300MCM as a parallel feeder

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Isosynchronous

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We are being asked to install a new 480 volt 3 phase 3 wire 600 amp copper feeder to a distribution board at a machine shop to replace an existing aluminum feeder that was damaged by a forklift. Our plan was to run 2 parallel runs of 300MCM THHN copper wire in (2) 3" EMT Conduits, approximately 150 feet in length, with one # 1 CU ground wire in each conduit. The existing 600 amp feeder circuit breaker is 80% rated and the terminations are rated for 75 Deg C. The ambient temperature in the shop is less than 40 Deg C. The maximum calculated full load current draw of all of the loads present on the existing the distribution board is 430 Amperes. Is 2 sets of 300MCM Copper wire feeders an acceptable installation?
 
Based on table 310.16 a 300 kcmil is rated 285 amp at 75* table line .

2 sets in parallel. 570 amp next size up OP 600 amp

Your installation is totally complain .
 
More then enough. At 75 deg. you have 570 amps. that is a good design size, unless you want to save money and go back to aluminum.You could use 400 XHHW in parallel and get 540 amps. or go to 500 XHHW and have over 600 amps. If the conductors are in a long run, the cost savings will be huge. I know some people are of the mind set that there is something wrong with aluminum, but when installed correctly I have not seen any failure of AL that would not have occurred in CU.
 
It will cost between 3 and 5 times more to go with copper.

I might also price this out with 3 sets of parallels instead of 2 if I was going with copper.

As copper wires sizes increases you get less 'amps per dollar' spent on the conductors.

Of course you do have to consider the cost of an extra raceway.

I have already done the math and know that for a 400 amp feeder one set of 600 copper in one PVC conduit costs much more than 2 sets of 3/0 in two PVC conduits ....this includes the additional labor to run the extra conduit.
 
iwire said:
It will cost between 3 and 5 times more to go with copper.

I might also price this out with 3 sets of parallels instead of 2 if I was going with copper.

As copper wires sizes increases you get less 'amps per dollar' spent on the conductors.

Of course you do have to consider the cost of an extra raceway.

I have already done the math and know that for a 400 amp feeder one set of 600 copper in one PVC conduit costs much more than 2 sets of 3/0 in two PVC conduits ....this includes the additional labor to run the extra conduit.
Click to expand...

I am in 100% agreement with you. The point of diminishing returns is exponential in wire size. And the ease of handling smaller conductors is worth the change.
 
Iso -
My response was pretty short - but that is just because I figured you alreeady had the details figured out.

And I did notice a couple of pertinent points in your OP.

1. The temperature in the shop is <40C. I translated that to say the conductors (the entire run) and the equipment could easily be 39C continuous. This gives a .88 derate on what ever cable you select. For example the 300kcmil you picked is good for 570 x .88 = 501.6A. That is a perfect fit. It allows you to use the existing 600A CB and of course is greater than the 430A load calc.

YMMV with Aluminum. For example, 400AL is 540 x.88 = 475A. Enough for your load, but you would have to change your breaker. 500AL would work and should fit in your 3" even if you went to rigid. However, 3-500 and a 1/0 in 3" is a drag (pun intended):roll:

One thing I am curious about. The original feeders were Al. Why the switch to CU? Qwner specification?

2. You are going to use EMT as opposed to PVC, which makes sense - the conductors are in an area subject to forklift attack. However, if the original conductors were also in EMT, you may wish to consider rigid for the added protection. I have not done the math - so I can't tell you if it is cost effective or not to go with smaller wire and add additional conduits.

You have a good plan. Don't let the value engineers take your plan apart.:grin:

and Merry Christmas to all -

It's been fun, but I'm suposed to be working today (and that's okay - it pays the bills:))

ice
 
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