Parrell Feeders with different amperages

[Flagstaff]

We have a 380 foot run of parallel 4" conduits. Each with 4 - 500 MCM XHHW CU conductors. The system is 480 volts. The conduits lengths are slightly different because of bends etc. so the wires are slightly different lengths. Maybe a foot more or less.

The amperage measured on the two A phase wires was 280 amps and 250 amps. B phase was 280 and 271, on C phase it was 270 and 265. If the wires are within a foot or so of each other why the large difference? In fact, A phase has a difference of over 10%.

I see this with large runs. Even parallel runs that are within a few inches of each other. Years ago our power company been installing A phase in one conduit, B in another and C in another making the wires virtually identical in length. I've noted this with their feeders as well.

Ideas on what is happening?

 

[nakulak]

I don't know the answer, but the idea of this happening and having catastrophic results always scares the farad out of me. We go to great lengths to feed the same exact amount of wire when we parallel.

 

[beanland]

Think Impedance not Resistance

Think Impedance not Resistance

Remember that the current will balance based on the impedance of the lines, not just the resistance. The reactance, the inductive part of impedance, is affected by the way the conductors lay in their own conduits and how they lay relative to each other in the adjacent conduits.

There will always be a difference between the conductors. However, there is enough design margin so that there should never be overheating.

In fact, if one conductor starts to get hot, its resistance will rise, forcing more current in the colder conductor. This negative feedback system will help balance the current in the conductors.

 

[Flagstaff]

do you mean the way the wires twist in the conduits? I wondered if this might have some impact on the impedance. I assume more twist in the wires means less impedance. I also wonder about the consistency of the wire itself. How consistent the copper purity is, how consistent the size of the conductor, that sort of thing.
Thanks for your ideas and input.

 

[brother]

If this is real issue, would not those 'cable overcurrent devices' (not sure exactly what they are called) help protect the wire individually?? Hopefully someone knows what im talking about.

 

[iwire]

If this is real issue, would not those 'cable overcurrent devices' (not sure exactly what they are called) help protect the wire individually??

I will take a guess that you mean cable limiters, but the NEC does not recognize that form of over current protection.

 

[jrannis]

Remember that the current will balance based on the impedance of the lines, not just the resistance. The reactance, the inductive part of impedance, is affected by the way the conductors lay in their own conduits and how they lay relative to each other in the adjacent conduits.

There will always be a difference between the conductors. However, there is enough design margin so that there should never be overheating.

In fact, if one conductor starts to get hot, its resistance will rise, forcing more current in the colder conductor. This negative feedback system will help balance the current in the conductors.

Very good description. You can actually see this happen if you have a little patience.

 

[brian john]

Did a job several years ago where the on site inspector beat up the contractor with the parallel feeder installation from the conduit layout, to the cable lengths, exact lengths, hypress connections laid out in the termination cabinets just so. And guess what the current was not balanced between the conductors. As Ben noted there are other factors you can not control that affect the current distribution between the conductors. The inspector was not happy.

 

[JohnJ0906]

In the OP's situation, the current on each conductor, although unbalanced, doesn't exceed the ampacity of the conductors.

How big a problem is this? Is this simply a curiosity, a catastrophe waiting to happen, or something in between?

 

[brother]

I will take a guess that you mean cable limiters, but the NEC does not recognize that form of over current protection.

Yes i think thats it 'cable limiter'. Too bad they are not recognized as a over current device cause thats what they do. I figure he can have those installed in 'addition' to his ocpd if hes concerned about the current being too much on one of the cables.

 

[don_resqcapt19]

brother,

Too bad they are not recognized as a over current device cause thats what they do.

Cable limiters don't provide overcurrent protection...they only provide short circuit protection, not overload.
Don

 

[drbond24]

In the OP's situation, the current on each conductor, although unbalanced, doesn't exceed the ampacity of the conductors.

How big a problem is this? Is this simply a curiosity, a catastrophe waiting to happen, or something in between?

This is my thought as well. The ampacity of the 500 MCM is 380 amps, so what is the problem in this case? I can see where it would be bad if the unbalance caused one of the parallel feeders to exceed its ampacity, but it shouldn't matter here.

 

[crossman]

IMSO, the amperage differences as given by the OP are absolutely inconsequential.

 

[jghrist]

It doesn't really seem like a large difference to me. In the worst phase, the currents are only about 5-6% different from the average. There will be some difference in contact resistance at the terminations.

 

[rattus]

Guess work:

Guess work:

I am guessing, but we are dealing with small impedances here, and otherwise insignificant variations may create noticeable differences in current.

There are possibly small differences in resistivity and wire diameter, especially in a long pull where one wire may stretch a little more than the other. Then there is the matter of termination resistance which, although quite low, may vary considerably between any two conductors.

Any corroboration on this theory?

 

[Flagstaff]

It's been my experience that feeders always have had a difference in the amps they each carried. I agree that it's not an issue, however the engineer is concerned and I'd hoped to get other's ideas on why this is happening. The fact is I have never seen identical readings on parallel feeders, no matter how closely they were in lengths. I agree that its likely related to impedance and slight variations in the wire and possibly the copper itself. I appreciate your interest and comments.

 

[VinceS]

How is the Delta load distributed, and Voltage per Phase

How is the Delta load distributed, and Voltage per Phase

Do you have information on how the load is distributed off the feeds? Transformers, inductive heating and the like. In looking into balanced AMP's I always start with distribution. You may have a large motor, post rewind, with a balanced current issue... Check any oversized drives with small motors, ( The drive A and C phase AMP loads can be as much as 50% larger than B phase)... I don't think you mentioned voltage per phase, except that it was a 480 VAC system. Do you have info on the voltage spread?

 

[VinceS]

How were the readings taken?

How were the readings taken?

Were both phases measured at the same time with meters that will read the same at that range? Loads change quickly using one meter may not give a result I would bet on.

 

[benaround]

VinceS, The OP is talking about the two conductors of the same phase.

 

[VinceS]

Yes, but..

Yes, but..

If the readings weren't taken at exactly the same time, or with a 3phase monitor such a fluke 434, the time between reading from the load may change...