Potential Feeder Imbalance

[jim dungar]

Citation?

For the OP, I guess pulling in one more conductor in each is too hard?

Cheers, Wayne

Are they install per section 250 requirements?

 

[ptonsparky]

To my way of thinking, there would be a grounded conductor from POCO transformer to the SE. Neutral load or not. After that comes the EG.

 

[wwhitney]

Are they install per section 250 requirements?

Not aware that section 250 would require anything other than tying the extra conductor to the EGC at both ends.

Cheers, Wayne

 

[wwhitney]

I'm having trouble seeing what the actual technical downside to the install in the OP would be. We can take as our baseline "worst acceptable imbalance" the isophase installation allowed under 300.3(B)(1) Exception. So if we had (3) 3" conduits in a row, we could install 3xA in the first, 3xB in the second, and 3xC in the third.

Now delete the third conduit and move the C conductors into the A and B conduits. This is going to decrease the mean distance between conductors of different phases, and is therefore an arrangement that seems to me closer to having all the conductors in a single conduit. Doesn't that mean that the imbalance in impedances will be less than in the isophase baseline case?

If someone with more physics that me can point me to the relevant equations for capacitance and inductance per unit length for parallel conductors at a distance d from each other, I'm happy to do the math comparing the various spatial arrangements.

Cheers, Wayne

 

[kwired]

I'm having trouble seeing what the actual technical downside to the install in the OP would be. We can take as our baseline "worst acceptable imbalance" the isophase installation allowed under 300.3(B)(1) Exception. So if we had (3) 3" conduits in a row, we could install 3xA in the first, 3xB in the second, and 3xC in the third.

Now delete the third conduit and move the C conductors into the A and B conduits. This is going to decrease the mean distance between conductors of different phases, and is therefore an arrangement that seems to me closer to having all the conductors in a single conduit. Doesn't that mean that the imbalance in impedances will be less than in the isophase baseline case?

If someone with more physics that me can point me to the relevant equations for capacitance and inductance per unit length for parallel conductors at a distance d from each other, I'm happy to do the math comparing the various spatial arrangements.

Cheers, Wayne

With isophase you have equal impedance on individual conductors of each parallel set.

With OP's described setup you have two conductors of one phase in one conduit and one conductor of same phase in the other conduit. The impedance difference will cause current to divide unevenly over those three conductors.

OP at very least needs to find a way to get every conductor of a phase in the same raceway. This could still lead to overall phase imbalance but won't result in much for unbalance on the individual conductors of any phase set. This may result in two phases in one raceway and one phase and grounded conductor in the other raceway. Maybe not ideal arrangement, NEC may or may not apply, probably better situation than what he currently has though.