600A Feeder Size

[mstrlucky74]

The engineer is calling for (2) sets of 4#300 w/1/0 in 3" conduit. If I am thinking about this correct the 75 degree column only gives you 285 amps x 2 would only be 570. I guess he's using the 90 degree column but that's unusual, right? Thanks.

 

[jumper]

Did not check your numbers, but if the calculated load is 570A or less then one may use 240.4(B) and install a breaker the next size up of 600A.

90C can only be used for derating, in most cases, final ampacity is still 75C.

 

[mstrlucky74]

Did not check your numbers, but if the calculated load is 570A or less then one may use 240.4(B) and install a breaker the next size up of 600A.

90C can only be used for derating, in most cases, final ampacity is still 75C.

Thanks, so when sizing feeders is not necessarily based of breaker/switch size? It's the calculated load?

 

[iwire]

Thanks, so when sizing feeders is not necessarily based of breaker/switch size? It's the calculated load?

Both, it really becomes a deisgn issue.

In your example at no time can a future contractor add load beyond 570 amps so in my opinion it really is a 570 amp feeder protected by a 600 amp breaker as allowed by 240.4(B).

Two key points

1) 240.4(B) does not increase the capacity of the conductors


2) 240.4(B) does not apply for feeders 801 amps and larger.

 

[kwired]

You have probably seen 500 copper for 400 amp circuits many times but never noticed it only has 380 ampacity @ 75C.

As long as the calculated load doesn't exceed the ampacity of the conductor you can protect it at next higher standard device - until you go past 800 amps.

Somewhat common mistake is thinking you can use 3 parallel 500's for a 1200 amp circuit or similar situation with 1600 or 2000 amp circuits.

 

[infinity]

For 600 amps you could use a #1 EGC in each raceway but I guess that some engineers feel that bigger is better.

 

[kwired]

For 600 amps you could use a #1 EGC in each raceway but I guess that some engineers feel that bigger is better.

Or you could even use metal raceways as the EGC.

 

[mstrlucky74]

Both, it really becomes a deisgn issue.

In your example at no time can a future contractor add load beyond 570 amps so in my opinion it really is a 570 amp feeder protected by a 600 amp breaker as allowed by 240.4(B).

Two key points

1) 240.4(B) does not increase the capacity of the conductors


2) 240.4(B) does not apply for feeders 801 amps and larger.

Thanks. Yeah that's interesting.

 

[mstrlucky74]

Ok so in this case I'd like to compare AL conductors to the #300's. A question.

The 2 sets CU #300's gives me 570A. AL #350 x2 would give me 560A. There a chance( don't know the connected load) that would be ok, right. Why I'm thinking that is because #250 CU x 2 sets gives 510A but #300's x2 were designed so the connected load falls between 520 and 570A. Am I right so far? Or do I need to go with #500 AL?

 

[kwired]

Ok so in this case I'd like to compare AL conductors to the #300's. A question.

The 2 sets CU #300's gives me 570A. AL #350 x2 would give me 560A. There a chance( don't know the connected load) that would be ok, right. Why I'm thinking that is because #250 CU x 2 sets gives 510A but #300's x2 were designed so the connected load falls between 520 and 570A. Am I right so far? Or do I need to go with #500 AL?

240.6 Standard ampere ratings comes into play as well.

Since standard sizes jumps from 500 to 600 your 510 amp set can also be protected with a 600 amp device - but the load must be 510 or less.

 

[mstrlucky74]

240.6 Standard ampere ratings comes into play as well.

Since standard sizes jumps from 500 to 600 your 510 amp set can also be protected with a 600 amp device - but the load must be 510 or less.

So if for some reason there's an overload of up to 600a on the conductors rated for 510 that seems like a problem to me. The breaker won't trip.

 

[infinity]

So if for some reason there's an overload of up to 600a on the conductors rated for 510 that seems like a problem to me. The breaker won't trip.

In order to use the next size up rule the load couldn't be more than 510 amps. How can there be an overload if the calculated load is 510 amps or less?

 

[kwired]

So if for some reason there's an overload of up to 600a on the conductors rated for 510 that seems like a problem to me. The breaker won't trip.

This:

In order to use the next size up rule the load couldn't be more than 510 amps. How can there be an overload if the calculated load is 510 amps or less?

and if someone later adds more load then they are the ones in violation.

Short circuits/ground faults will still draw high level current and your device should trip.

 

[mstrlucky74]

This:



and if someone later adds more load then they are the ones in violation.

Short circuits/ground faults will still draw high level current and your device should trip.

Ok got it, thanks. Infinity guess I'm thinking short circuit or ground fault ....guess breaker would trip in that instance as current would be extremely high.

 

[iwire]

Ok got it, thanks. Infinity guess I'm thinking short circuit or ground fault ....guess breaker would trip in that instance as current would be extremely high.

Correct, during a fault the current will spike so high that overcurrent protection will open very quickly. Its the same idea as equipment grounding conductors that are smaller than the circuit conductors they only have to deal with high currents for such short time no damage happens to the conductors.

 

[mjmike]

Actually, this is a 4-wire circuit. Depending on the voltage, the neutral might be a CCC. In that case, the 300 is only for 256A per set totaling 512A paralleled.

 

[infinity]

Actually, this is a 4-wire circuit. Depending on the voltage, the neutral might be a CCC. In that case, the 300 is only for 256A per set totaling 512A paralleled.

According to the diagram in the OP this feeder is feeding single phase apartment panels, not likely that the neutral would be a CCC for derating purposes.

 

[kwired]

According to the diagram in the OP this feeder is feeding single phase apartment panels, not likely that the neutral would be a CCC for derating purposes.

Look again. The supply side feeder says it is 208/120 three phase 4 wire. The feeders to the individual units are only 208/120 single phase 3 wire, but I think the 600 amp three phase feeder is what the thread is supposed to be about.

 

[iwire]

Look again. The supply side feeder says it is 208/120 three phase 4 wire. The feeders to the individual units are only 208/120 single phase 3 wire, but I think the 600 amp three phase feeder is what the thread is supposed to be about.

Right and unless that feeder is supplying a data center it is unlikely that the neutral must be treated as a CCC.

 

[infinity]

Right and unless that feeder is supplying a data center it is unlikely that the neutral must be treated as a CCC.

My thoughts exactly.