Neutral current

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the formula for an unbalanced 3 wire neutral current is

I=sqrt[(L1^2 + L2^2)-(L1 X L2)]

just out of hypothetical conversation with a fellow electrician, what if it was grossly unbalanced and you had 200 amps on L1 and No AMPS on L2. This is basically a 120/240 service to a house.

I said I suppose the neutral would burn up since we use 4/0 4/0 2/0 aluminum for our services. :(. since using table 310.15(B)(6) allows this for dwellings, 2/0 is only good for 150 amps. but the actual calculation lets you derate the neutral. Of course this would be a long shot since something would have to be GROSSLY wrong for an unbalance current to that degree. ;)

So what do you guys think?
 

don_resqcapt19

Moderator
Staff member
Location
Illinois
the formula for an unbalanced 3 wire neutral current is

I=sqrt[(L1^2 + L2^2)-(L1 X L2)]

just out of hypothetical conversation with a fellow electrician, what if it was grossly unbalanced and you had 200 amps on L1 and No AMPS on L2. This is basically a 120/240 service to a house.

I said I suppose the neutral would burn up since we use 4/0 4/0 2/0 aluminum for our services. :(. since using table 310.15(B)(6) allows this for dwellings, 2/0 is only good for 150 amps. but the actual calculation lets you derate the neutral. Of course this would be a long shot since something would have to be GROSSLY wrong for an unbalance current to that degree. ;)

So what do you guys think?
The code rules require that the grounded conductor be sized for the maximum unbalanced load. Remember that line to line loads do not add any neutral current.
 

chris kennedy

Senior Member
Location
Miami Fla.
I think your install should be planned acording to this.

210.11 Branch Circuits Required.

(B) Load Evenly Proportioned Among Branch Circuits. Where the load is calculated on the basis of volt-amperes per square meter or per square foot, the wiring system up to and including the branch-circuit panelboard(s) shall be provided to serve not less than the calculated load. This load shall be evenly proportioned among multioutlet branch circuits within the panelboard(s). Branch-circuit overcurrent devices and circuits shall be required to be installed only to serve the connected load.
:)
 

cpal

Senior Member
Location
MA
If you are drawing 200A thru a 200A breaker there is a good chance the device will clear as a overload (hopefully minutes instead of hours)some one here will most likely add info on actual time trip curves. I'm not sure what the values are but if you installed copper. That wire can withstand apporximately 1 amp for 5 seconds for every 30 cir mil (or 42.25)
at 75 deg C. Ref the IAEI Green Book adapted from Ustice Soares. I m guessing most of the service entrance is outside or underground so the conductors can handle a little more than that listed in table 310.16. maybe long enough to get that hot breaker in the enclosed panel to clear.

but I'm sure some one else will have another twist.
 

gar

Senior Member
090926-2057 EST

brother:

You are describing a single phase residential 200 A service with a center tapped transformer.

If you equally load both sides with loads that have the same power factor, then there is no neutral current.

If you load only one side to 200 A and zero on the other side, then the neutral current is 200 A.

If you put a purely capacitive load on one side of 200 A, and a purely inductive load on the other side of 200 A, then the neutral current is 400 A.

The neutral current can be theoretically anywhere between 0 and 400 A.

In the real world under most normal conditions the neutral current will be substantially below 200 A.

.
 

infinity

Moderator
Staff member
Location
New Jersey
just out of hypothetical conversation with a fellow electrician, what if it was grossly unbalanced and you had 200 amps on L1 and No AMPS on L2. This is basically a 120/240 service to a house.

I said I suppose the neutral would burn up since we use 4/0 4/0 2/0 aluminum for our services. :(. since using table 310.15(B)(6) allows this for dwellings, 2/0 is only good for 150 amps. but the actual calculation lets you derate the neutral. Of course this would be a long shot since something would have to be GROSSLY wrong for an unbalance current to that degree. ;)

So what do you guys think?
I agree with your calculation. The question is how long would the #2/0 conductor last with a load of 200 amps.
 

iMuse97

Senior Member
Location
Chicagoland
2/0 AL rated for 150 amperes, means: it can handle that load without deterioration of the insulation. I've seen a lot of wire that size starting to lose insulation integrity at the terminations as a result of overloading, SO--
Just from field experience,
I'll say that 2/0 AL neutral would last a long time (years), if the load was not continuous.

If it was a continuous load, it would still last a few months before significant problems began to show at insulation near the terminations.
 
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Smart $

Esteemed Member
Location
Ohio
the formula for an unbalanced 3 wire neutral current is

I=sqrt[(L1^2 + L2^2)-(L1 X L2)]

... This is basically a 120/240 service to a house.

...
Hmm... I believe that is the rudimentary neutral current formula (neglects unequal power factors) for a 3-wire circuit originating from a 3-phase, 4-wire wye source.

The basic neutral current formula for a 3-wire circuit originating from a 3-wire split phase source is:
IN = |IL1–IL2|
 

iwire

Moderator
Staff member
Location
Massachusetts
The code rules require that the grounded conductor be sized for the maximum unbalanced load. Remember that line to line loads do not add any neutral current.
If there was 200a worth of 120v load connected to one line, the neutral could not be smaller than the line conductor.
I agree, the NEC does not allow loading the neutral beyond it's capacity.

Assuming NEC compliance the neutral is in no danger, if we assume the NEC will be ignored every installation is a danger.
 
the formula for an unbalanced 3 wire neutral current is

I=sqrt[(L1^2 + L2^2)-(L1 X L2)]

just out of hypothetical conversation with a fellow electrician, what if it was grossly unbalanced and you had 200 amps on L1 and No AMPS on L2. This is basically a 120/240 service to a house.

I said I suppose the neutral would burn up since we use 4/0 4/0 2/0 aluminum for our services. :(. since using table 310.15(B)(6) allows this for dwellings, 2/0 is only good for 150 amps. but the actual calculation lets you derate the neutral. Of course this would be a long shot since something would have to be GROSSLY wrong for an unbalance current to that degree. ;)

So what do you guys think?

I agree with Imuse. Not that my opinion is calculated, just from experience in the field. It is an overload, so there are no magnetic forces, just heat.
 
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