How many amps can my sub-panel hold?March 6, 2012 11:47 AM   Subscribe

Panel to Garage sub-panel. How many amps can my sub-panel hold? (You are not my electrician.) I will be hiring a licensed electrician, I’m just curious as to what to expect.

The main house electrical service panel is connected to a sub panel in a detached garage.

The circuit on the main panel feeding the garage has a two pole breaker labeled 30 amps.

The garage sub panel has a one 20 amp circuit and one 15 amp circuit. There is room on the panel to add two more circuits.

My questions are:

It is my understanding that the circuit to the garage can be either a single 240 volt or two 120 volt. Is my understanding correct?

If this is correct, how many amps can I add to the garage sub-panel if it’s single 240?

Alternatively,

How many amps can I add to the garage sub-panel if it’s two 120?

Finally,

If this is completely wrong, let me know.
posted by otto42 to Home & Garden (3 answers total)

It is my understanding that the circuit to the garage can be either a single 240 volt or two 120 volt. Is my understanding correct?

Not quite. The two-pole breaker in the main panel is connected to both 120V legs of the service coming into your house. Presumably the conductors from that breaker are in turn connected to the 2 sides of the sub-panel. Those 2 legs can be used to make either 120v or 240v circuits, depending on the type of breakers you install. A single-pole breaker in the sub-panel will connect to only one of those legs, and give you a 120v circuit. A double-pole breaker will connect to both legs and give you a 240v circuit. Make sense?

The breakers you put in the sub-panel shouldn't be any larger than the 30-amp breaker in the main panel that feeds it. I'm not sure what code and convention have to say about the relationship between that 30-amp breaker and the total amperage of all the breakers in the sub-panel, but from an electrical standpoint it doesn't necessarily matter. You could fill the sub-panel with 30-amp breakers and connect them all to 30-amp loads, and it would work as long as you only turned on one load at a time.
posted by jon1270 at 12:21 PM on March 6, 2012

This is all according to Canadian Code:

First while technically you could run two 120V circuits out to the garage legally you aren't allowed to with the loads you outlined and it would be foolish anyways. Barring some exceptional circumstance your electrician is going to install a 240V service fed by a four conductor wire.

Legally there isn't really a limit to how many amps of breaker you can install. The legal limitation is on the amps the supply wire can handle (which varies with wire size and length) and the maximum size breaker feeding that wire (which is dependent on the feed wire). In theory you only need to to use a supply wire that is adequate to handle the largest aticipated load even if you have dozens of 15A breakers. I've seen a garage with 16-15A circuits fed by a 40A breaker.

how many amps can I add to the garage sub-panel if it’s single 240?

This entirely depends on the wire feeding the panel. If you need to run new wire I'd plan for a wire large enough for a 40A load for an average garage. That's way more than you need at the moment but it would allow you or the next owner to run the lights/fridge and a decent size welder.
posted by Mitheral at 1:11 PM on March 6, 2012

A 2pole 30Amp breaker is 240v. So, you have 240v in the garage. The 20Amp and 15Amp breakers in the garage are only using one hot wire, thus they are 120v.

As to how many circuits a 30A breaker can feed - that depends. It is a question of load. If the entire panel pulls more than 30A, than you trip that breaker, and the whole sub-panel shuts off. But, once you have 30A over there, you can divide it up between several circuits. You can have ten 15A circuits - but that does not mean you suddenly have 150Amps. Each 15A circuit can only pull 15A, then it will trip and turn off that one circuit. If all ten circuits pull more than 30A combined, then you will trip the 30A circuit and turn off the entire sub-panel.

Generally, electricians do load calculations to determine how many circuits can reasonably be supported by by a main breaker. It is a complex calculation.

If you want more power to the garage, you need to run new wire. There is a 30A breaker because the wire is 10gauge. 10 wire can hangle 30Amps. Over 30Amps can cause 10gauge wire to melt and become a fire hazard. If you want 40Amps in the garage, you need to replace the wire feeding the garage with 8 gauge wire. Replacing the feeder wire to the garage might be very difficult.
posted by Flood at 1:48 PM on March 6, 2012

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