neutral current

[socalelect]

in a 120v ckt the neutral compleats the ckt correct? so if thats the case wouldnt the neutral show roughly the same about of current as the ungrounded conductor feeding the ckt ?


heres my example several 120v circuits from a 120/240V single phase service i was doing some troubleshooting for another issue
so i amp clamped everything L1 had approx. 6 amp load L2 had aprox 5.5 amp load the loads on these are very very small elec motors shouldnt the neutral be carrying back approx some where around the sum of the L1 & L2 amperage ? :?:?

 

[iwire]

shouldn't the neutral be carrying back approx some where around the sum of the L1 & L2 amperage ? :?:?

No, it should carry the difference of the each leg.

Examples

L1=10 amps L2=10 amps means N=0 amps


L1=0 amps L2=10 amps means N=10 amps


L1=15 amps L2=5 amps means N= 10 amps

 

[jumper]

in a 120v ckt the neutral compleats the ckt correct? so if thats the case wouldnt the neutral show roughly the same about of current as the ungrounded conductor feeding the ckt ?


heres my example several 120v circuits from a 120/240V single phase service i was doing some troubleshooting for another issue
so i amp clamped everything L1 had approx. 6 amp load L2 had aprox 5.5 amp load the loads on these are very very small elec motors shouldnt the neutral be carrying back approx some where around the sum of the L1 & L2 amperage ? :?:?

You do not add the differences between L1 and L2, you subtract. You have .5A in your scenario.

If L1 was 6A and L2 was 3A, you would have 3A.

If L1 was 6A and L2 was 6A, you would have 0A.

These are approximations, other factors may apply, but they are usually close for a lot of stuff.


Three phase neutral current is different.


Edit- I type too slow.

 

[socalelect]

i must be having a bad day , i feel kind of like a moron :dunce: at this point

but heres my next question
what if you only had loads on L1 how does that affect the neutral current

how do the loads on opposite L1 vs L2 cancel out neutral current against eachother when the loads are only 120V ?

 

[Dennis Alwon]

If it is a multiwire branch circuit on a single phase then the shared neutral will read the difference between Phases. If one phase is not used then the load will be that of the other phase. This is why you can use a #12 neutral for 2 hot conductors on opp. phases. The current on the neutral will never be more than 20 amps. If the hot conductors were on the same phase then you would need to add the sum of the loads/

 

[jumper]

what if you only had loads on L1 how does that affect the neutral current

Your neutral current would then be equal to L1 current

how do the loads on opposite L1 vs L2 cancel out neutral current against each other when the loads are only 120V ?

L1 and L2 are displaced by 180 degrees. This results in the cancellation on the neutral. The current will flow on the hot conductors and the neutral only carries the imbalance.

 

[iwire]

what if you only had loads on L1 how does that affect the neutral current

L1 77 amps L2 0 amps results in 77 amps on the neutral. 77-0=77

how do the loads on opposite L1 vs L2 cancel out neutral current against eachother when the loads are only 120V ?

They start to run in series with each other.



Here is one example

For now ignore load C.

Close the black switch and you can follow the circuit from black through load A, through the neutral and continue on into load C and out on the red.

If both load A and load B where the same, there would be 0 current on the neutral going back to the source and in fact could be removed.

 

[dkarst]

i must be having a bad day , i feel kind of like a moron :dunce: at this point

but heres my next question
what if you only had loads on L1 how does that affect the neutral current

how do the loads on opposite L1 vs L2 cancel out neutral current against each other when the loads are only 120V ?

No need to feel badly... everyone has this question at some point. The reason is because the voltage on L1 and L2 are 180? out of phase with respect to the neutral (i.e. usually the center tapped transformer). It helps some people to think of the current going "out" on L1 returns on L2 for two balanced loads and therefore nothing returns on neutral.

edit... wow people are fast today...

 

[iwire]

edit... wow people are fast today...

Maybe so but you provided a piece that had not been mentioned.

 

[jumper]

Maybe so but you provided a piece that had not been mentioned.

Yep. Now all we gotta do is wait for the rest of the EEs to show up and this thread will be 300 posts long.

 

[iwire]

Yep. Now all we gotta do is wait for the rest of the EEs to show up and this thread will be 300 posts long.

