Res. Feeder Ampacity

[willmc]

Scenario: 400A service, 1 factory installed 200A main breaker feeds buss, 1 field installed 200A (maximum) main breaker available for feed thru (Siemens MC3040MB22). Contractor opts to feed 2 subpanels inside residence with #1 SE cable. 1 feeder is landed at the feed thru breaker, 1 feeder is landed at breaker on buss. Buss breakers also feed 2 A/C's and a pool. What determines the 2 subpanel feeder ampacities, 310-16 60 degree column or 310-15 (6)- residential exception??? (2001 California Electric Code which is '99 NEC) Thanks in advance

 

[curt swartz]

Re: Res. Feeder Ampacity

willmc,

Table 310-15(b)(6) is for the service entrance, service lateral or feeder conductors that serve as the main power feed to a dwelling unit. Since neither of these feeders are the main power feed to the dwelling I don't feel you can make use of these reduced sizes. These feeders should be sized using table 310-16. If there is not an temperature rating on the breakers or service equipment you would need to use the 60C column but I would guess that Siemens rates these 2-pole 100 amp breakers for 75C wire so #1 AL would be fine.

Curt

 

[curt swartz]

Re: Res. Feeder Ampacity

willmc,

I just checked and a Siemens Q2100 is listed for 60/75C conductors. I do have a question though. What breaker is the contractor using in the field installed main position? According to Siemens the panel you specified requires a QPM for the field installed main position. The only 2 sizes of QPM breakers I show as being available are 150 and 200 amp.

Curt

 

[willmc]

Re: Res. Feeder Ampacity

Curt, That factory installed main is a QPMH with a 22K interupter rating although I've seen some with a QNRH (QNHR??)which Reno at the Siemens office and their 2003 Speedfax verified. I'm questioning more the 310-16 / 310-15(6) choice. I thought the 60 degree rating was a given because of the cable assembly. 338-4(a) & 336-26 + 310-10. Please correct my understanding if incorrect and Thank You all

 

[curt swartz]

Re: Res. Feeder Ampacity

willmc,

I don't have a copy of the 96 NEC in front of me but can see why you are getting a little confused by the 99 NEC. 338-4 says the type SE cable used for interior wiring shall comply with parts A and B of article 336. Included with part B is 336-26 which limits the ampacity of type NM cable to 60C. 336-26 is new to the 99 NEC and I am not sure where it was located in previous additions but the 60C temperature limitation does not apply to type SE cable. If you happen to have a copy of the 2002 NEC look at 338.10(B)(4)(a) which excludes 334.80 (the 60C limit of NM cable in the 2002 NEC).

My concern was with the field installed main breaker not the factory installed main. I just checked and the QN series breaker has replaced the QPM and QPJ series but is still only available in 150, 175 and 200 amp sizes. You said the one of these 100 amp feeders was going to be supplied by the field installed main.

Curt

 

[willmc]

Re: Res. Feeder Ampacity

Curt, Thank you, I was focusing on the ampacity of the wire and missed the main breaker requirements. The higher rating on SE cable makes sense (my '02 NEC is at work). My stand was 310-16 60C column. I like being able to justify backing away from too rigid an interpretation. So correct code call is - use proper main breaker, Size conductors @ MB per T310-15(b)(6), Size conductors @ secondary breakers per T310-16 75C & 240-3(b).