Feeder Cable Sizing Question

[SparkOmatix]

I have a Bus Bar (BB) some 650 feet away from the MCC. The full load on the BB will be 260A. This is 3ph 480V. Would it be more prudent to run a single set of 350MCM conductors (THHN) out to it...or parallel 2 sets of 3/0? This will be in galvenized rigid conduit.

I will also have three other similar setups; however, the loads will be 750ft, 900ft, and 1050ft from the MCC.

 

[wareagle]

Assuming the load is contineous, 260 x 1.25 = 325 amps. The 350 only has an ampacity of 310 amps. So the choices would be 500 kcm(380 amps) of 2 - 2/0(350 amps) or 2 -3/0(400 amps) VD using the 2/0 would be 3.1%, 2.4% for 3/0 and 1.8% for 500 kcm

 

[WoodrowJWeen]

My choice would be a set of 500MCM fed by a 400A breaker. Gives you a little spare capacity for future loads. Simpler pull than two sets of small conductors in one pipe.

 

[wareagle]

SparkO
How about a response.

 

[SparkOmatix]

What I've done is take the FLA (183.1) of the largest motor and multiplied it by 125%. This came up to some 228.88A. I added to this value the remaining motor FLA's (77A) which resulted in some 306 Amps.

I'm going to use THHN.

Given that 350MCM is good for 350 Amps. I initially chose this size.

I ran through the voltage drop calculation and found that (3) three 350MCM cables with a 2/0G stretched 650" through rigid conduit will have a VD of 3.9% Thus, I will run a 3" conduit and pull (3)-1/C 350MCM with 2/0 ground.

Have I done this correctly?

I should never exceed 5% when dealing with VD, yes?

Thanks for your help guys!

 

[davidbeach]

No, you can't use THHN at the 90C rating because you don't have terminations that can be used at 90C. You have to use the 75C ampacity column and 350 is good for only 310A.

 

[wareagle]

Quote "Given that 350MCM is good for 350 Amps. I initially chose this size. "

AS David said, you can not use the 90C rating for ampacity rating. You must use the 75C rating instead. The terminations on your gear are rated at 75C. You can not use 350 THHN. You need 500 kcm. Lucky we caught this.

 

[waross]

When running multiple conductors per phase it is common to run each set in a separate raceway. Otherwise you lose your advantage when you de-rate the conductors. (More than three current carrying conductors in a raceway or cable.)

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[wareagle]

SparkO
What do you think?

 

[burnt2x]

I guess SparkO is thinking about that problem (if he already bought the 350MCM)!

 

[wareagle]

I was hoping that he would have responded by now.

 

[wareagle]

SparkO
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