Feeder runs for 600A service

[katwalatapan]

Hello,

My query is with regards to selecting feeder size and runs from service mast to main disconnect switch for 1 phase 3 wire 600A service.

1. Is it required to have 2 runs of 3#300MCM cable from mast to 600A main disconnect switch in 4" PVC conduit? Is it general practice to split feeder runs for higher rated service? Would it be acceptable if there is availability of 1000 kcmil and appropriately sized conduit?

Thank you.

 

[rcw retired EE]

Multiple parallel runs of smaller cable are usually less expensive and easier to install than a single large cable.

For a 600 Amp Service, two 350 kcmil copper wires per phase would be sufficient per the NEC for most cases (ampacity = 2 x 310A = 620 Amps. A single conductor 600A needs a 1250 kcmil (595 amps) or a 1500 kcmil (625 Amps) to get 600 amps, assuming the service terminations require 75C rated wires. That assumes you can even find a 1250 kcmil cable termination for a service panel.

Note that two 350 kcmil = 700 kcmil of copper. With a single conductor it takes 214% copper cross section (1500/700) to achieve a 0.8% increase in ampacity (620 Amps to 625 Amps). More copper = more costs. Cost of the extra conduit and terminals is minimal.

Many contractors would install three #4/0 AWG wires per leg. (Ampacity = 3 x 230A = 720 Amps.)

But, if you have the 1000 kcmil wire already and can adjust the overcurrent protection to the 545 Amp rating, it could work.

 

[katwalatapan]

Thank you RCWilson for the response.

On a related topic.

1. Is it possible for a 50KVA transformer to service 600A single phase 120V/240V service? Or that would also have to be upgraded?

2. Is it general practice to use two conductors per phase for connection between service mast and main disconnect? Any other benefit except cost savings due to use of less copper and less labour costs?

Thank you.

 

[davidbeach]

Never try to make any connection between the size of the service and the size of the transformer provided by the utility. There is no correlation, and the utility doesn't have to follow NEC rules for transformer sizing.

 

[rcw retired EE]

"2. Is it general practice to use two conductors per phase for connection between service mast and main disconnect? Any other benefit except cost savings due to use of less copper and less labour costs?"

My experience:

<200 Amps: 1/c
300-400 Amps: 1/c 25% of time, 2 per phase 75%
> 400 Amps: 2 or more per phase.

 

[racobb]

If you ever have the opportunity to install and terminate 1000 MCM conductors you will understand the advantage of the parallel option!

Alan

 

[rcw retired EE]

Agreed. 1000MCM bends as easily as an aluminum baseball bat. The cable and bat are about the same size, only the bat has a thinner shell and is hollow. (slight exaggeration.)