Ampacity of Parallel Conductors

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Dennis Alwon

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500 MCM has 1/2 the cir. mil of 1000 MCM. So if I parallel 2- 500 MCM cables I should have the same area as one 1000MCM cable.

Chapter 9 Table 8 verifies this fact -- 500 MCM has an area of .519 sq. in multiply by 2 and you get 1.038 sq. inches. Now 1000 MCM has an area of 1.042 sq. inches. Pretty close anyway-- not sure why they are different,
but that's not my question.

500 MCM is rated for 380 amps at 75 C. I always assumed if I parallel the 500 MCM cables then it would be good for 760 amps---(Forget the derating issues.) or is it only good for 545 amps (same as 10000 MCM at 75 C)?
If it is good for 760-- why the difference?
 

Dennis Alwon

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LarryFine said:
Because two conductors have more surface area (as in cooling) than one twice the area.
That's what I thought but it seems like such a big difference I thought I was missing something. Thanks
 

coulter

Senior Member
Yeah, that bugged me too until the light came on one day. Following Larry's reasoning:

Conductor cross sectional area is porportional to the diameter squared. A(cross) = pi x D^2/4

Conductor surface area is porportional to the diameter. A(surface) = pi x D

So if the cross sectional area goes up by 2X, the diameter (and surface area) goes up by sqr(2) (about 1.4X)

Heat transfer is porportional to surface area, so allowable current in 1000kcmil should be 1.4 X 500kcmil

380 X 1.4 = 532A pretty close.

That only took me 30 years to figure that out. :)

carl
 

Dennis Alwon

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hardworkingstiff said:
Put 2 sets of paralleled conductors in the same conduit and the 500s go from 760-amps to 608-amps.
One does not have to put 2 sets of parallel conductors in one raceway.
 

bcorbin

Senior Member
The only time I ever almost designed something with parallel sets in the same conduit:

Four existing service conduits under building foundation...all with a set of 400's in them. To upgrade for increased service, I would have needed four sets of 500's, but wanted to leave them a spare conduit by going with 2 sets of 350's in three of the conduits (total, six sets). Plus...some contractors in my area get grumpy if they have to pull 500's.

Ended up just bringing a whole new service to the building, leaving the old one alone. Was the best bang for the buck in that instance.
 

bcorbin

Senior Member
Yeah...I've heard on more than one occasion that it takes them "special equipment" to pull 500's. I have no idea what that means, though.
 

iwire

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IMO it means the electrical contractor does not own, or does not plan on renting the equipment necessary to be bidding on that work and should not be considered for the job.
 
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