320 amp service

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enireh

Senior Member
Location
Canyon Lake,TX
I have a 320 amp service feeding a 200 amp and a 125 amp panel. Double lugs on load side of meter. Ok to feed 200 with 2/0 coppper and the 125 with #2 copper?
 
mopowr steve

mopowr steve

Senior Member
Location
NW Ohio
Occupation
Electrical contractor
Without knowing all the particulars of your install, 3/0 thhn rated for 75* lugs in your meterbase is probably more accurate. As well as #1 awg thhn for the 125a.
Where and how those conductors are ran makes a difference.
 
Dennis Alwon

Dennis Alwon

Moderator
Staff member
Location
Chapel Hill, NC
Occupation
Retired Electrical Contractor
enireh said:
thank you my question is, is it ok to feed the 200 with 3/0 and the 125 with #1 from the meter?
meter is double lugged on load side
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Sure since 3/0 is rated 200 amps and $1 is good for 125 amps- I assume copper. You just cannot take advantage of the residential deduction on services in this case
 
kwired

kwired

Electron manager
Location
NE Nebraska
GerryB said:
If it's residential I would say ok perTable 310.15(B)(7)
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iwire said:
No you can't use that table in this instance as neither set of conductors carries 100% of the load.
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Can use (B)(7) if each feeder is feeding an individual dwelling unit, say in the case of a two family unit.

But two feeds to a single dwelling unit and you can not use (B)(7), though you could for the common supply to the meter.
 
kwired

kwired

Electron manager
Location
NE Nebraska
ptonsparky said:
Are these both going to a single residence? Never tried two laterals to a house, but then I am thinking a pole mounted meter remote from the house.
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If two breakers were on the pole and both supplied the same building that wouldn't be permitted, but a single breaker (or no breaker and just tied to meter lugs making them service conductors) supplying up to six disconnects at the house is ok, and it could be multiple parallel conductors to each of those six disconnects as well.

Clear as mud isn't it?

ADD:

Forgot who I was talking to, clear as manure slurry isn't it?
 
electrofelon

electrofelon

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Location
Cherry Valley NY, Seattle, WA
lets not forget the next size up rule (240.4(B)) which often in effect allows the dwelling conductor sizes to be used even on conductors that do not carry the entire load of the dwelling unit. In the OP's case, 2/0 at 75 degree falls on a standard size so that case doesnt fly. #2 at 75 degree is 115 so 125 would be the next size up so that works.
 
kwired

kwired

Electron manager
Location
NE Nebraska
electrofelon said:
lets not forget the next size up rule (240.4(B)) which often in effect allows the dwelling conductor sizes to be used even on conductors that do not carry the entire load of the dwelling unit. In the OP's case, 2/0 at 75 degree falls on a standard size so that case doesnt fly. #2 at 75 degree is 115 so 125 would be the next size up so that works.
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What you said is the general rule, but doesn't apply when using provisions allowed by 310.15(B)(7).

If we are able to use 310.15(b)(7), within that section it says "shall be permitted to have an ampacity not less than 83 percent of the service rating" (or "feeder rating" in the next subsection). So we essentially size a conductor that normally would be allowed to be protected at 200 amps to 83% of that, and since it says "not less than 83 percent" it automatically rules out any "next size up" actions. Your base is 200 amps overcurrent device that you have already selected you are just allowed to go down to 83% here on conductor ampacity.

Outside of (B)(7) applications you size the conductor to the load and then apply overcurrent device to the conductor, next standard size up is permitted for 800 amps and less. But with (B)(7) we are selecting an overcurrent device first and matching a conductor at no less then 83%.

200 x .83 =166. We need a conductor from table 310.15(16) that has a 75C ampacity of at least 166 in that case - 1/0 is less then that 2/0 is greater then that, so we must use at least a 2/0 conductor - this is before factoring in any other ampacity adjustments if needed.
 
kwired

kwired

Electron manager
Location
NE Nebraska
electrofelon said:
lets not forget the next size up rule (240.4(B)) which often in effect allows the dwelling conductor sizes to be used even on conductors that do not carry the entire load of the dwelling unit. In the OP's case, 2/0 at 75 degree falls on a standard size so that case doesnt fly. #2 at 75 degree is 115 so 125 would be the next size up so that works.
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After reading again, it is possible I took what you intended to say the wrong way.

You may have been comparing results of standard method of determining needed conductor size to option allowed result by (B)(7).

If so some cases do yield same size conductor as a possibility. Like your mentioned #2 with a 125 amp breaker.

Also common is 4/0 aluminum with a 200 amp breaker. Works out as minimum size required using (B)(7) but also is still allowed to be on a 200 amp breaker as long as load calculation is not over 180.
 
ptonsparky

ptonsparky

Tom
Location
NE (9.06 miles @5.9 Degrees from Winged Horses)
Occupation
EC - retired
kwired said:
If two breakers were on the pole and both supplied the same building that wouldn't be permitted, but a single breaker (or no breaker and just tied to meter lugs making them service conductors) supplying up to six disconnects at the house is ok, and it could be multiple parallel conductors to each of those six disconnects as well.

Clear as mud isn't it?

ADD:

Forgot who I was talking to, clear as manure slurry isn't it?
Click to expand...
(Pretty clear right now, no agitation or manure)

I was hinting ... Sometimes I can be subtle.
 
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