Parallel conductors from transformer to swgear

[bigfish92]

We have to run underground 11 sets of 4 #500KCM from transformer to 480 switchgear (3750 amps). Can we parallel the same phase WITH more than 3 conductors IN A CONDUIT without derating?

The problem we have is defending the need to derate. Our client says we should NOT have to derate.
He gives us the example of 2 #1000 KCM IN PARALLEL(same phase), you do not derate OR 4 #600 KCM IN PARALLEL (SAME PHASE),which has the same amperage (AS 2 #1000 KCM) and thus heating is the same and no derating required.

Which is right? DO WE DERATE OR NOT?

[ May 14, 2003, 09:28 PM: Message edited by: bigfish92 ]

 

[bigfish92]

Re: Parallel conductors from transformer to swgear

Correction: Our client says we do not have to derate

 

[Nick]

Re: Parallel conductors from transformer to swgear

The size of the conductor has nothing to do with it. You derate based on the number of current carrying conductors in the raceway. In a large underground ductbank you need to watch out for heating between conduits. If the circuit is loaded heavily and the conduits are not properly arranged, conductors can overheat.

 

[bennie]

Re: Parallel conductors from transformer to swgear

Do you have lug space to terminate this number of cables?

 

[bob]

Re: Parallel conductors from transformer to swgear

Originally posted by bigfish92:
[QB] We have to run underground 11 sets of 4 #500KCM from transformer to 480 switchgear (3750 amps). Can we parallel the same phase WITH more than 3 conductors IN A CONDUIT without derating?


Are you asking if you can parallel the SAME PHASE
in the same conduit? If so you need to put each phase A,B and C in each conduit and neutral if necessary. 300.30A 392.8D

 

[jmc]

Re: Parallel conductors from transformer to swgear

We have to go back to basics. When a current flows through any resistance, heat is developed. A copper or aluminum conductor has a specific resistance per foot at a specific ambient temperature. Each conductor develops heat that impinges on every conductor within the conduit, duct, or cable assembly. For example, 500 Kcmil, cu has a resistance of 0.0258 ohms per 1000 ft. If each conductor is loaded to its capacity (380 Amperes), for a 400 foot run, the heat generated by EACH current carrying conductor is I squared R or (380x380)x 400x(.0258/1000) = 1,490 watts. spread along 400 ft. or 3.7 watts per foot. If you place more than three current carrying conductors in one raceway, the heat developed will cause the heat in the conduit to exceed the insulation rating of the conductors. If you perform the same calculation for 1000 Kcmil conductors, the result is (545x545)x400x(.0129/1000)= 1532.6 watts for 400 ft or 3.8 watts per foot. Regardless of the conductor size, once u exceed 3 current carrying conductors in a raceway or cable assembly u are REQUIRED to use the adjustment factors in Table 310.15(B)(2).

 

[don_resqcapt19]

Re: Parallel conductors from transformer to swgear

He gives us the example of 2 #1000 KCM IN PARALLEL(same phase), you do not derate OR 4 #600 KCM IN PARALLEL (SAME PHASE),which has the same amperage (AS 2 #1000 KCM) and thus heating is the same and no derating required.

The ampacity of 1000 kcmil at 75?C is 545 amps, the ampacity of 600 kcmil is 420 amps. Two 300 kcmil in parallel is 590 amps. Two 600 kcmil has a much higher permitted ampacity than a single 1000 kcmil. If you match the ampacity of the conductors in parallel with the ampacity of a single larger conductor, the smaller conductors in parallel will always produce more heat per foot than the single larger conductor. Derating is always required when there are more than 3 current carrying conductors in a raceway.
Don