3 ph service 240/120

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hawkinselec

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Can the hi-leg on a 240/120 3 ph. service ,wire size be reduce.
I have a 800 amp service 240/120 3 ph. parallel 500 mcm w/ 1-4/0 thwn,
was turn down by the inspecter , was not aware of this in code
 
Why wuld you be allowed to reduce it? 800 amps is 800 amps so the conductor must be able to carry the current. What am I missing?
 
hawkinselec said:
parallel
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At first glance I thought by virtue of the fact that it was a parallel installation that it would not be possible. If this service has a singular main rated at 800 amps then I believe the inspector made the right call.

If this is a service with multiple mains then 230.90(A) exc. #3 might just allow what you have done. 230.42(A) requires that the service conductors be sized to handle the load. I do not see where the individual phases are required to be the same size. 310.4(B) requires that the conductors comprising each phase be the same.

All this being said I'm not sure where the NEC would prohibit your installation as long as you do not have service equipment with a singular main.

Pete

(I have a feeling I'm about to get "schooled" big time:))
 
hawkinselec said:
Can the hi-leg on a 240/120 3 ph. service ,wire size be reduce.
I have a 800 amp service 240/120 3 ph. parallel 500 mcm w/ 1-4/0 thwn,
was turn down by the inspecter , was not aware of this in code
Click to expand...

And your reason for wanting to do this? Is is because no single phase loads can be utilized on the high phase thus limiting the need for 800 amp capacity and/or the cost savings? Wanting to utilize smaller pipe?

I don't get it and even if legal, I doubt I'd try it. I get giddy with downsized neutrals. Downsized phase conductors would completely steal the remainder of the little sleep I do get.
 
One situation where high leg delta is used is where you have lots of single phase 120/240 load, with only a small amount of three phase loading. If you have 700A of single phase load and 100A of three phase load, then it makes perfect sense to size the 'high leg' for the three phase loading.

With that said, you would need to meet all of the requirements for properly protecting all of the conductors, follow all of the rules for parallel conductors, etc.

For example, when you use parallel sets of service conductors, each parallel set must have the same characteristics, and each must have the complete phase set. Thus you couldn't have, for example, 2 pipes, one with 3x500kcmil and one with 3x500kcmil + 1x4/0, and then parallel the 500's between the two pipes.

Similarly, I don't think that you could terminate the 4/0s on an 800A breaker.

But I don't see a problem with two pipes, each with 3x500kcmil and 1x4/0, terminating at a 600A two pole breaker and a 200A three pole breaker. Using the 200A to supply your three phase loads and the 600A for the single phase loads.

-Jon
 
3 ph service 240/120

3 ph service 240/120

the service 240/120 3 ph, has voltage A120 B 120 C 195 there is only 1/2 the amps are used
 
[The panel is MLO with 3-200 1ph. and 1-200 3ph feeding 3 small ac units
thanks



QUOTE=winnie;1205555]One situation where high leg delta is used is where you have lots of single phase 120/240 load, with only a small amount of three phase loading. If you have 700A of single phase load and 100A of three phase load, then it makes perfect sense to size the 'high leg' for the three phase loading.

With that said, you would need to meet all of the requirements for properly protecting all of the conductors, follow all of the rules for parallel conductors, etc.

For example, when you use parallel sets of service conductors, each parallel set must have the same characteristics, and each must have the complete phase set. Thus you couldn't have, for example, 2 pipes, one with 3x500kcmil and one with 3x500kcmil + 1x4/0, and then parallel the 500's between the two pipes.

Similarly, I don't think that you could terminate the 4/0s on an 800A breaker.

But I don't see a problem with two pipes, each with 3x500kcmil and 1x4/0, terminating at a 600A two pole breaker and a 200A three pole breaker. Using the 200A to supply your three phase loads and the 600A for the single phase loads.

-Jon[/QUOTE]
 
For that matter one could have a fused disconnect and the middle phase could be fused for the parallel 4/0. NO??? Nothing in 310.4 says the characteristics must be the same between phases.
 
hawkinselec said:
Can the hi-leg on a 240/120 3 ph. service ,wire size be reduce.
Click to expand...
Absolutely, as long as it's sized large enough for its load, and is properly protected for its size.

I've done it several times. In one case, a POCO field engineer had to get the OK from his boss.

We had worked together on another job like that, but he must have forgotten or something. :roll:
 
When I was a 2nd or 3rd term apprentice we were doing a service equipment & conductor change over spring break at an elementary school. It was 240/120 w/ high leg. Being a lower term apprentice I got stuck unrolling our parallel wire cuts and prepping the pull... I remember thinking "Why is this conductor so much smaller than the others?"

Turns out that the 800 amp service only had a small amount of three phase load (some air handlers I think) and the rest was all lights and plugs and other single phase stuff. I was weirded out by this for a while, until one of my classroom instructors led us through the calculation of a similar situation.

Works great if you're pretty positive that no major additions to the three phase load are going to happen somewhere down the road.
 
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