short circuit across 3 sets of wire

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Grouch1980

Grouch1980

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New York, NY
Hi all,
I think this question has a simple answer... say you have a feeder that's composed, for example, of 3 sets of 3#300 MCM wire (3 phase), protected by a 3 pole, 800 amp breaker.

When you have 600 amps of load current flowing through the feeder, it's 600 amps on each phase (A, B, and C)... however it's 200 amps on each parallel wire, for the total of 600 amps on each phase wire.

If you have a short circuit current of 50,000 amps flowing through the feeder, i'm assuming it flows the same way as normal current? 50k on each phase, but 16,666 amps on each parallel wire... is this correct?

Thanks.
 
Grouch1980 said:
Hi all,
I think this question has a simple answer... say you have a feeder that's composed, for example, of 3 sets of 3#300 MCM wire (3 phase), protected by a 3 pole, 800 amp breaker.

When you have 600 amps of load current flowing through the feeder, it's 600 amps on each phase (A, B, and C)... however it's 200 amps on each parallel wire, for the total of 600 amps on each phase wire.

If you have a short circuit current of 50,000 amps flowing through the feeder, i'm assuming it flows the same way as normal current? 50k on each phase, but 16,666 amps on each parallel wire... is this correct?

Thanks.
Click to expand...
It really depends on where the short is. If it's a short at the load, then your asumption above is correct, with sutble differences in each conductor's impedance causing the current to be shared proportionally. If it's a short in one set of feeders out of three (someone core drilled through one of three conduits for instance) then the short circuit would only be seen by one set of cables. The other sets would see none of the short circuit current. This assumes your conduits are arranged with all phases in each conduit, and not (for instance) an isolated phase duct bank.


SceneryDriver
 
SceneryDriver said:
It really depends on where the short is. If it's a short at the load, then your asumption above is correct, with sutble differences in each conductor's impedance causing the current to be shared proportionally. If it's a short in one set of feeders out of three (someone core drilled through one of three conduits for instance) then the short circuit would only be seen by one set of cables. The other sets would see none of the short circuit current. This assumes your conduits are arranged with all phases in each conduit, and not (for instance) an isolated phase duct bank.


SceneryDriver
Click to expand...
Yep, got it. yeah, i was thinking of the short being at the load end, which would then subdivide the short circuit currents between all the parallel wires of each phase.... with the subtle differences as you mentioned. ok great. thanks!
 
A short in one conduit might be seen by all parallel sets: feeding directly from the supply through one conduit to the short, and feeding through the other parallel sets to the far end and back to the short.

Jon
 
SceneryDriver said:
It really depends on where the short is. If it's a short at the load, then your asumption above is correct, with sutble differences in each conductor's impedance causing the current to be shared proportionally.
Click to expand...
I'd agree with this.

If it's a short in one set of feeders out of three (someone core drilled through one of three conduits for instance) then the short circuit would only be seen by one set of cables. The other sets would see none of the short circuit current.
Click to expand...
I'd disagree here. Short-circuit current would still flow on all conductors; the shorted conductor would carry slightly more.
 
Grouch1980 said:
hmmm... do not follow this one.
Click to expand...
All conductors are still connected together at both ends, so some fault current would flow from the source to the fault through the non-shorted conductors to the load end, and then back to the fault on the shorted conductor, in addition to the fault current on the direct-from-the-source conductor.

Draw it out if it helps.
 
LarryFine said:
All conductors are still connected together at both ends, so some fault current would flow from the source to the fault through the non-shorted conductors to the load end, and then back to the fault on the shorted conductor, in addition to the fault current on the direct-from-the-source conductor.

Draw it out if it helps.
Click to expand...
You're right, Larry. I was simplistically picturing our hypothetical core bit cleanly slicing and shorting the conductors only from the feed end. In reality, the short would be fed through the other feeder sets from the load end as well.


SceneryDriver
 
LarryFine said:
All conductors are still connected together at both ends, so some fault current would flow from the source to the fault through the non-shorted conductors to the load end, and then back to the fault on the shorted conductor, in addition to the fault current on the direct-from-the-source conductor.

Draw it out if it helps.
Click to expand...
NOW i got it! Yes i see what you both meant now. Thanks for the feedback.
 
Grouch1980 said:
Hi all,
I think this question has a simple answer... say you have a feeder that's composed, for example, of 3 sets of 3#300 MCM wire (3 phase), protected by a 3 pole, 800 amp breaker.

When you have 600 amps of load current flowing through the feeder, it's 600 amps on each phase (A, B, and C)... however it's 200 amps on each parallel wire, for the total of 600 amps on each phase wire.

If you have a short circuit current of 50,000 amps flowing through the feeder, i'm assuming it flows the same way as normal current? 50k on each phase, but 16,666 amps on each parallel wire... is this correct?

Thanks.
Click to expand...
One additional consideration here is the possible damage to the common bus connection on the load end of the conductors. I have had the experience of the buss melting or the connectors melting for large services supply at 1000 amps and larger. I now install cable limiters to protect the good cables and the buss bars.
 
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