Different sized conductors in a parallel feeder

[mak134]

We recently discovered a parallel 600 amp feed that had two 500 MCM compact aluminum cables on the A and B phases and one 500 and one 750 mcm cable on the C phase. Will this cause the c phase to carry more current creating an unbalanced load? Is this a code issue?

 

[infinity]

It's a code issue but the C phase won't carry more current it's just the current on the C phase will divide differently on the two different size conductors.

 

[Carultch]

We recently discovered a parallel 600 amp feed that had two 500 MCM compact aluminum cables on the A and B phases and one 500 and one 750 mcm cable on the C phase. Will this cause the c phase to carry more current creating an unbalanced load? Is this a code issue?

It would be a legal installation to put 2 sets of 500 kcmil on Phase A, 2 sets of 600 kcmil on Phase B, and 2 sets of 750 kcmil on Phase C, provided that 2x500 kcmil is the appropriate size. And this is true, even if the loads are balanced. One reason you might want to do this, is if you do have imbalanced current, and need to curtail voltage drop differently among the phases.

The only reason why there would be slightly more current on the phase with larger conductors, is if it curtails voltage drop to simple resistive loads, balanced across the phases, such that there is more voltage delivered to the load and therefore more current. But remember, it won't increase the delivered voltage to the load any more than a ~5% improvement, if sized according to NEC recommendations on voltage drop.

What is not a legal installation, is dissimilar sizes in parallel on the same phase. Because it means that a disproportionate amount of current could flow among the conductors, and therefore you cannot take credit for the assumption that current divides itself uniformly across the sets. You have to preserve the symmetry as much as practical, when working with parallel conductors. Same length, same size & type, same conditions of use, same termination method, etc. The "same length" rule is probably the one that is the least practical to know you've done correctly and enforce, because it is difficult to get the conductors exactly the same length.

 

[Cow]

We recently discovered a parallel 600 amp feed that had two 500 MCM compact aluminum cables on the A and B phases and one 500 and one 750 mcm cable on the C phase. Will this cause the c phase to carry more current creating an unbalanced load? Is this a code issue?

It's wrong, but how long has the installation been in service?

 

[don_resqcapt19]

While it is a code violation, I don't see it as a real world issue. There would be unequal current division between the 500 and and 750 that are connected in parallel, but neither conductor would be operating beyond their rated ampacities of 310 amps for the 500kcmil and 385 amps for the 750. A quick calculation based on a 600 amp load shows that the 500 would carry 252.5 amps and the 750 would carry 347.5 amps.

 

[drcampbell]

This is most likely a safe & adequate installation.

The impedance of 750 kcmil cable is about 70% that of 500 kcmil. All other things being equal, the current will divide inversely proportional to the impedances, about 42/58%. At a total current of 600 amps, the 500 kcmil conductor will carry about 250 amps and the 750 kcmil will carry about 350 amps, well within the ampacity of each conductor.

 

[GoldDigger]

This is most likely a safe & adequate installation.

The impedance of 750 kcmil cable is about 70% that of 500 kcmil. All other things being equal, the current will divide inversely proportional to the impedances, about 42/58%. At a total current of 600 amps, the 500 kcmil conductor will carry about 250 amps and the 750 kcmil will carry about 350 amps, well within the ampacity of each conductor.

Correct. The case that the code is trying to avoid is where the two conductors are calculated so that each of them is carrying its full rated load assuming that the resistances are inversely proportional to the base ampacities.
Now if you change the resistance of one of the wires in the pair (making it longer or shorter, for example, or adding in a bad contact resistance to just one) then the current will not divide as planned and one or the other (but not both) of the conductors will be overloaded.

 

[ibarrola]

While it is a code violation, I don't see it as a real world issue. There would be unequal current division between the 500 and and 750 that are connected in parallel, but neither conductor would be operating beyond their rated ampacities of 310 amps for the 500kcmil and 385 amps for the 750. A quick calculation based on a 600 amp load shows that the 500 would carry 252.5 amps and the 750 would carry 347.5 amps.

This is most likely a safe & adequate installation.

The impedance of 750 kcmil cable is about 70% that of 500 kcmil. All other things being equal, the current will divide inversely proportional to the impedances, about 42/58%. At a total current of 600 amps, the 500 kcmil conductor will carry about 250 amps and the 750 kcmil will carry about 350 amps, well within the ampacity of each conductor.

Hi... Quick question, what table did you guys use for the impedance of the cable? In order to calculate the 42/58%...
Thanks,
Yogi

 

[Smart $]

Hi... Quick question, what table did you guys use for the impedance of the cable? In order to calculate the 42/58%...
Thanks,
Yogi

Appears Chapter 9 Table 8. There'd be less differential if they used Table 9 values.

 

[drcampbell]

Table 9, with a great deal of rounding off.

 

[ibarrola]

Appears Chapter 9 Table 8. There'd be less differential if they used Table 9 values.

Table 9, with a great deal of rounding off.

Much appreciated...:thumbsup:

 

[don_resqcapt19]

I also used Chapter 9, Table 9.