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Wiring Your New House Part 2 – Sizing Your Electrical Service

March 6, 2007

Ok, I changed the title for part 2 of this series. The title “planning” was too general and would have made this post too long.

Now that you have completed your research, it’s time to get started on your house. But, what size electrical service do you need? Well, there is a calculation to determine this. Open up your National Electrical Code book to article 220. Here you will find everything you need to calculate the minimum sized electrical service for your new house.

Let’s get started with the general lighting and general use receptacle load. To determine this you need to know the square footage of your new house measured from the outside dimensions of the house. This measurement does not include open porches, garages, or an unfinished basement not adaptable for future use. However, if you do have such a space that is suitable for conversion to family rooms, bedrooms, home offices, or other habitable areas, these spaces need to be included in the general lighting load calculation.

Let’s assume your house is 2,000 square feet with a gas furnace and water heater. So, according to NEC table 220.12, you calculate your general lighting and general use receptacle load at 3 volt-amperes (VA) per square foot. This calculation equals (3VA x 2,000 square feet) 6,000 VA. So what does this mean? Well, 6,000 VA ÷ 120 volts = 50 amps. This means you need a minimum of 3 – 20 amp circuits or 4 – 15 amp circuits or a combination of the 2 totaling 50 amps or more for general lighting and general use receptacles.

Now, according to NEC section 210.11(C)(1) you are required 2 small appliance circuits. Each small appliance circuit shall be calculated at 1,500 VA [NEC section 220.52(A)]. You are also required 1 laundry circuit [NEC section 210.11(C)(2)] calculated at 1,500 VA [NEC section 220.52(B)].

So far we have:
General lighting and receptacles = 6,000 VA
Small appliance circuits = 3,000 VA
Laundry circuit = 1,500 VA
Total = 10,500 VA

Now we need to refer to NEC table 220.42 for lighting load demand factors. These demand factors shall apply to the portion of the total branch circuit load calculated for general illumination. They shall not be applied in determining the number of branch circuits for general illumination. That said, the demand factor percentage for the first 3,000 VA or less is 100%. The demand factor percentage from 3,001 VA – 120,000 VA is 35%. Now we can use these numbers to determine our net load.

Total load from above = 10,500 VA
3,000 VA at 100% = 3,000 VA
(10,500 VA – 3,000 VA) 7,500 VA at 35% = 2,625 VA
Net load = 4,875 VA

Now we need to add in the range and dryer loads to determine the net calculated load.

Net load from above = 4,875 VA
Range = 8,000 VA
Dryer = 5,500 VA
Net load = 18,375 VA
18,375 VA ÷ 240 volts = 77 amps

So, according to the calculation above you need a 77 amp service minimum. However, NEC sections 230.42(B) and 230.79 require service conductors and disconnecting means rated not less than 100 amperes. So, you need a 100 amp service minimum. However, I recommend installing a 200 amp service. This will allow for future growth, like a hot tub, air conditioner or whatever you desire, at a later date without needing to upgrade your electrical service.

If you need further clarification or have questions please submit them in the comment section of this post.

Tune in for part 3 tomorrow.

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Comments

13 Responses to “Wiring Your New House Part 2 – Sizing Your Electrical Service”

  1. Art Wong on April 2nd, 2007 5:59 pm

    Hi there,

    I am considering a new home which is approx 3000 sq ft. The builder is only providing 100 amp service. Other builders in the same area specify 200 amp. These new homes are all over $450K.

    Is there something odd about the 100 amp house.

    Thanks,
    Art

  2. Administrator on April 3rd, 2007 12:27 pm

    Hello Art,

    Thank you for your electrical question.

    This is a very conservative builder or electrician. If you do a load calculation following the example I provided above you should come up with an answer of approximately 99 amps. A 100 amp service will barely work. However, there is no room for future growth.

    If there is an electric water heater, air conditioning or anything else not listed above going into the home, the 100 amp service is too small.

  3. Barbara Eason on April 5th, 2007 6:07 pm

    Hello,

    We are about to upgrade an older 1600sf 2 bedroom, 2 bath house. Central heat (gas) and air and whirlpool tub will be added.

    Will we also need 200 amp service or would 150 amps do?

