NEC code question

Discussion in 'General Electronics Chat' started by cpu_user, Oct 31, 2011.

  1. cpu_user

    Thread Starter New Member

    Jan 16, 2011
    Let's assume that a home has a 200 AMP 240VAC electrical service.
    If the house heating requirement requires a 150 amps of electric heaters at 240VAC; does that mean I really only have 50AMPS of available current for other loads?

    My knee jerk reaction is to say YES, but not all the heaters could be on at one time. Are we to assume the above 150 AMP load to be worst case scenario and have to install another service in tandem to handle the rest of the house applicances?

    If anyone knows the NEC codes to this situation, I would appreciate the citing!

  2. #12


    Nov 30, 2010
    The devil is in the details.

    If a house heating requirement requires 150 amps of electric heaters at 240VAC, the wiring has to be rated at 125%, or 187.5 amps. Or did you mean the breaker for your furnace is 150 amps and the load is, by code, not more than 80% and that's 120 amps (or less) being used for heat.

    230-2 Number of services. A building or structure served shall be supplied by only one service. 230-2 Exception 4(b) when the supplying utility feels like it (needs 2 services due to large load). Exception 5: Buildings of large area, by special permission (probably by the local governing authority).

    Then there is the non-concurrent use exception when it is IMPOSSIBLE for all the heaters to be on at the same time. Well, is it?

    There's a start. Now you know some of the questions to ask.
  3. cpu_user

    Thread Starter New Member

    Jan 16, 2011
    Sorry about the lack of clarity in my questions.

    The total amount of electric radiation was 150A.(Arbitrary number for this example). If we have several rooms with electric heat(baseboard) again hypothetically all size correctly for the individual rooms. All having the REMOTE possibility of turning on at the same time although unlikely. Does that mean that I only have 50A remaining in the sized main panel for other appliances such as the frig, oven, etc.??? Its highly unlikely that I would have all the electric on at the same time.

    Is this a potential problem or would it require two services?
  4. #12


    Nov 30, 2010
    If the total amount of electric radiation is 150 amps and everything is on at once, then you have at least 187.5 amps of breakers dedicated to the heaters and there is 50 amps left before the main breaker trips. You could use that up with all the heaters in the electric stove turned on and the water heater running. Then there is the clothes dryer at 24 amps and a microwave oven at 10 amps of 120VAC, and lighting at 36 amps (calculated on a building of 2880 square feet and 3 watts per square foot as described in the NEC).

    If your main breaker ever trips, then you should consider installing more service. In other words, I have never seen anybody shell out a thousand dollars for more power if the main never trips.

    One of my personal problems is that I am a problem solver. If you don't have a problem, I don't fix it. Another person with a different attitude might be more helpful in this hypothetical case.

    In this case, the house would hold at 70 degrees when it is 0 degrees outside if the average insulation value was 2.62.

    If you live where the temperature gets to zero degrees, you can be sure the house does not have R2 insulation value in the attic and the walls. That is one reason why I can't get serious about this. Your hypothetical 2880 square foot house would be illegal in every state and nearly impossible to build that badly.
  5. GetDeviceInfo

    Senior Member

    Jun 7, 2009
    I can't comment on the NEC, but the CEC provides a base wattage per sq.ft. for residential, plus other loads, some of which may be derated. Heating devices can be derated after 10K watt, but that depends if you have an electric or gas stove. Branch circuit sizing is subsequently described as continuous or non, with continuous derated to 80%.

    Sorry can't help, but I suspect the answer is deeper than the info provided.
  6. mcgyvr

    AAC Fanatic!

    Oct 15, 2009
    If you have 200 apples and your heater takes 150 apples you have 50 apples left.
    #12 likes this.