300Amp Challenge

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chris kennedy

chris kennedy

Senior Member
Location
Miami Fla.
Occupation
60 yr old tool twisting electrician
I got a call from one of our guys today. Customer has a 300A 1? genny. A 2" schedule 40 was slabbed a while back for this. House has a 600A service.

This is what I got:

300A = 350kcmil
3 350 thhn = 1.5726 in.?
1 #4 thhn= .0824 in?
1.5726+.0824=1.655 in?

2" sched 40 =1.316

That doesn't work

Parallel:

2/0 = 195?.8=156A (as per 310.15(B)(2)(a)

6 2/0 thhn = 1.3338 in?

Already over without the #4 EGC.

As a note this is a manual transfer switch operation. I would have to guess that the actual load is over 300A.

Anybody have a miracle up their sleeve?:grin:
 
chris kennedy said:
I got a call from one of our guys today. Customer has a 300A 1? genny. A 2" schedule 40 was slabbed a while back for this. House has a 600A service.

This is what I got:

300A = 350kcmil
3 350 thhn = 1.5726 in.?
1 #4 thhn= .0824 in?
1.5726+.0824=1.655 in?
Click to expand...

Chris can you not downsize the neutral to 1/0 or something smaller than 350
 
Dennis Alwon said:
Chris can you not downsize the neutral to 1/0 or something smaller than 350
Click to expand...
That was an idea, but with the load over 300A on a 600A service I don't know. As stated this is a manual transfer switch so the HO will have to do his own load shedding.

What is the art. that allows the size of the grounded conductor to be reduced?
 
Dennis,

I was thinking the same thing, yet Chris will still be over by .13. How big a deal this will be is the question.
 
chris kennedy said:
I'm not going to listen to you until you give me my dollar back.
Click to expand...

Okay I'll email it to you.

250.24(C)(1) tells you it can't be smaller than but does not limit you to the size of the ungrounded conductors
 
My company's standard feeder schedule for a 300 amp 3-wire system requires a 3 inch conduit, three 350's, and a #4 EGC. I think your choices are,

(1) Replace the conduit,

(2) Find a way to locate the generator and route its cables without going underground,

(3) Essentially derate the generator by using smaller conductors and replacing the generator breaker with a smaller size (you could certainly get 200 amps of conductor in that existing conduit, and you might be able to squeeze in 250 amps worth),

(4) Get the owner to replace the generator with a smaller one, or

(5) Bring in only two conductors plus ground, essentially making this a 120 volt single phase, instead of a 120/240 volt single phase generator.
 
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