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#### beachfront71

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Hello and TIA.

Just wanted to check in for a second opinion.

We are adding AirC to our house and need to run a new 30a line for the condenser outside.

Easiest way to do this would be to run from the subpanel due to ease of access.

Subpanel is 100amps and has 17 20a and 10 15a breakers on it right now with 6 empty double breaker expansion slots still available.

Based on that limited info.. would adding a 30amp breaker to this be possible/safe?

Again, appreciate the responses.
Electrician is coming out this weekend and I want to have access ready when he gets here.

#### Jim Port

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13,487 Posts
You do not add up the breakers to determine panel capacity. The correct method is a demand load calculation .

#### joed

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You have the space. But it depends on what those 27 breakers do now. If they are for baseboard heaters it might not work. If they are for unused or lightly used receptacles then it probably will work.

#### beachfront71

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I will dig into it more but all of these breakers are outlets and lights for the house minus a few that are in the main panel.. This includes the fridge, microwave and can really think of any other heavy users minus the vaccuum cleaner or hair dryer..

All of the big stuff like the kitchen Range, electric dryer etc are on the main panel.

#### beachfront71

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Is there a DIY formula for load calculation?

I found something like -- #circuits x amp rating x .8 x 120v = VA

VA / 208volts / 1.732 = AMPS

but even that math goes over 100 AMPS for my box

#### Daniel Holzman

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##### Civil Engineer
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The National Electric Code (NEC) has the required procedure for doing a load calculation. The formula you presented is for three phase 208 volt power. Not sure where you are located, but I doubt you have three phase power, if you are in the United States you probably have split phase 120/240 volt single phase. I am not an electrician, I don't feel comfortable discussing load calculations, there may be an electrician on the forum willing to discuss the procedure with you.

#### beachfront71

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Im not doing the work.. but I am trying to figure out if it will work so I can do any prep w/ regards to making this clean and easy .. IE drywall, etc..

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