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Discussion Starter · #1 ·
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
 

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When using just two hots of a three phase supply the neutral current is about the same as the currents on the hots. This is because the hots are 120 degrees apart not 180.
 

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Here is a code section that relates to it

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).
 

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In your example (1A and 2A) the neutral current should be about 1.7A
 

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So, if each of the two circuits are drawing 20amps each, the neutral would have approx. 22amps?
Actually, it would have 20 amps.



In = SQRT((Ia*Ia+Ib*Ib+Ic*Ic)-(Ia*Ib+Ib*Ic+Ic*Ia)) (posted after BBQ did below)
 

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Discussion Starter · #14 ·
Troubling Update

After more thorough testing I am even more troubled.

Quick recap: My service 120/208, hot to hot is 208.

With an 8.5 amp toaster oven on one circuit and 11 amps on the other circuit which feeds the fridge (about 1 amp) and space heater (temp) my neutral current reads 19.2 amps. I'm using an Amprobe ACD-15 Pro clamp on ammeter. This condo has been wired with mostly MWBCs ever since it was built from what I can tell. Have the neutrals been overloaded the whole time?! I'd really like to get on the feed to my panel which is just a subpanel but I just don't have the room.


Thanks,

Don
 

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Discussion Starter · #15 ·
It's always something stupid and simple...

I'm both embarrassed and happy to say that after further troubleshooting I discovered that the outlet we wired temp for testing was miswired and being fed by the wrong circuit. This led to all loads being on the same leg. I was a little lazy in my initial troubleshooting and made too many assumptions. One assumption was that there was a problem in the first place.

I think the initial problem I saw with the 3 amp neutral was simply due to low inductive loading along with meter creep.

Final Reads Ignoring Fridge (<1 amp) which is always on:

CKT 1 on 2 off - In=9.1A
CKT 1 off 2 on - In=11.9A
Both CKTs on - In=12.0A

Thanks for the replies.
 
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