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I would like to know what size and type of feeder wire and feeder breaker do I need for my basement subpanel where I am planning to add six 15 Amp circuits, and three 20 Amp circuits. This will be 2' away from the main service panel. It will be for lighting and receptacles for bedrooms, a large living area, and a bathroom. I plan on running a 65" HDTV with sound system and 2 video game consoles; one of the rooms will be an office with a PC and printer. The two bedrooms will each have a 32" TV. The bathroom has only an exhaust fan, shower light and vanity light. There will be no heavy equipment.

  • You should give an indication of what types of loads you will be running. For example, do you intend on having a huge stereo system and a giant HDTV? The amperage of the circuits is mostly irrelevant because the subpanel's feeder circuit doesn't need to be sized to handle all possible loads on the subpanel, just the expected load plus a margin of error. You might be able to get away with a 60A feeder, but if your expected loads are high, a 75A, 90A or 100A feeder might be more appropriate. The wire size you need is based on how much current you want to carry, as well as the distance this run h – cathode Mar 31 '15 at 5:15
  • Can you include the square footage of the area being serviced by the panel? – Tester101 Mar 31 '15 at 9:29
  • The total area will be 1300 sq. Ft. The large living area is about 450 sq. Ft and the rest almost equally divided on 3 other rooms. The bathroom is about 50 sq. Ft. – Angel Mar 31 '15 at 16:34
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National Electrical Code says that for general lighting and receptacle circuits, you can use 3 volt-amperes per square foot to calculate the load. However, the square footage is calculated from the outside dimensions of the floor area. So when calculating the area, don't forget to include the wall thickness.

National Electrical Code 2014

Chapter 2 Wiring and Protection

Article 220 Branch-Circuit Feeder, and Service Calculations

220.82 Dwelling Unit.

(B) General Loads. The general calculated load shall be not less than 100 percent of the first 10 kVA plus 40 percent of the remainder of the following loads:

(1) 33 volt-amperes/m2 or 3 volt-amperes/ft2 for general lighting and general-use receptacles. The floor area for each floor shall be calculated from the outside dimensions of the dwelling unit. The calculated floor area shall not include open porches, garages, or unused or unfinished spaces not adaptable for future use.

Let's say you're going to be servicing a 1300 square foot area with the panel, and it sounds like you're only planning for 120 volt general lighting and receptacle circuits. The load would be calculated as follows:

3 volt-amperes * 1300 sq.ft. = 3900 VA
3900 VA / 120 Volts = 32.5 amperes

In this scenario, you'd be able to use a 40 ampere double pole breaker in the main panel, and 8 AWG copper or 6 AWG aluminum conductors to feed the panel. However, you may want to leave room for future expansion, so the minimum might not be what you're looking for.

You may in fact want to use a 60 ampere double pole breaker, and 6 AWG copper or 4 AWG aluminum conductors. Even though you only need half that capacity, the cost difference might be worth it if you want the option for future expansion.

  • Thanks Tester101. Does it matter which side of main panel i install the feeder braker. Do i have to keep the load balanced in main braker and subpanel? How much load does the feeder breaker accounts for in case i do want to go ahead and use a 60amp breaker for possible future needs? – Angel Mar 31 '15 at 19:12
  • You'll be installing a double pole breaker in the main panel, so you don't have to worry about balanced loads there. Just make sure in the second panel, you don't put everything on one leg. – Tester101 Mar 31 '15 at 19:40
  • It sounds like you might want to contact a local Electrician to do this install for you, or do a bit more research on electrical panels, services, and feeders. – Tester101 Mar 31 '15 at 19:43
  • Great! Thanks. I have actually called the City Building Dpt. to consult on details and they literally sent me to homedepot saying "they would know better the specs". At that point i thought their inspections would not be muchb9f assurance. I appreciate your help. – Angel Mar 31 '15 at 22:07
  • @Angel This answer might be worth a read. – Tester101 Apr 1 '15 at 10:10

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