# Residential Calculation whether to use 220 volts or 240 volts when dividing watts to determine the amperage

#### gunnpe

##### New User
Question: When determining the wire amperage size when tying in multiple forced air heaters, is 220 volts or 240 volts used when dividing into watts to get the correct amperage for sizing my wire in a residential setting?

#### LarryFine

##### Master Electrician Electric Contractor Richmond VA
Welcome to the forum.

I would use the stated power and voltage numbers to calculate the operating resistance.

Then I would calculate the operating power using the current at the actual applied voltage.

#### Dennis Alwon

##### Moderator
Staff member
It depends. If the voltage is 240 at the site and the unit is "X" watts at 220v then you have to do a little math to get the correct info for sizing on resistive loads

#### LarryFine

##### Master Electrician Electric Contractor Richmond VA
It depends. If the voltage is 240 at the site and the unit is "X" watts at 220v then you have to do a little math to get the correct info for sizing on resistive loads
Isn't that what I said?

Staff member

#### Dennis Alwon

##### Moderator
Staff member
To the op -- the difference wont be very large in your case. Going from 240 to 208 is about 75% decrease in wattage.

If you need help with the math Larry has volunteered to help you. LOL

#### don_resqcapt19

##### Moderator
Staff member
220.5 (A) Voltages.
Unless other voltages are specified, for purposes of calculating branch-circuit and feeder loads, nominal system voltages of 120, 120/240, 208Y/120, 240, 347, 480Y/277, 480, 600Y/347, and 600 volts shall be used.