Conductor size for 100 amp sub panel?
- Thread starter vliposky
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Does 310.15 (B) (6) apply?
Does 310.15 (B) (6) apply?
My plan is to feed a new basement located 100 AMP service (with 100 AMP breaker) with #2, then from ANOTHER 100 AMP breaker in the primary service panel to feed a 100 AMP sub panel (with yet another) 100 AMP breaker (total 3) that will provide power to all circuits (except HVAC & H2O circuits which will be fed from the primary service panel) in this second floor dwelling unit of this residential duplex.... (The first floor unit have it's own 100 AMP service & no sub panel etc...) Does 310.15 (B) (6) apply under these circumstances???? ( I already have #4 CU installed so I want to use what I have if I can....)
Does 310.15 (B) (6) apply?
My plan is to feed a new basement located 100 AMP service (with 100 AMP breaker) with #2, then from ANOTHER 100 AMP breaker in the primary service panel to feed a 100 AMP sub panel (with yet another) 100 AMP breaker (total 3) that will provide power to all circuits (except HVAC & H2O circuits which will be fed from the primary service panel) in this second floor dwelling unit of this residential duplex.... (The first floor unit have it's own 100 AMP service & no sub panel etc...) Does 310.15 (B) (6) apply under these circumstances???? ( I already have #4 CU installed so I want to use what I have if I can....)
FrancisDoody
Senior Member
- Location
- Durham, CT
After reading the highlighted sentence from above it only makes sense that a feeder need not be larger than the service entrance conductors to a dwelling unit.
- Location
- Windsor, CO NEC: 2017
- Occupation
- Service Manager
I'm a little foggy on what you're up to.
If a feeder comes from a panelboard beyond the service (as in, the feeder does not start at the service) then you may not apply 310.15(B)(6).
Hope that helps,
If a feeder comes from a panelboard beyond the service (as in, the feeder does not start at the service) then you may not apply 310.15(B)(6).
Hope that helps,
- Location
- Lockport, IL
- Occupation
- Semi-Retired Electrical Engineer
charlie b said:
charlie b said:
Dennis Alwon said:
Thought you were going there. Now I need to ask what version of the NEC applies in the jurisdiction under discussion? The reason I ask is that that phrase was revised in 2005. It no longer says that the feeder need not be BIGGER than the service. What it now says is that the feeder need not have a higher ampacity. But ampacity depends on the ?conditions of use.? So it is possible that a feeder might need to have a larger size conductor, in order to get the same ampacity as the service conductors.
By the way, I think 310.15(B)(6) does not apply. Neither of the two ?sub-panels? under discussion appears to be the ?main power feeder? to a separate dwelling unit.
radiopet
Senior Member
- Location
- Spotsylvania, VA
Agree with Charlie.......the statement of 310.15(B)(6) is for the dwelling itself to which is being fed, the definition of the feeder in the example is from the service disconnect to the lighting and appliance panel...so if the service disconnect was on the lets say outside.....and run to a " remote distribution panel " inside as part of the main power feeder setup...then 310.15(B)(6) could apply.......not for feeders actually feeding other remote distribution panels not part of the main power feeder to a different structure.
6) 120/240-Volt, 3-Wire, Single-Phase Dwelling Services and Feeders. For dwelling units, conductors, as listed in Table 310.15(B)(6), shall be permitted as 120/240-volt, 3-wire, single-phase service-entrance conductors, service lateral conductors, and feeder conductors that serve as the main power feeder to a dwelling unit and are installed in raceway or cable with or without an equipment grounding conductor. For application of this section, the main power feeder shall be the feeder(s) between the main disconnect and the lighting and appliance branch-circuit panelboard(s). The feeder conductors to a dwelling unit shall not be required to be larger than their service-entrance conductors. The grounded conductor shall be permitted to be smaller than the ungrounded conductors, provided the requirements of 215.2, 220.22, and 230.42 are met.
