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Re: electrical subpanel

From: info@builderswebsource.com
Category: Electrical/Lighting
Remote Name: 63.198.181.43
Date: 18 Oct 2000
Time: 10:03 PM

Comments

The electrical code provides formulas for derating subpanels and branch/feeder circuits depending on what's connected. Without getting into too much detail, you first need to compute the total expected load (continuous loads + non-continuous loads). Typically continuous loads must add a 25% extra cushion.

You say that not many things will be connected. Consider all the following things that might be found in a garage:

Washer, dryer, water heater, automatic door opener, outdoor lighting timers, pumps, lights, central vacuum, outlets, etc.

In order to properly design your electrical system, you need to account for the above conditions.

Now, if you only plan to install enough for general lighting and small power tools, then you can get by with less. I always like to plan for the future, because you never know what other power needs you'll have.

If you're adding a new subpanel, I'd put in at least a 70A rated panel, or even a 100A or 125A rated panel. The subpanel itself doesn't cost that much and you don't have to fill it with breakers. This gives you capacity for future expansion. The wiring rating to the subpanel must exceed the highest total current current you expect to draw in the future. So, if at some point in the future, you need 100A, you need 100A wiring installed now.

If you go with a 70A metal panel, you'll probably need to use at least #4 copper wire from the subpanel to the main switch panel (hot, neutral, and ground). Now, just because you install a 70A or greater subpanel doesn't mean you need a 70A breaker on the main panel. The key point is that in no case should the main breaker be GREATER than the maximum safe load to the branch circuits. For example, you wouldn't want a subpanel and its accompanying wiring to be rated at 70 amps, but protected with a 100 amp breaker. This would create an unsafe condition, potentially allowing an overcurrent on a branch circuit that fails to trip a breaker -- hence resulting in a fire hazard.

Whatever you do, you need to be sure that the main panel can handle the extra load you're providing in the garage.

There are too many variables to give you an exact answer without knowing all the loads and what your existing panels and wiring is like. But for what you're trying to do, it sounds like a 70A system is more than plenty.

If you think you might need 220V in the garage, I'd consider running two hot wires, neutral, and a ground. That way you can balance the loads and have 220V available for possible other applications in the future. Then, just use a dual-gang circuit breaker on your main panel, such as a dual 30-A breaker.

If you do this work yourself, read up on the local codes. For example, don't put the subpanel in the fire wall between the house and the garage. Also, respect mandated clearance requirements. And, when working with electricity, always shut it off completely before rewiring. It's so easy to make a fatal error if you're not careful.

If you want to do the job yourself, but still save some money, then it's worth hiring an electrical for an hour just to give you some pointers and confirm that that what you're doing meets code and is safe.

Happy lighting!

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