standard cable sizes

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mshields

mshields

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Location
Boston, MA
From a practical standpoint, what cable sizes in Aluminum and Copper are readily available. e.g. should I avoid 600kcmil because it's not commonly available?

Also, what's the ceiling, would an electrician prefer to see 750kcmil cable or to use parallel conductors in multiple conduits?

Thanks,

Mike
 
We almost always deal with copper conductors. The largest size, if given a choice, that we would prefer would be 500 kcmil. It's isn't so heavy that you can't work with it and it can be bent by hand. We have installed 750 kcmil copper conductors in the past but the extra weight and difficulty in bending them at their terminations makes me wish for 500 kcmil conductors when I'm installing them. For a 400 amp feeder I would use 500 kcmil conductors in one conduit as opposed to 2 sets of #3/0 in two conduits.
 
Mike as you are from the same areas as I am I can tell you that 750 Kcmil and below is definitely readily available and larger may also be.

Like Trevor most of our work is with copper, although we regular use 600 copper in 4" conduits.

Use what suits the job, even if it means the conductor is not 'on the shelf' a job that requires that size conductor will usually have the lead time yo get the conductors.

For us the issue is usually trying to get switchgear fast enough as that is not 'on the shelf'.
 
Mike,
You should avoid the larger sizes because in many cases they are not cost effective. Parallel 350s are good for 600 amps, but you have to go to 1500kcmil to get 600 amps in a single conductor. I expect that once you are over 500, it is cheaper to go parallel.
Don
 
jtester said:
On Thursday we priced paralleled 3/0 vs 500 kcmil. The parallel 3/0 was over $2.00 per foot cheaper.

Jim T
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Jim
I assume you used two conduits. Is the total installation cheaper using 3/0?
 
Bob,
No need for two conduits as long as the calculated load is 360 amps or less. 8 3/0s fit in the same size conduit as 4 500s.
Don
 
don_resqcapt19 said:
Bob,
No need for two conduits as long as the calculated load is 360 amps or less. 8 3/0s fit in the same size conduit as 4 500s.
Don
Click to expand...


That may be true, but it would require twice as many terminations on each end. The labor alone could wipe out any potential savings on the copper cost difference.
 
I also try to draw the line at 500KCM, but sometimes the 600's seem to save a set of conductors. For example, if you have a 800A breaker, 2 sets of 500 don't quite cut it.

As a side note, I used to spec. 3.5" conduit. The electricians keep telling me 3.5" is special order and 4" is just as cheap. (At least for EMT)
 
bob said:
Jim
I assume you used two conduits. Is the total installation cheaper using 3/0?
Click to expand...

Bob

There was an existing 4" conduit where we were replacing a feeder. We designed paralleled 3/0 and then wondered if 500 kcmil would be cheaper. The only difference was conductor pricing, the contractor doing the work charged the same price for labor.

Jim T
 
jtester said:
Bob

There was an existing 4" conduit where we were replacing a feeder. We designed paralleled 3/0 and then wondered if 500 kcmil would be cheaper. The only difference was conductor pricing, the contractor doing the work charged the same price for labor.

Jim T
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One thing you need to keep in mind is that by paralleling in one conduit you will encounter derating issues. If your feeders are three phase the parallel #3/0 will have a derated ampacity of 360 amps. The single set of 500 kcmil conductors will have an ampacity of 380 amps.
 
You also have to keep in mind what the Utility will allow from the secondary side of the Transformer. We have one utility (KCPL) that will not allow anything larger than 500MCM to be terminated without special permission.

Then again we have another utility (Weststar) in the area that will only allow a maximum of ( 8 ) sets to be terminated on the secondary of the utility transformer which means to get 4000A we need to provide ( 8 ) sets of 900 MCM.

-Ed
 
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ed downey said:
Then again we have another utility (Weststar) in the area that will only allow a maximum of ( 8 ) sets to be terminated on the secondary of the utility transformer which means to get 4000A we need to provide ( 8 ) sets of 900 MCM.

-Ed
Click to expand...

I think you may find that this requirement is driven by the fact that it is an equipment limitation. Padmount transformer bushings typically cannot handle more then 8 conductors per terminal.
 
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