Load center calculations

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  #1  
Old 09-23-15, 01:17 PM
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Load center calculations

Hey fellow diyers! Been a while since I posted, but I have a new issue.

Recently we moved and purchased a older home, like 101 years old. The previous owners replaced as much of the knob and tube wiring as possible. They also updated to a 100 amp box, installed by a electrician. Nothing is labeled and I am working through and tracing wires and what they power. Now here is the question.

The house INCLUDING basement is roughly 3200 square feet. There are 4 bedrooms, living, dining, a extra sitting room, two baths, front porch, rear porch, and a two stall detached garage. Gas furnace, central air, electric dryer, washer, fridge, dishwasher, built in microwave with range hood.

We have a spa brought from the other house we want to wire up, I also have a 220 compressor I want to wire up in the garage.

Now any load calculations I do WITHOUT adding in the two new circuits show the 100 amp service to small?! I would assume then that wanting to add new circuits would really be to small. Or am I able to add a sub panel and power the two new circuits from there?
 
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Old 09-23-15, 01:24 PM
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My sense is that the 100A service is borderline for 3200 sq. ft. with some electric appliances, even before considering the spa. You can do a "demand load calculation" where you plug in exact values for major loads like the air conditioner and cooking appliances to know for sure. There are several spreadsheets online that can help walk you through it. You'll also need to know exact specs on the spa and compressor measured in watts or horsepower to complete the calculation.

You'll also need to identify the type of feed to the detached garage, and if it has 240V available, do a calculation on that panel to see if it is adequate to support the compressor motor.
 
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Old 09-23-15, 01:55 PM
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I will search out a demand load calculator. The garage is currently fed off just a single 20 amp breaker. It has to be buried somewhere, I can see it come out of the main panel and into the floor sill plate.

Per the few load calculators I did find, I believe you figure them at watt x amp for the volt-amp rating
Therefore a 120 volt applianc at 15 amps is 120 x 15 = 1800 volt-amps
 
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Old 09-23-15, 01:57 PM
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Old 09-23-15, 02:18 PM
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Thankyou ray, that is the one I am working with. Calculating everything up I am coming up with needing 190 amps. Even taking out the spa and compressor I am coming up to 142 amps. Now I understand that tha would be if EVERYTHING was running at the same time, and that is very unlikely. Heck even the compressor and spa won't run all the time.

I am going to say my best option is to upgrade this to a 200 amp panel. If that isn't a option at this time, could I run a sub panel still to power these other two? Even as a temporary solution?
 
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Old 09-23-15, 06:16 PM
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A subpanel only adds breaker space. It does nothing to add capacity.
 
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Old 09-24-15, 07:24 AM
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The demand load calculation applies reduction factors to the loads to account for the fact that not everything will be on at the same time. For example, it counts the first 3000W of lighting at 100%, then the rest at 35%. Similar in cooking appliances and fixed appliances. But you should still think of it as a Thanksgiving calculator -- all the guest rooms are full, the hair dryers are running in both bathrooms, fridge and freezer are at full power, laundry is going, and all the cooking equipment is running, then you try to get away from the family for a little bit and head out to the garage to flip on the big screen and compressor to make it sound like you're doing something important.
 
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