I am wiring a subpanel for a workshop. I will have a 2HP Dust Collection system on a dedicated circuit, lighting on a dedicated circuit, and two 20 amp receptacle circuits for now. I may want to expand to a 240V circuit for a 3HP table saw at some point as well, and perhaps add an air compressor.

Anyway, a 50 Amp circuit will probably be enough and I know that I should use 6-3 w/G to feed the subpanel and isolate the neutral bus bar from the ground bus bar in the subpanel.

But if I choose to go with a 60 Amp subpanel feed instead, what size wire do I use? I know 6 AWG is good to 55 Amps. Yet I have a furnace with two backup strip heaters on 60 Amp circuit breakers. Each looks like the electrician used 6-2 cable for the circuit although I can't find any writing on the sheath that I can tell for sure. And I've never seen any 4-3 NM-B cable.

I have a number of very good books on the subjecdt, but none address this issue beyond stating that I should use SER cable to feed subpanels at or above 100A and NM-B 6-3 for a 50 Amp.

Thanks in advance.

Rob

Oh, BTW, in Lowes today, I purchased a subpanel with a 125 amp rating. The guy in that department said that I was "supposed" to use cable consistent with that rating even though I plan to feed the panel with a 50 (or 60) Amp circuit. That didn't sound right at all. Am I confused or was he?

 

6/3 supports either a 50-amp or a 60-amp subpanel (using the next larger standard size rule in 240.4(B)).

The information you got at Lowes is certainly not the stupidest thing I've ever heard coming from a home improvement store, and at least it isn't dangerous, but the advice you got was both stupid and wrong anyway. He's confused. You're not.

 

Thanks John.

As an "advanced amateur," I am aware of the 240.4B rule as it was mentioned in one of my books, but I wanted to make sure I wasn't misapplying it in my case.

Rob

 

You can go ahead with the #6 cable for the load you have planned at this time. Personally I would run #2 to allow for expansion of your shop in the future up to a 100A feed. Wire's pretty cheap, saves running it over a few years down the road if your projects take off.

 

A dust collector and a 3 HP table saw may have a problem starting on only a 60 amp line. Even if you start one and wait for it to come to speed, the second may trip the main in the sub-panel. 100 amp is more usefull in a full up wodd shop.

 

I have a 3 hp and a 5 hp compressors on a 60A sub with a total of 42 run amps, it stars them just fine and the 5 is the secondary. Never have a problem, but of course bigger is always better.

 

I have a 3 HP unisaw installed on a 60 amp panel for a while. It need 45 amps to start repeatedly, and even after it was up to speed the dust collector would trip the breaker about 25% of the time. A lot depends on the motor quality. Better quality motors have a higher kVA locked rotor code and take more power to start up. The unisaw motor would be >60 HP using the 'maximum developed' method. It will also cut through 3 inches of hard maple, or even ironwood without slowing.

 

Thanks guys. I got the 6-3 cheap and will go with that for now. In my old house, I had a 40 amp subpanel, and never had a problem with the same equipment I plan to use now.

However, I will always have access to the run between the Service Entrance (Two 200 Amp panels) and my workshop subpanel, so if necessary I can replace the subpanel feed and buy a new breaker. As I stated earlier, my subpanel is rated at 125A.

Rob

 

Art. 430.63 allows the rating of the Feeder C-B to exceed the rating of the Feeder conductors where the Feeder supplies a "combination" load consisting of motors, liting fixtures, and appliances. It's possible to have a 100 amp C-B protect Feeder conductors rated at 60 amps if the Feeder is supplying motors.

The 40 amp "difference" would be the full-load current of the largest motor mutiplied by a factor of 250% (Table 430.52).

The FLC of a 240 volt, 3-HP, single-phase motor is approx 20 amps. If this motor was supplied by a Feeder with a 30 amp liting/appliance load, the rating of the C-B would be (250% X 20 )+ 30 = 80 amps. The ampacity of the Feeder would be (125% X 20) + 30 = 55 amps.

 

Where did you get a good price on the 6/3.

I think when I got about 10 feet at lowes, it was about $1.00 a foot.
Thanks, Tony D.