EGC sizing for parallel conductors increased for voltage drop

[PWDickerson]

I would appreciate it if someone would double check me here. I have a fused disconnect used as service equipment at a 277/480 system. Disco is 200A rated with 110 amp fuses. Max continuous load is 83A. The disco feeds a 75 kVA transformer 610' away. We will be using XHHW-2 AL conductors. Terminals are rated for 75C. Minimum size for ampacity per 310.15(B)(16) is 1/0. Minimum size EGC per 250.122 is #4. Due to voltage drop we will be installing 3 parallel runs of 1/0 phase conductors. I am trying to size my EGC correctly. Here is my math.

Phase conductors are effectively increased in size by a factor of 3
1/0 AWG conductor cmil = 105,600 per Table 8, Ch 9
#4 AWG conductor cmil = 41740 per Table 8, Ch 9
3 x 41740 = 125,220
Next size up is 2/0 at 133,100 cmil per Table 8, Ch 9

250.122(A) states that the EGC is not required to be larger than the circuit conductors supplying the equipment. The way I read this is that my effective circuit conductor size is 105,600 x 3 = 316,800 cmil, so I would still be required to run a 2/0 EGC to protect my 1/0 phase conductors. Is that right?

If I am wrong about my interpretation of 250.122(A), then it seems I could use a 1/0 EGC.

Here's the rub. When I enter all this information into the Parallel Conductor and Voltage Drop Calculator at electriciancalculators.com, it comes up with a #1 EGC.

Any help would be appreciated.

 

[infinity]

Since your circuit conductors are in parallel the phase conductor size is equal to 3*1/0 or (105600 * 3 = 316800 kcmil) not #1/0, meaning that a #2/0 in each raceway would not be larger than the total equivalent parallel circuit conductor size.

 

[charlie b]

I don't know anything about that calculator. But I agree with your math, and with the selection of a 2/0.