Shared Neutral Current Additive 120/208?

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Smart $

Smart $

Esteemed Member
Location
Ohio
donniet1977 said:
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.

Lol, someone caught me. I did post this on 2 or 3 other forums. I figure that enough quantity is bound to generate some quality.

Thanks,

Don
Click to expand...
IMO, 19.2A on the neutral is quite high for the described loads. A toaster is a resistive load, and a space heater is resistive or mostly resistive if it has a fan. Only the fridge and space heater fan would be reactive loads and should not push neutral current anywhere near 19A.

While L1 to L2 may measure 208, hot to hot can also be 0 (L1 to L1, or L2 to L2). Make sure the circuit hots are not connected to the same line.
 
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GoldDigger

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Location
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If you had not made the phase to phase voltage measurement at the sub panel, I would suspect that somebody goofed and ran two wires of the same phase to it by mistake.
And, yes, if your observations are correct you do have overloaded neutrals!
Are there any 208 volt line to line loads in the unit? If there are, what are the current readings there?


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D

donniet1977

Member
Location
Hermosa Beach, CA, USA
cadpoint said:
So you just supplied a new circuit up to a point and left the existing wiring ?
Click to expand...

Well...not exactly. Before I touched anything, there were four 1/2" flex conduits coming from the panel. Each had a 3 wire circuit. So there were 4 MWBCs in the panel. This was original. I then bought my tandem and quad breakers to add the required circuits to bring the condo up to code.
 
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donniet1977

Member
Location
Hermosa Beach, CA, USA
GoldDigger said:
If you had not made the phase to phase voltage measurement at the sub panel, I would suspect that somebody goofed and ran two wires of the same phase to it by mistake.
And, yes, if your observations are correct you do have overloaded neutrals!
Are there any 208 volt line to line loads in the unit? If there are, what are the current readings there?


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Click to expand...


No.
 
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GoldDigger

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donniet1977 said:
Well...not exactly. Before I touched anything, there were four 1/2" flex conduits coming from the panel. Each had a 3 wire circuit. So there were 4 MWBCs in the panel. This was original. I then bought my tandem and quad breakers to add the required circuits to bring the condo up to code.
Click to expand...
You did verify the bus connections of your tandem and quad units didn't you?
Remember that on a quad the MWBC pairs will typically be the inner two and the outer two handles.


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cadpoint

cadpoint

Senior Member
Location
Durham, NC
Smart $ said:
IMO, 19.2A on the neutral is quite high for the described loads. A toaster is a resistive load, and a space heater is resistive or mostly resistive if it has a fan. Only the fridge and space heater fan would be reactive loads and should not push neutral current anywhere near 19A.

While L1 to L2 may measure 208, hot to hot can also be 0 (L1 to L1, or L2 to L2). Make sure the circuit hots are not connected to the same line.
Click to expand...

cadpoint said:
So you just supplied a new circuit up to a point and left the existing wiring ?
Click to expand...

My point was that you didn't ring out the existing run of wires to assure that only one circuit is here, and only one circuit is there. :)

Bothersum group of wires this one is ... :)
 
mivey

mivey

Senior Member
mivey said:
English only please.

Can't forget the phasors as they are the answer.

Let's make a wild guess and say there is a refrigerator pulling about 1.14 amp on one phase and a spinning unloaded 3/8" drill on the other phase pulling about 2.14 amps. I do not have a drill with me to measure so assign a 0.1 pf to the drill and a 0.8 pf to the refrigerator. Now the neutral current is 2.71 amps.

I'll buy the power factor and the reading inaccuracies combined but not the readings alone because the phasors are key factors, IMO.
Click to expand...
I see the OP has posted some new information but I'll not let my good measurements go to waste, especially since I had to work up a sweat:
Drill pf = 0.3
Refrigerator pf (cooling) = 0.965
so:
2.14A at 0.3 pf and 1.14A at 0.965 pf gives a neutral current of 2.85 amps.
 
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GoldDigger

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donniet1977 said:
Yes, I'm on the outer two of a quad. I have 202V screw to screw on these circuits.
Click to expand...

OK.
It is time for some redundant sanity checks, since I am about to lose mine:

Put the amp clamp around the two phase wires only.
1.If it doesn't match the neutral current, the wiring is strange.
2. If it does match, call for an exorcist with an oscilloscope.

One way that case one might come about is if both heaters also have connections to the other phase somewhere and you are controlling both by switching one. (120V fixed heaters are less common where 208 is available.)

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mivey

mivey

Senior Member
donniet1977 said:
Quick recap: My service 120/208, hot to hot is 208.
Click to expand...
Well then we are back to the original analysis from cadpoint before that electrician, and I would not want to point out iwire by name, said you had 208 volts (watch me be wrong again).

You must have the feeds on the same phase. Take an extension cord from one receptacle over to the other receptacle and measure the voltage between the two receptacle hots.
 
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donniet1977

Member
Location
Hermosa Beach, CA, USA
It's always something stupid and simple...

It's always something stupid and simple...

:slaphead: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. :ashamed1:
 
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donniet1977

Member
Location
Hermosa Beach, CA, USA
Spot On

Spot On

mivey said:
I see the OP has posted some new information but I'll not let my good measurements go to waste, especially since I had to work up a sweat:
Drill pf = 0.3
Refrigerator pf (cooling) = 0.965
so:
2.14A at 0.3 pf and 1.14A at 0.965 pf gives a neutral current of 2.85 amps.
Click to expand...

I think this is EXACTLY what happened and got me off on my tangent. I just went ahead assuming that I had a problem.
 
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ggunn

PE (Electrical), NABCEP certified
Location
Austin, TX, USA
Occupation
Consulting Electrical Engineer - Photovoltaic Systems
One lesson that I can never seem to learn is this one: Check all the simple stuff first.
 
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