Need Help Installing a Sub-Panel in the Garage

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Old 08-12-14, 05:11 PM
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Question Need Help Installing a Sub-Panel in the Garage

Hey guys,

so ive recently started to build my garage workshop and i am going to need to add a 220V sub-panel. My current main panel is a 200A service with lots of empty space. I recently got rid of an electric dryer (30A breaker) and my electric stove (50A breaker) and switched over to gas. I also have 2 empty 30A breakers while the rest are filled with the house's normal lighting/plug loads.

Here is what i need to add:

A 230V air compressor which requires a 30A breaker
A 240V tig welder which requires a 30A breaker
A 220V automotive lift which requires a 25A breaker
several 120V receptacles (lets say 10 total receptacles on 2 separate breakers)


My HVAC unit also has a 60A sub-panel run to it from the main panel except there is only two 15A breakers inside of it (both only using 120V legs). One breaker is for the humidifier which i never use so that leaves a single 15A breaker fed from a 60A panel. Can this be shrunk down to a 30A sub-panel to leave more room for circuits in the garage?


So what size breaker and what size sub-panel do i need to add to my main panel to cover all of this? For the air compressor i am thinking of using the existing breaker/wiring from my stove (50A but can be shrunk to 30A) because i want the compressor to sit in the basement directly below where the stove used to be and there is plenty of slack in the wire for it.

I also dont need to run all of these items at the same time but i assume i should size the panel accordingly as if i were to. I also need to know what size wire to run to the garage. Its easily an 80 foot pull between the main panel and where i want to put the sub-panel so take that into consideration.

I do have some electrician friends but i want at least a decent idea of what im doing before i ask them to come over and help.


thanks in advance!
 
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Old 08-12-14, 05:41 PM
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Is the shop attached or detached?

Note: Nominal voltage is 240 not 220.
 
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Old 08-12-14, 06:31 PM
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the garage is attached to the house
 
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Old 08-12-14, 06:54 PM
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i am going to need to add a 220V sub-panel.
It will have to be a 120/240 volt subpanel if you want to add 120 volt circuits to the subpanel.
 
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Old 08-12-14, 09:37 PM
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Is the garage attached or detached? I just did a detached one of these a week ago. Ran 2-2-2-4 URD cable on a 100A breaker to a 100A 20 space panel.

My HVAC unit also has a 60A sub-panel run to it from the main panel except there is only two 15A breakers inside of it (both only using 120V legs). One breaker is for the humidifier which i never use so that leaves a single 15A breaker fed from a 60A panel. Can this be shrunk down to a 30A sub-panel to leave more room for circuits in the garage?

Why not leave it alone? It'll eat up 2 spaces in the main panel anyway, regardless of what breaker size is in there.
 
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Old 08-13-14, 04:28 AM
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You should do a "load analysis" before starting the project. The guidelines are in the National Electric Code. Among other things it calls for 3 watts per square foot of living area, figuring out the watts for each built in appliance from the name plate information, 80% of the watts for certain appliances considered intermittent use, 100% for electric heat or air conditioners.

Most likely you will install a 100 amp at least 12 slot (120/240 volt) subpanel in the garage. Run 2 gauge copper NM cable (or conduit with THHN wire), 4 conductors (hot, hot, neutral, ground) from the main panel. Put a 60 amp breaker in the main panel slots.

When you add more circuits or a subpanel, the rest of the system stays the same, i.e. not shrunk. The sum of the amperes ratings of the breakers inserted in a panel may exceed the amperes rating of its main breaker or its breaker in a supra-panel (feeding panel). But the main breaker for the panel may not exceed the panel's total amps rating shown on its label.

Your HVAC panel could not support the HVAC unit plus the compressor that might start up at any time plus one of the other large pieces of equipment you are tending.

Not ruled out could be the need for a new larger (incoming) service from the power company, that's why you need a load analysis.
 

Last edited by AllanJ; 08-13-14 at 04:55 AM.
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