way the contactors were wired up. Do think somewhere I have a photo of the front so I will look it up one day.
cool to see it explained this way... so does that mean that the power company 'meters' actually measure each phase individually for calculation of how much to charge us for power?
Yes, and then they add the 3 values together. Unless you're an industrial installation that gets charged for the imbalance, then there's a fourth meter on the neutral.
I have a question, if in 2 phase circuit you have exact the same current in the neutral is no current used in consumer? and if so why we pay for the production of current and not just transport and distribution.
What amperage/current will there be when 1 heater is higher then the other 2? Is it the same as when there is only heater on?
Well explained, but any electrician who does not know this should not be let loose with a screwdriver!
New people train to join the industry every year - knowledge is power.