How do you calculate the AMPS for a single family dwelling electrical service entrance?

Answer 1

To figure service sizing requirements you must follow the directions below. If you do not know how to size a dwelling then you should probably be getting in touch with a licensed master electrician. Electricity is dangerous if you do not know what you are doing. Some locations however do allow you to do your own. This work requires a permit in most cases and you should be getting in touch with your local building department before you begin any work!

To get started on calculating a service size for a single family dwelling you must add the following:

Add 3 volt-amperes per square foot of floor area for general lighting and general use receptacle outlets.

Plus

You must add 1,500 volt-amperes per 20-ampere-rated small appliance and laundry circuit.

Plus

You must add the nameplate volt-ampere rating of all fastened-in-place, permanently connected or dedicated circuit supplied appliances such as ranges, ovens, cooking units, clothes dryers and water heaters. There are default numbers here if you have not installed your appliances or you do not have an exact model number and ratting yet. The default numbers would be applicable if you are wiring in an outlet for these.

This will give you a subtotal. Apply the following demand factors to the subtotal.

The minimum subtotal for the loads above shall be:

100 percent of the first 10,000 volt-amperes

Plus

40 percent of any portion of the sum that is in excess of 10,000 volt-amperes

Plus the largest of the following:

1 - Nameplate rating(s) of the air-conditioning and cooling equipment.

2 - Nameplate rating(s) of the heating where a heat pump is used without any supplemental electric heating.

3 - Nameplate rating of the electric thermal storage and other heating systems where the usual load is expected to be continuous at the full nameplate value. Systems qualifying under this selection shall not be figured under any other category in this table.

4 - One-hundred percent of nameplate rating of the heat pump compressor and sixty-five percent of the supplemental electric heating load for central electric space-heating systems. If the heat pump compressor is prevented from operating at the same time as the supplementary heat, the compressor load does not need to be added to the supplementary heat load for the total central electric space-heating load.

5 - Sixty-five percent of nameplate rating of electric space-heating units if less than four separately controlled units. Forty percent of nameplate rating of electric space-heating units of four or more separately controlled units.

This gives you a total volt amperage required. The minimum total load or amps will be the volt-ampere sum calculated above divided by service entrance volts (usually 240V). If you get a number that falls between an available service equipment size then you must go up to the next available size.

These instructions will get you through the calculations needed for the International Residential Code. Your code may vary and it is best to consult you local building official for verification of this information.

Good luck and MOST important… BE SAFE!

Terry