Single phase Existing Dwelling Service Size
- Thread starter wmeek
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mbrooke
Batteries Included
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- United States
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- Technician
I would think 310.15(B)(7) considering its both a dwelling and both 300 and 350amp services are recognized in the table. The only difference being parralleled conducters to get your 350kcmil equivalent in the table. Looking further no exclusion comes out either.
Im 90% sure in the above in that its the way the AHJ allows here, but it might be interpreted differently where your district.
Im 90% sure in the above in that its the way the AHJ allows here, but it might be interpreted differently where your district.
- Location
- Chapel Hill, NC
- Occupation
- Retired Electrical Contractor
There is nothing in 310.15(B)(7) that allows parallel conductors. IMO, you must use Table 310.15(B)(16)
texie
Senior Member
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- Fort Collins, Colorado
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- Electrician, Contractor, Inspector
Agreed. You have a 300 amp service.
mbrooke
Batteries Included
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- United States
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- Technician
Couldnt the parallel conducter tables also apply considering your getting an equivalent though to the dwelling table?
jumper
Senior Member
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- 3 Hr 2 Min from Winged Horses
Look at the heading of 310.15(B)(7).
(7) 120/240-Volt, 3-Wire, Single-Phase Dwelling Services
and Feeders.
I would say that pretty means this section does not apply for parallel feeds to a dwelling.
- Location
- Illinois
- Occupation
- retired electrician
CMP 6 does not agree. The following is a proposal and panel action for the 1996 code
Note 3 to the ampacity tables in the 1993 code was the equivalent of 310.15(B)(7) in the 2011 code.
mbrooke
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- United States
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- Technician
Not saying anyones wrong, but what the reasoning though behind not allowing the table for parallels? Heat build up?
That leaves a big question on how to determine the proper size parallel conductors. For example, 1/0 is okay for a 175 service, 350kcmil okay for a 350A service. So is parallel 1/0 okay for a 350A service? If we look at Table 310.15(B)(16) 1/0 is rated 150A, 350kcmil is rated 310A... thus two 1/0 would not equal or exceed the rating of a single 350kcmil.
- Location
- Illinois
- Occupation
- retired electrician
The CMP did not give any guidance with their answer. I would say that parallel 1/0s would be good for a 350 amp service based on their comment.
jumper
Senior Member
- Location
- 3 Hr 2 Min from Winged Horses
But, I thought that the basis for that table is historical data from POCOs for 3W 240 1P services.:?
- Location
- Illinois
- Occupation
- retired electrician
"3W" defines the system, not the number of conductors used in the system.
jumper
Senior Member
- Location
- 3 Hr 2 Min from Winged Horses
- Location
- Lockport, IL
- Occupation
- Retired Electrical Engineer
Agreed. More specifically, the word "ampacity" does not appear anywhere in that table. The table is a "permission granted" opportunity, not a "here is another way to look at ampacity" opportunity.
I once submitted a proposal similar to the one Don quotes above. Mine was rejected for essentially the same reason, and with essentially the same lack of clarification. What I would like the CMP to do is to change the title or instructions of the table to explicitly state that the numbers represent allowable ampacities, so that we can clearly parallel conductors in order to get higher ampacities. Right now, all it says is that for this specific installation you can use this specific wire.
I think someone mentioned in another thread, there is an accepted proposal to change the section in the 2014 NEC edition... anyone know what that entails?
jumper
Senior Member
- Location
- 3 Hr 2 Min from Winged Horses
It is a calculation now and the table is gone. The main power rule for feeders is gone also.
(7) 120/240 Volt, Single-Phase Dwelling Services and
Feeders.
For service and feeder conductors of 120/240-
volt, single-phase, individual dwelling unit one-family,
two-family, and multifamily service ratings from 100 am-
peres through 400 amperes, an adjustment factor of 0.83 of
the service ampere rating shall be permitted to be used to
determine the size of the ungrounded conductors. The
grounded conductor shall be permitted to be smaller than
the ungrounded conductors, provided that the requirements
of 215.2, 220.61, and 230.42 are met.
[ROP 6?49a]
Yes, but note the word I emphasized. It will only affect feeders requiring an ampacity greater than 83% of the service rating.
jumper
Senior Member
- Location
- 3 Hr 2 Min from Winged Horses
I probably just need more coffee, but I do not understand.
I probably need some caffeine too. It's not really a big issue. Let's take a 200A service as an example.
200A ? 0.83 =166A
No feeder supplied by this service will be required to have an ampacity greater than 166A.
jumper
Senior Member
- Location
- 3 Hr 2 Min from Winged Horses
Okay, I get your point.
What I was referring to was that now you can drop a load or two at a meter/panel disco and use the calc. for a main feeder to the house panel and not get grief/flack over it.
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