    Thank you
    Barbara

  4. Administrator on April 6th, 2007 12:38 pm

    Hello Barbara,

    Thank you for your electrical question.

    150 amps will work, but I recommend 200 amps. There is not much difference in price to install 150 amps verses 200 amps. However, the 200 amp service will allow room for future growth and add more to the resale value of your house.

  5. Joseph Graham on April 10th, 2007 9:42 am

    How did you get load calculations for the range and dryer, or are those amounts a given? Also, how do you calculate the load for a refrigerated air unit?

  6. Administrator on April 14th, 2007 3:14 pm

    Hello Joseph,

    Thank you for your electrical question.

    I got the 5000 watts for the dryer from NEC section 220.54 which states “The load for household electric clothes dryers in a dwelling unit(s) shall be either 5000 watts (volt-amperes) or the nameplate rating, whichever is larger, for each dryer served.”

    I go the load for the range from NEC table 220.55.

    You calculate a refrigerated air unit by adding the full load amps on the nameplate of the unit to the load calculation.

  7. John Gee on June 16th, 2007 9:05 pm

    Hello
    I want to install a tankless water heater in our yet to be delivered Manufactured Home. (1900 sq ft) It uses 112.5 amps alone.
    Our range will be gas. We plan on a heat pump. We also have a shop and a well pump. Should this water heater be on a separate line coming from say a circuit box below the meter base ? Also, don’t I need a 300 amp service?
    Thanks a lot
    John

  8. Mark on May 2nd, 2008 3:12 pm

    I’m making an office out of an old basement room. But I want to make sure I have enough power to run two computers and attached equipment. Plus some other small pieces of office equipment. I’m going to put it on it’s own circuit, however, I’m not sure whether I should put the one room on one 20 amp circuit or perhaps two 15 amp circuits. What would be recommended?

    thanks!

    Mark

  9. dan decol on August 3rd, 2008 2:45 pm

    I am looking just to replace my central air conditioner. This one is working, but my home always feels hot, then cool. My electric bill is ridiculous now and I thought maybe I should just replace the old unit. My home is 1600sf, so how big of a unit should I go with so as not to be under sized as well as over sized.
    Thanks again.
    Dan D.

  10. Ed Sabo on September 29th, 2010 12:19 pm

    I am remodeling a small lake house , 450 square feet. The furnace is gas, the stove is gas but i am going to put in an electric plug as well as an option for buyers when we sell. There is a well (220) and I will be adding a hot tub in the detached garage as well as an a.c. unit. There is one bedroom, 1 bath, a kitchen and living room. The garage is an over sized 1 car.

    My question is on the size service I should install. Due to the small size and no plans to expand it my thought was that a 100 amp service, 30 slot would be fine and leave a few open spaces for addition later on.

    In your opinion, would a 200 amp panel be overkill? I know i want extra spaces but would the load evr demand higher?

    For the sake of arguement, lets say the hot tub is on at the same time as the furnace, the fridge kicks on and the stereo, tv and lights are on (it is a lake house and I am certian there will be a party or two at some point when the remodel is finished)

    I know the cost factor is minimal, and am not too concerned with a little more money up front, i just dont know that i would ever need 200 amps for such a small house.

    What would you do if it were your place?

    Thanks in advance.

  11. lemlem kassahun on February 15th, 2012 6:00 am

    I need housing instalation

  12. Waqar Ahmed on April 22nd, 2012 7:25 am

    Hi
    In my house I have 200 Amp panel in garage that feeds Ist floor and second floor of my house. Out of this panel in basement 100 Amp Sub panel is installed. I plan on finishing basement. Can I feed following from this panel in basement?
    Lighting 100 watts light bulbs 50 bulbs
    receptacles for rec room,Den,wet bar,bathroom, and 3 rooms.
    50amp 2o8v oven
    Referigrator, Dish Washer, Electric Dryer, Garbage disposal,
    120V 15Amp heater In bath room.
    208v 15amp breaker heater for Garage.
    furnace 15amp breaker.
    I would appreciate if you calculate this for me.
    waqar
    Sump pump 15Amp breaker

  13. Katherine Carr on April 5th, 2013 9:10 am

    Thanks for your help. I’m glad to have a place I can go for a quick review.

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