Replacing and Adding Subpanels

Old 09-23-04, 09:45 AM
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Replacing and Adding Subpanels

I recently built a shed in the back yard and now need to run power out to it.

The old workshop, in the house basement, has a dedicated subpanel with two 20 amp breakers. There is no room for additional breakers. The subpanel is being served by 8/3 Romex that is protected by a 40 amp 2 pole breaker at the main box. Is it safe for me to assume that I have 240 volts in the subpanel?

I'd like to tap into this subpanel to run power to the shed. It eliminates needing to snake feeder wire through the house.

Ideally, I'd like to run 240 volts out to the shed. I don't have the need for 240 volts right now, but I'd like to keep my options open down the road.

Assuming I have 240 volts in the subpanel, would I simply need to replace that subpanel with one that will accept the two 20 amp breakers that are currently hooked up and a new 40 amp 2 pole breaker to protect the line going to the shed? Is it also safe to assume that I can then run new 8/3 feeder to the shed giving me 240 volts?
Old 09-23-04, 07:11 PM
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If the information you provided is correct, you could replace the workshop sub-panel with a larger one (you need 4 breaker slots). It sounds like you do indeed have 240 volt. There are lots of threads on installing a new sub-panel. Maybe you can do a search? You could certainty run 8/3 cable to the shed, but it would need to be rated for direct burial (UF cable), if that's the route you plan to take. However, given the current load on the original 40 amp circuit, you would only be able to supply the shed with 20 amps at 240 volt, and so would need to be protected by a double 20 amp breaker at the workshop panel. This may not meet the requirements of the new panel in the shed.

How far away from the workshop sub-panel is the shed? You can still use 8/3 cable for the run, but it maybe a little over kill, For 20 amps, 12/3 would be sufficient if the run is less than 100 feet. I would split the difference and run 10/3 to the shed. This would give you some protection from voltage drop.

If you really need 40 amps at 240 volt at the shed, unfortunately, you would need to either install at least 8/3 cable all the way back to the main panel (again if total run is less than 100 feet), or update the workshop panel to at least a 60 amps feed using 6/3 cable from the main panel.

Hope this helps.
Old 09-23-04, 07:48 PM
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You are providing power to a shed from a 8/3 feeder which gives you a maximum of 9.6KW, of which up to half of that is already used in the existing subpanel, leaving not much for the shed. Even though it is easier to run the feeder to your shop from the subpanel, it might be much wiser to run it from the house panel...unless running a hand drill and a light bulb is all you ever plan to use the shed for.

To understand whether your plan is merely a poor idea, or perhaps a really bad idea, we need to know more about what kind of electrical stuff you plan to have in your shed.

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