Shared Neutral Current Additive 120/208?

I'm remodeling my kitchen and am bringing it up to code. My service is 120/208. I wired up my first multiwire circuit. I decided to read neutral current flow with a clamp on ammeter to make sure everything was cool. I had a 1 amp load and a 2 amp load on the two hots. I expected to see something less than 2 amps on the neutral but was surprised to see the full 3 amps. The hots are fed from opposite busses. Can anyone help me figure this one out?




Thanks,


Don
 

Sahib

Senior Member
Location
India
I think your supply 120/208V is tapped from 3 phase supply and not 120/240V single phase supply. As such, there is no wonder you get 3 amps in the neutral.
 

iwire

Moderator
Staff member
Location
Massachusetts
I'm remodeling my kitchen and am bringing it up to code. My service is 120/208. I wired up my first multiwire circuit. I decided to read neutral current flow with a clamp on ammeter to make sure everything was cool. I had a 1 amp load and a 2 amp load on the two hots. I expected to see something less than 2 amps on the neutral but was surprised to see the full 3 amps. The hots are fed from opposite busses. Can anyone help me figure this one out?
When you use just two phases of a 3 phase wye supply the current on the neutral is about the same as the current on the the hots. It is normal.


I think your supply 120/208V is tapped from 3 phase supply and not 120/240V single phase supply.

Are you sure?


. My service is 120/208.
 
Last edited:

Sahib

Senior Member
Location
India
I did not say the same, I said about the same, please look the words up that you do not understand.
Even 'about the same' does not make sense: in one hot it is 1A and in the other hot it is 2 A and in the neutral it is 3A.
Are you sure? :D
I am sure it is single phase supply tapped from POCO's three phase supply.:slaphead:
 
Last edited:

iwire

Moderator
Staff member
Location
Massachusetts
Here is a code section about current carrying conductors that goes along with this discussion.

310.15(B)(5)(b) In a 3-wire circuit consisting of two phase conductors
and the neutral conductor of a 4-wire, 3-phase, wye-connected
system, a common conductor carries approximately the same
current as the line-to-neutral load currents of the other conductors
and shall be counted when applying the provisions of
310.15(B)(3)(a).
 

mivey

Senior Member
Here is a code section about current carrying conductors that goes along with this discussion.
310.15(B)(5)(b) ...a common conductor carries approximately the same current as the line-to-neutral load currents of the other conductors
Approximately the same as either one but not the sum unless you have something weird like a pure resistive on one and pure reactive on the other or some similar mix of peculiarity.

1@0d + 2@120d = 1.73@90d

1@0d + 2@30d = 2.91@20.1d
 

iwire

Moderator
Staff member
Location
Massachusetts
Welcome to the Forum

The breaker should be on 2 & 4, not 1 & 2. :)
I agree: the breakers would be on the same phase. But what about the common trip?
The OP did not say what circuit numbers he was using but he does have 208 volts between the two breakers.

How do I know this? My supernatural powers tell me so.

































Or it could be this post from the same person on another forum. :D

That was my first thought but I get 208 hot to hot. Very strange.
 

iwire

Moderator
Staff member
Location
Massachusetts
Approximately the same as either one but not the sum unless you have something weird like a pure resistive on one and pure reactive on the other or some similar mix of peculiarity.

1@0d + 2@120d = 1.73@90d

1@0d + 2@30d = 2.91@20.1d
Engineers could make tying ones shoes into a complicated drawn out affair. :)



Do you really think his amp clamp is dead nuts accurate and that his true readings were 1, 2, and 3 amps with no decimals?
 

mivey

Senior Member
The OP did not say what circuit numbers he was using but he does have 208 volts between the two breakers.

How do I know this? My supernatural powers tell me so.
Cheater!:D

Well then I vote for unbalanced reactive load (I can't picture where that would come from in a kitchen unless the GD was just running empty for a load test) or a harmonic load like from an electronic ballast.
 

mivey

Senior Member
Engineers could make tying ones shoes into a complicated drawn out affair. :)



Do you really think his amp clamp is dead nuts accurate and that his true readings were 1, 2, and 3 amps with no decimals?
I already considered that and dismissed it as something an electrician would think so it must be invalid.:D

Besides, that would be a heck of a rounding issue since both phase readings would have to be off by a good bit, relatively speaking. Say 2 amps and 3 amps were actual, then the neutral might be rounded to 3 instead of an actual 2.65. No, I'd rather assume the OP knows how to read a meter until we find out differently.

Maybe the meter is not reading true RMS for some electronic loads?
 

iwire

Moderator
Staff member
Location
Massachusetts

:D

I would rather call it using all the resources I have available.


I already considered that and dismissed it as something an electrician would think so it must be invalid.:D
:D


Besides, that would be a heck of a rounding issue since both phase readings would have to be off by a good bit, relatively speaking.
1.999 = 2, it is electrician rounding.

If I open two lighting fixture boxes and only 1.99% of the parts are there .... it equals one. :p




Seriously though my amp clamps are not good at measuring accurately at low levels even my true RMS one.
 

kwired

Electron manager
Location
NE Nebraska
There was no cheating, Op said his service was 120/208 in the second sentence of OP.



Don, if you were to bring in the third phase and share that neutral in your multiwire circuit - then you would have zero current on the neutral if all three phases are the same current.

The harmonic currents on the neutral of non linear loads however are additive in a wye system.
 

iwire

Moderator
Staff member
Location
Massachusetts
There was no cheating, Op said his service was 120/208 in the second sentence of OP.
The 'cheating' was the fact I knew he was on two phases, not the same phase.



The harmonic currents on the neutral of non linear loads however are additive in a wye system.
True, but even without non-linear loads he would have neutral current.
 

charlie b

Moderator
Staff member
Location
Seattle, WA
I think we are overthinking this. Forget the phasor additions and the issue of how close is "about" right. The measurements were 1 and 2 amps, but exactly how accurate were those measurements? Can a clamp on ammeter really distinguish 0.9 amps from 1.1 amps, so that we can say with confidence that the actual current was, in fact, 1.0 amps and not 1.1? We don't know how the measurements were taken, nor with what instrument, and we don't know the degree to which the loads were balanced on the panel as a whole (i.e., we don't know phase angles).
 
Top