Tell you what, if someone tries to drag the single phase / two phase disagreement into this I will remove it. (Perhaps into it's own thread but not in this one. )

 

[jumper]

Here is a good article on neutral current. It has some theory and sizing requirements.

http://ecmweb.com/nec/code-basics/electric_code_basics_14/

 

[rattus]

Yep. Now all we gotta do is wait for the rest of the EEs to show up and this thread will be 300 posts long.

Naw, you guys got it down!

 

[iwire]

OK, the EEs came in and went in depth about neutral current, which is fine and many may find it interesting but lets let this thread continue as it was. A more general discussion of neutral currents for electricians, I don't think we are serving the OP well with math tricks.

I have moved the in depth discussion into its own new thread.

[h=3]Neutral currents in depth[/h]

 

[jim dungar]

OK, the EEs came in and went in depth about neutral current, which is fine and many may find it interesting but lets let this thread continue as it was. A more general discussion of neutral currents for electricians, I don't think we are serving the OP well with math tricks.

I have moved the in depth discussion into its own new thread.

Neutral currents in depth

I take offense to this. I simply responded to statements made in posts by Dennis and Rattus.

You cannot discuss how two currents 'cancel each other' without discussing the fundamentals of current flow and the resulting math. The underlying concept is Kirchhoff's Current Law (KCL) which says 'the current into a junction equals the current out of it'.

Think of KCL like: 'what goes in, must come out'. So how do you have two currents into the neutral conductor and nothing coming out? Easy answer, the two currents are actually flowing in different directions.

 

[iwire]

I take offense to this.

Jim, I meant no offense at all.

But at the same time we have at least two ongoing threads, a couple of closed ones all discussing roughly the same issue.

Not every post has to be in such minute detail and it is entirely off putting to those that are just starting out. We crawl before we run.

I did not toss your posts into oblivion I simple gave them there own thread. I did put up a pretty clear warning in post 11.

Now all that said if you want we can take this to the mod forum for a consensus among mods.

 

[ronaldrc]

L1 77 amps L2 0 amps results in 77 amps on the neutral. 77-0=77



They start to run in series with each other.



Here is one example

For now ignore load C.

Close the black switch and you can follow the circuit from black through load A, through the neutral and continue on into load C and out on the red.

If both load A and load B where the same, there would be 0 current on the neutral going back to the source and in fact could be removed.

Bob I had been in the Electrical trade 40+ yrs. before I retired and I have never seen that circuit configuration
that is very interesting. Just lucky I guess I didn't do that somewhere.

I could see how someone could mistakenly think to them self , hey I can wire it up like this and switch that 240
volt circuit with two switches from two different locations. Thats why it pays to draw a diagram before attempting to
a final hookup before or really thinking about it. Never know when your going to do something like that.

Thanks for showing me something new.

Ronald

OK, the EEs came in and went in depth about neutral current, which is fine and many may find it interesting but lets let this thread continue as it was. A more general discussion of neutral currents for electricians, I don't think we are serving the OP well with math tricks.

I have moved the in depth discussion into its own new thread.

[h=3]Neutral currents in depth[/h]

 

[iwire]

Bob I had been in the Electrical trade 40+ yrs. before I retired and I have never seen that circuit configuration
that is very interesting. Just lucky I guess I didn't do that somewhere.

I could see how someone could mistakenly think to them self , hey I can wire it up like this and switch that 240
volt circuit with two switches from two different locations.

I found the diagram on the web and it was close enough for my needs.

You could think of the switches as breakers but really in many cases the NEC does allow a 240 load to be controlled with a single pole switch. It is when it is the disconnecting means that all ungrounded conductors have to be switched.

 

[ggunn]

Yep. Now all we gotta do is wait for the rest of the EEs to show up and this thread will be 300 posts long.

Well, OK, since you asked so nicely... ;^)

Actually, L1 and L2 are technically not 180 degrees out of phase, they are reverse polarity from each other. For a simple single frequency sine wave there is no functional difference, but for a more complex waveform the two relationships are very different.

 

[rattus]

Well, OK, since you asked so nicely... ;^)

Actually, L1 and L2 are technically not 180 degrees out of phase, they are reverse polarity from each other. For a simple single frequency sine wave there is no functional difference, but for a more complex waveform the two relationships are very different.

We are of course only considering pure sine waves.