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I have a 150 amp service in my home. I recently added a tool room, and would like to isolate that room with its own GFCI breaker. How do I determine if that it is doable?

I have perhaps 5 empty slots in the main panel. Does 150 amp service mean that all the breakers amperage in total have to equal 150 amps or less?

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See the section Summing Up Breaker Ratings Means Nothing in this answer to the question, Do the breaker amp readings sum up to dictate my current panels total / possible amperage?. This answer, to the question How do you plan capacity for electrical circuits? might also be useful. –  Tester101 Feb 19 at 14:50
    
You can read all that but the simple answer is yes and no. Unless that added room is a grow room you will be fine. –  Justin K Feb 19 at 15:18
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This article does a good job explaining load calculations. Residential Service Calculations in the National Electrical Code. Calculate the volt-ampere value, then divide by 240 volts, and you'll get the service size required. Next add the additional items you'll be adding, and make sure you're still under 150 amperes. –  Tester101 Feb 19 at 16:01
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Mike Holt has a Residential Load Calculations Spreadsheet (xls) available on the Technical Calculations and Formulas section of his website –  Tester101 Feb 19 at 16:05

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