Paralleling Size

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mbrooke

mbrooke

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At what point do you typically start paralleling conductors? What is the most economical? Anyone ever used wire over 600MCM?

Typically over 400amps is where paralleling begins, and sizes over 600MCM are just never used.

I have however seen some "odd" prints where it was below 4/0...

I'm curious what others do/think on the matter.
 
We use up to 750 kcmil copper all of the time. Rarely do we parallel anything smaller than 250 kcmil.
 
infinity said:
We use up to 750 kcmil copper all of the time. Rarely do we parallel anything smaller than 250 kcmil.
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Same here except we would parallel as small as 3/0 depending on the particular situation.


Roger
 
We typically don't specify single conductors larger than 500 kcmil and the AL/CU and other choices are as Rob said. It's a discussion with the client and cost/benefit of designing for now or for the future.
 
mbrooke said:
If you had to choose, would you rather pull sets of 3/0s or one 600MCM? I've seen a few prints choose that instead.
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3/0 but the pulling is not as much an issue as forming and terminating the large conductors verses the small.

Roger
 
Being a one man show, I usually prefer more smaller sets. In fact my preference would often be go smaller/more sets than the number of ports on the gear. I parallel 4/0 and 250 AL often. 350 AL is not too bad and has a nice round 250A ampacity. I once did an (approx) 400 amp sub with two runs of 4/0 MC cable, figured 500-4 MC wouldn't have been possible to do by myself.
 
When I issue a design, I refer to a table that gives wire and conduit for various circuit ratings. I don't know where it came from, but I have verified that all the tabulated numbers are valid. It tells me that a circuit that would need more than a 500 MCM copper should be installed with smaller, parallel conductors. I have never yet received an RFI from an EC asking if they can use 750's instead of paralleling smaller wires. If I had received such an RFI, I think I would be willing to approve it.
 
charlie b said:
When I issue a design, I refer to a table that gives wire and conduit for various circuit ratings. I don't know where it came from, but I have verified that all the tabulated numbers are valid. It tells me that a circuit that would need more than a 500 MCM copper should be installed with smaller, parallel conductors. I have never yet received an RFI from an EC asking if they can use 750's instead of paralleling smaller wires. If I had received such an RFI, I think I would be willing to approve it.
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Thats what I frequently see, 500MCM is the cut off. However I'm seeing an exponentially increasing number of new prints where the cutoff is 4/0. Spotted just today:


https://imgur.com/kh6yN1n

Are smaller parellel sets more efficient?
 
Smaller conductors use less copper per amp, so if you parallel smaller conductors you use less copper for the same ampacity.

Larger conductors have more copper per amp, so for the same total ampacity larger conductors will have lower voltage drop.

-Jon
 
winnie said:
Smaller conductors use less copper per amp, so if you parallel smaller conductors you use less copper for the same ampacity.

Larger conductors have more copper per amp, so for the same total ampacity larger conductors will have lower voltage drop.

-Jon
Click to expand...


Good comparison, thank you! I was about to ask regarding voltage drop but you answered that question to.

Regarding the EGC, do you ignore table 250.122 and just size it to half of the mm2 area of the phase conductors?

For example if I have 3/0 CU per phase I'll multiply 85.01 by two getting 42.5mm2 which brings me to a #1 equipment ground.
 
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