Panel feed via double lugs with multiple wire sizes?

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ces

ces

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Northern California, USA
I'm feeding two 225 amp adjacent panels with 400 amp wiring -500kcmil and want to double-lug a 225A breaker in the first to feed a 225A breaker in the second.
As I recall the tap rule allows downsizing to 225A for 10ft and even 25ft in this case. My question is can I mix 500kcmil and 3/O wire on the same lugs?

I could also put a gutter below and splice 3/O up to each panel -this might be easier perhaps cheaper in the long run than wrestling with the 500's?

Any thoughts on this appreciated
Gary Conway
 
The double lugs are fine as you described as long as they're rated for the conductors you're installing. If I'm understanding your configuration, you want to have the 500 Kcmil feeder into the line side of a 225 amp CB with another set of conductors feeding an adjacent panel. I don't know how easily you will find the CB with the lugs you've described. You will need a minimum conductor size of #4/0 cu to feed a 225 amp CB. #3/0 cu. is only good for 200 amps.
 
infinity said:
I don't know how easily you will find the CB with the lugs you've described. .
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That's exactly what I was just thinking. It might not even exist. That type of job normally calls for a piece of gutter.
 
Thanks i've specified lugs like this before -(2) sets of 500 on the first panel breaker and (1) set on the second. BUT that doesn't mean it got installed that way, or that they aren't hard to come by at best.

I'm still wondering about combining different wire sizes on one set of lugs -I mean mechanically is it a good bond if one wire is much smaller than the other?

I spoke to the installing contractor after making the post (i just push paper i'm afraid) he agreed that wrestling 500's was a bad choice so we will go with (2) sets of the 3/O from a pullbox. 3/O's are fine for 225A per NEC 240.3(B) which is copied below. A lot of people miss this one.
I like to think I do fairly well understanding the code, but where I frequently need help is understanding the practical side of what actually works in the real world, including what is more cost effective.
I'm new to this forum and look forward to looking into it more.

Thank you for the comments
Gary Conway

2002 NEC 240.3
(B) Devices Rated 800 Amperes or Less. The next higher
standard overcurrent device rating (above the ampacity of
the conductors being protected) shall be permitted to be
used, provided all of the following conditions are met:
(1) The conductors being protected are not part of a multioutlet
branch circuit supplying receptacles for cordand-
plug-connected portable loads.
(2) The ampacity of the conductors does not correspond
with the standard ampere rating of a fuse or a circuit
breaker without overload trip adjustments above its rating
(but that shall be permitted to have other trip or
rating adjustments).
(3) The next higher standard rating selected does not exceed
800 amperes.
 
ces said:
I spoke to the installing contractor after making the post (i just push paper i'm afraid) he agreed that wrestling 500's was a bad choice so we will go with (2) sets of the 3/O from a pullbox. 3/O's are fine for 225A per NEC 240.3(B) which is copied below. A lot of people miss this one.
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In that case, why not supply each panel with its own 3/0 feeder?
 
I spoke to the installing contractor after making the post (i just push paper i'm afraid) he agreed that wrestling 500's was a bad choice so we will go with (2) sets of the 3/O from a pullbox. 3/O's are fine for 225A per NEC 240.3(B) which is copied below. A lot of people miss this one.
Click to expand...


CES, the section you posted is 240.4(B) and I don't see how 240.4(B) applies. #3/0 Cu THHN is rated for 200 amps which is a standard size listed in 240.6. You can't go up to the next size 225 amps. You would need to use #4/0 cu.
 
infinity said:
CES, the section you posted is 240.4(B) and I don't see how 240.4(B) applies. #3/0 Cu THHN is rated for 200 amps which is a standard size listed in 240.6. You can't go up to the next size 225 amps. You would need to use #4/0 cu.
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I agree, unless perhaps the OP is talking about a residential 400 amp. Many inspectors will let you use Table 310.15(B)(6) for a dwelling unit. In this case 3/0 copper is good for 225. But I tend to think there is something else going on based on his post of art. 240.4(B)
 
I may be missing something here, but it appears to me that 240.4(b)(2) requires that 3/0 THHN be protected by a 200A breaker since its ampacity in table 310.16 is 200 A (75 degrees C column).
 
infinity said:
CES, the section you posted is 240.4(B) and I don't see how 240.4(B) applies. #3/0 Cu THHN is rated for 200 amps which is a standard size listed in 240.6. You can't go up to the next size 225 amps. You would need to use #4/0 cu.
Click to expand...

infinity, oops my mistake you are absolutely correct. Thank you.
 
LarryFine said:
In that case, why not supply each panel with its own 3/0 feeder?
Click to expand...

I would not be surprised if (2) 225A feeders (I stand corrected that the required size is 4/O not 3/O) is more economical than one 400A feeder.

In this case we have 250ft of existing 3-1/2"C. While (8) 4/O will fit in a 3-1/2"C the 70% derating would require upsizing to 350kcml
 
ces said:
I would not be surprised if (2) 225A feeders (I stand corrected that the required size is 4/O not 3/O) is more economical than one 400A feeder.

In this case we have 250ft of existing 3-1/2"C. While (8) 4/O will fit in a 3-1/2"C the 70% derating would require upsizing to 350kcml
Click to expand...


Actually the #4/0 in one raceway may still work. #4/0 THHN has a 90 degree C ampacity of 260 amps. If the supply is 3 phase, 4 wire the neutral will not count as a CCC unless the load was more than 50% non-linear which is unlikely. So you would derate at 80% instead of 70%.

260 amps(80%)=208 amps. You can round up to the next standard size (225 amps) if you connected load doesn't exceed 208 amps. The 250' may be more of a problem due to voltage drop.
 
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