800A in 2-3" RGS

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beanland

beanland

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Vancouver, WA
I am sure this has been discussed before, but I cannot find the reference.

I have two existing 3" RGS buried in concrete. I would like to pull eight 4/0 copper THHN-2 and one 1/0 copper ground in each conduit. In the conduit, I can use the 90C rating for 4/0 of 260A but have to derate at 80%. That means I get 416A capacity in each conduit at 90C.

At the terminations, I must use the 75C rating but the conductors are in air, not conduit, so I can use the 75C rating of 360A each. So, each set of parallel 4/0 provides 576A rating, more than the limiting rating in the conduit.

So, since I have two 3" RGS, I can feed the two sets of eight 4/0 copper THHN-2 with an 800A OCPD and meet code. Agree?

The only problem will be finding an 800A OCPD that accepts four 4/0 conductors.
 
If there are 8 ccc in the conduit you must derate at 70%. The wire in free air can be smaller sized if you want.
 
beanland said:
Two of the conductors are neutrals and per 310.15(B)(4)(a) need not be counted.
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I got confused-- there are 2 sets of 4 wires in each conduit. Gotcha... so can you terminate the wires in a trough and then use a polaris connector to the breaker with 2 sets of 500KCM or 600kcm?
 
Doesn't "buried in concrete" lead to a 75 deg start for the derating before other factors are applied? (wet location). Or is this above grade somehow?
 
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Thhn, thwn, xhhw

Thhn, thwn, xhhw

Good point. THHN is 90C damp location. THWN-2 is 90C rated wet location. XHHW-2 is 90C rated wet location.

Yes, per NEC 100, all below grade locations are wet. Therefore THWN-2 or XHHW-2 are required to be able to use 90C rating.
 
90c

90c

Per the polaris literature they are rated 90C.

However, once the conductors are out of the conduit, they are operating under the 75C rule/rating.
 
I don't think you understand how derating works. If you are using 90C rated conductors you can use 90C column for derating purposes. In your case the 4/0 conductors have an ampacity of 260 amps using the 90C column of Table 310.16. Since you have 6 current caring conductors you need to derate by 80% which reduces the ampacity of these conductors to 208 amps. Since the derated ampacity is less than the ampacity for the same 4/0 conductor at 75C (230 amps) you can use the full 208 amps. Each pair of parallel conductors in each conduit will have an ampacity of 416 amps.
Your terminations only need to be rated for 75C


You should not be using Table 310.17 (single conductors in free air) for any portion of this circuit.
 
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Enclosure = Raceway?

Enclosure = Raceway?

OK, good clarification. That makes sense.

However, are conductors located in an enclosure considered in a conduit/raceway (310.16) or in free air (310.17)? NEC 100 defines a raceway basically as a conduit. Enclosures are defined totally differently. Based on the definitions, I have assumed that if it is not in a conduit(aka raceway), it is in free air, we do not have a third choice.
 
Conduit

Conduit

The conduits are underground RGS from a main switchboard to a remote panel. They are buried (possibly in concrete), then rise through a concrete floor, and terminate above grade at the enclosure.

The conductors are in the conduit for part of the run and in enclosures on both ends of the run. Where the conductors exit the conduits they are in the enclosures, not in the conduits anymore.
 
I think you will find that the conductor rating in the conduiit will be the max rating for the entire run.
 
beanland said:
Good point. THHN is 90C damp location. THWN-2 is 90C rated wet location. XHHW-2 is 90C rated wet location.

Yes, per NEC 100, all below grade locations are wet. Therefore THWN-2 or XHHW-2 are required to be able to use 90C rating.
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Hey thats right, Bob Badger pointed that out to me about a month ago, but I guess it didn't sink in all the way yet....:roll:
 
It's not 8. It's 6 as the grounded conductor which only carries the unbalance current of the other phases doesn't count. And neither does the grounding conductor.

Dennis Alwon said:
If there are 8 ccc in the conduit you must derate at 70%. The wire in free air can be smaller sized if you want.
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Just curious,what is the calculated load of the remote panel?
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beanland said:
OK, good clarification. That makes sense.

However, are conductors located in an enclosure considered in a conduit/raceway (310.16) or in free air (310.17)? NEC 100 defines a raceway basically as a conduit. Enclosures are defined totally differently. Based on the definitions, I have assumed that if it is not in a conduit(aka raceway), it is in free air, we do not have a third choice.
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Conductor ampacities under the NEC are, for the most part, determined by the wiring method used between enclosures and the like.

Free-air wiring is generally considered to be that which has substantial air-space around wires... but the amount of air-space is not specified by the NEC. Typically, this is for open conductors (i.e. not a jacketed multiconductor cable or wires in a raceway, for the majority of their length). The NEC does have some conductor-to-conductor spacing requirements, though.
 
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