Sub-Sub Panel - 30amps/#10NM or more?

[madosta]

So I upgraded my little Siemens Main Lug Panel to a Main Breaker Panel and have the old main lug panel that I'm thinking about putting across the garage from the main panel. 40 feet or so - this would give me more options for plugs and a welder plug on that side of the garage.

Now, I only have 60amps to the main panel, so I was thinking rather than pull #8THHN or #6NM that I would pull a #10-3NM-B and protect it at 30amps.

It will only have occasional use with a welder plug (30amps) and some lights maybe over the work bench if I keep the workbench there and a 4 or so 20amp recepts circuits.

Anything wrong with that?

 

[Aceman]

Sounds like a good idea to me. It's always easier to run one cable to an area for power than several separate wires. Plus it puts the necessary breakers right where you are working.

Depending on how many loads are running simultaneously you might need to go up a size or two.

 

[madosta]

Eh it just so happens that I'll be able to run 3/4 to it easily. Wasn't sure I could but I'll probably pull some #8. Thanks.

Sent from my Nexus 5 using Tapatalk

 

[slimpickins]

Sounds like your main service might be on the small side here. Disclaimer: I'm not an electrician, so you need to talk to one...

But for what its worth:
If you pull a #10-3 (which is rated at 30 amps) to a sub panel, and you run a welder at 30 amps off the sub panel, you won't be able to run anything else at the same time on the sub panel because it will trip the feeder breaker in your main panel.

In my area, there are no "rules" regarding the size of sub panels, (check with an electrician for your local code) however, you have to understand that your sub panel feeder has to handle the TOTAL load that you expect to put on the line at any given time. i.e. if you want to run your welder at 30 amps, and a few lights (so you can see what you're doing) and perhaps have an air compressor plugged in that might cut in. You add up all of the loads that you anticipate will be drawing power at the same time, and your feeder to the sub panel should be the next size above that. i.e. if you calculate your total potential load as 38 amps (at 220V) then you should run AT LEAST a 40 amp sub panel. Local codes may say you need to go to a 60 amp.

Keep in mind that your sub panel rating is "at 220V" and any loads at 110V are added at half their actual rating. (A 30 amp 220V sub panel, gives you 60 amps at 110V ... 30 amps on each of the 2 110 v. legs.)

When you are running sub panels, its easy to wire these in such a way that you will trip that sub feeder breaker in your main panel, because you're exceeding the rating of one leg of the sub feeder.

If you do not completely understand what I've written here, then you need to hire an electrician.

 

[jontar]

I did this in Alberta Canada, I ran a #2/3c with gnd Teck cable from my 32cct Culter Hammer Main Panel in the House, using a 100amp 2pole breaker, to my garage 125amp 16cct Culter Hammer Sub-Panel (seperate neutral and Gnd lugs). Then went to my Garden Shed witha 30amp 2pole breaker and a #10/3c with gnd. to a 4cct Square D.

The Shed only has a few keyless lights and (1) inside 15amp plug and (1) 15amp outside GFI and (1) 20 amp outside GFI and then finally a 220v 20amp outside plug for using the table saw on that side of the yard.

Typically on small garage /shed services the inspectors and the CEC let you get away with a lot compared to commerical and industrial because typically there is only one guy at a time using the service, and it verys hard for him to use more then one thing at a time. Most inspectors around here would allow a small 30a welder to be serviced out of the small panel with the #10 feeder. I personally would pull over an #8 and put it on a 50amp breaker, but aslong as the #10 is protected properly (and isn't a long run) a 30amp breaker you should be fine.

As far as the CEC is concerned, since your largest load would be the welder (and it has a duty cycle) you are allowed to over size the breaker, I don't think you should if your not an electrician, and can't do a service calculation.

A service calculation, btw isn't adding up the appliances at face value as done above either, otherwise my small shed would require 50 amps on one phase and 40 amps on the other not including the lighting load...

 

[jontar]

The NEC just like the CEC has a seperate section to deal with Welders, just as it does Motors and other devices. Since they are covered in seperate sections with seperate rules, they are very hard for a home owner to use proper and apply.

Typically most home owner welders would have a duty cycle below 50% and most likely either 30% ,20% , 10%, I won't tell you at what valve you can apply the duty cycle to your service, but it isn't a 100% of your welders rated current.