6) 120/240-Volt, 3-Wire, Single-Phase Dwelling Services and Feeders. For dwelling units, conductors, as listed in Table 310.15(B)(6), shall be permitted as 120/240-volt, 3-wire, single-phase service-entrance conductors, service lateral conductors, and feeder conductors that serve as the main power feeder to a dwelling unit and are installed in raceway or cable with or without an equipment grounding conductor. For application of this section, the main power feeder shall be the feeder(s) between the main disconnect and the lighting and appliance branch-circuit panelboard(s). The feeder conductors to a dwelling unit shall not be required to be larger than their service-entrance conductors. The grounded conductor shall be permitted to be smaller than the ungrounded conductors, provided the requirements of 215.2, 220.22, and 230.42 are met.
bradleyelectric
Senior Member
- Location
- forest hill, md
Why not use 60A breaker for the sub panels? The #4 is oversized for the breakers, but wont hurt. The 60A breakers cost less then the 100A breakers, and you have already pulled the major loads from the main service. Did you calculate the load that will be on the sub?
- Location
- New Jersey
- Occupation
- Journeyman Electrician
vliposky said:
No it doesn't. Since the #4 NM is already in place you'll need to use the 60 degree C rating of #4 in table 310.16. You'll need to use a maximum of a 70 amp CB for the #4. Unless your calculated load is greater than 70 amps. In that case you'll need to change something.
- Location
- Chapel Hill, NC
- Occupation
- Retired Electrical Contractor
I am totally confused the op's original question seems to be very different now. Anyone care to explain what we are looking at. Lost-----
- Location
- New Jersey
- Occupation
- Journeyman Electrician
Dennis Alwon said:
New 100 amp service fed with #2's and with a piece of #4/3 NM cable to a subpanel. He wants to use table 310.15(B)(6) to size the OCPD for the subpanel feeder.
- Location
- Chapel Hill, NC
- Occupation
- Retired Electrical Contractor
charlie b said:
Thank you Charlie--- I had been thinking about this all day today and I realized that the fact that NM cable was being used it would change what I was thinking. I just got the chance to get in and read what has transpired.
- Location
- Chapel Hill, NC
- Occupation
- Retired Electrical Contractor
infinity said:
It's more than that--- he said something about 3 100 amp breakers---Is the actual service 100 amp or is terminology being used incorrectly
Got room on the lost boat?Dennis Alwon said:
I think we went from a "simple" 100A sub being feed from a 100A main in a SFH:
(Post #1 )vliposky said:
...to a 2-family or multi-family dwelling? unit:
(Post #22 )vliposky said:
I drew this picture...obviously it's not right...but that's what I am getting from the post..
- Location
- Chapel Hill, NC
- Occupation
- Retired Electrical Contractor
Quite a change from the OP. Given the new info I would agree that Table 310.15(B)(6) does not apply. A #2 copper will be sufficient if the load is less than 95 amps. The #4 would have to be fused at 70 amps
- Location
- Chapel Hill, NC
- Occupation
- Retired Electrical Contractor
It ain't ridiculous. I appreciate the effort.
bradleyelectric
Senior Member
- Location
- forest hill, md
I'm thinking 100A service switch feeding a 100A panel with a 100A breaker in it feeding a 100A breaker that is being used to backfeed a MLO panel, but I haven't seen it to verify that mess. That's why I was thinking just put a 60A breaker in the Panel being used as the service to feed the 4-3 NM running to the MLO panel that I think is only feeding general lighting. Again nobody verifying this, just what I'm conjuring up in my wild imagination. I see Celtic came up with a MCB instead of the backfed breaker I had the guy buying, still neither is neccesary and you can still feed a 100A MCB panel with a 60A breaker if that is what exists or you have 1 sitting in the truck in good shape from the service upgrade you did yesterday if you want. Not condoning using that old panel, but we do tenant refits all the time where the new tenant uses the existing service without it being tested or reconditioned.
Ok, I did another reread and it seems he possibly has 2 100A MCB panels sitting in a basement. panel 1 feeds the first floor unit. panel 2 feeds the second floor unit HVAC, HWH, and a piece of 4-3 NM run to the second floor. He wants to put the 4-3 on a 100A breaker and have it feed a subpanel that has a 100A breaker of some configuration with unknown load. My suggestion is do a load calc. and make sure a 60A breaker will suffice and call it a day.
Ok, I did another reread and it seems he possibly has 2 100A MCB panels sitting in a basement. panel 1 feeds the first floor unit. panel 2 feeds the second floor unit HVAC, HWH, and a piece of 4-3 NM run to the second floor. He wants to put the 4-3 on a 100A breaker and have it feed a subpanel that has a 100A breaker of some configuration with unknown load. My suggestion is do a load calc. and make sure a 60A breaker will suffice and call it a day.
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