Is Larger Panel Needed

Old 06-09-14, 06:01 AM
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Is Larger Panel Needed

I have a similar question. I have 150 amp service and I'm looking to add 7500 watts (40amps) of electric baseboard heaters to the basement. My panel has 30 slots and currently has 6 open. It will have 4 open slots after I add the double pole 40 amp breaker.

I ran through the Single Family Dwelling Load Calculator as accurately as I could and with the 7500 watts of heat came out with 118 amps. Is it simply OK if the calculated amps is lower than the main breaker rating or is there some % of the full capacity that you need to stay underneath?

Thanks for your help.
Old 06-09-14, 10:19 AM
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The question is is not the size of the panel but the rating of the Service Conductors, in this case 150 amperes which gives a power-capacity of 240 x 150 = 36000 Watts. Presuming the maximum load should be limited to 80 % of the rating of the Service Conductors , then the maximum load is 150 x .8 x 240 = 29000 Watts ( approx. ).

For a load with either cooling, heating , or both, the maximum load will occur during extreme hi/low temperature conditions. You can calculate the "average" load during such periods with exact KWHR meter-readings in a give time-period.

We will use a time-period of a week for the KWHR readings which equals 168 hours. The first reading is at "Time Zero" and the second reading at "Time 168" which indicates a power-consumption during this period of 2000 KWHR's. The Kilo-Watt value is 2000 / 168 = 12 KW's = 12.000 Watts.

12,000 Watts / 240 volts = 50 amps; we have calculated that during this time-period the current on the Service Conductors had a "constant" value of 50 amperes.

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