Electricians advice needed

The determining factor is the breaker feeding the sub-panel

You can if you put it on a 40 amp breaker --- You DO need a feed breaker for the panel.

Don't you meed to run four wires?

It will be overkill if you are installing a 120/240 2/4 slot panel. That is 40a per phase as Clare said but you do need 3 wire plus ground (4 wires total) as Dean said. GE does make that 2/4 panel and I have seen it at about $14 each plus

So you're saying a 40A feed breaker and the sub panel can have 2 20A breakers?

Yes if you are feeding it with a 2 pole 40 (240v) and you are using single pole branch circuit breakers (four 120v circuits)

If in the US you will need 4 wires, 2 hot, 1 neutral, 1 ground. You put in a breaker in the main panel and that determins the size of the wire you will need to use. At the sub panel you could most likely install 10 of the 20 amp breakers if you wanted to. You would not be able to use all of them at that current as the brakere at the main would trip first. Similar to most houses. If you total up all the breakers most likely they will be larger than the main breaker. Not many are going to use everything at once.

It can have 4. Look at your common 100 amp breaker panel - room for 32 breakers - even at 15 amps each, that's WAY over 100 amps. It's over 200.

Even if it's only an 18 slot panel - the range and drier alone acount fot 90 amps??? Add cental air for another 20.

This looks like what the OP wants if he only wants 4 circuits It is 3R rated but you can use it inside.

That's an odd thing on the terminology. We deal with 3 phase 480. The underground cable we bury is called quad. One wire is marked as the ground from the factory. #4 has all the wires the same size. Cable larger than that has the ground one size smaller than the other three.

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It's leftover from the pre required-grounding days in the Code when there was mostly (say) 14/2 or whatever and then there was 14/2 w/g which, of course, has three wires in the cable but couldn't call that

If ground had been required from the git-go, common nomenclature would likely have been different and just counted the total number...

Thanks...that was my assumption, but I wanted "clare"ification!

You do need to consider what the total load of the panel will be at any given time. Will you be running machines constantly or intermittently? You do not have to total up the rating of the individual circuit breakers.

You need four wires for a sub-panel and I think a ground rod for lightning protection is a good idea.

John Grabowski

Thanks John, besides a circuit for fluorescent and LED lighting...there will be only one (table saw, miter saw, grinder, etc.) used at a time.

If you do run that #6 in pretty much anything but Romex, (limited to the 60c column) you can hang a 60a breaker on it and put in a pretty nice sub panel. THHN in pipe would be an example of a wiring method that would get you into the 75c column (65a). At that point I would be looking at a 6 or 8 slot panel for a few bucks more than that 2/4 GE I linked the other day.

always better to go BIG when planning electrical upgrades. while up sizing may cost a bit more, its cheaper than upgrading later

Do not use a ground rod with the subpanel; aside from being a NEC code violation, it can induce current in the grounding conductor. There should only be one grounding point in the system, and that should be at the service entrance.