Sub-panel in garage.

[browntown]

My main panel is in the garage. There is a 240v circuit in the main panel running in conduit to what used to be a hot tub. The conduit ran inside the garage, then through the walls, then under the house, to outside the house to a cheap disconnect box.

I'm keeping the conduit inside the garage and pulled the rest of the cable inside the garage. The idea is that the 3/4" grey pvc conduit will terminate at a sub-panel, I'll be putting in. (I've noted all the warnings about not bonding neutral to ground bars in the sub-panel)

Within that subpanel, I'd like to have two 240v circuits. One that will run flex conduit to a hard wired compressor, the other to a 240v outlet for plugging in the drill press, or switching out to a welder, etc.

The current breaker in the main panel feeding this wire is 50A. The wire doesn't specify gauge, but if I had to guess it is 8/3wGrd (two hots, one neutral, one bare ground).

Is this wire thick enough to switch out the main panel breaker to 60A, and then have two 30A breakers at the subpanel or one 40A, one 20A? Or is 50A at the top of the wire's capacity?

For reference here are a couple pics of the existing setup, the 50a breaker, the conduit running along the inside wall of the garage, and a picture of the wire.

The hot conductors are 1/4" in diameter (this is including insulation) and have 7 strands of copper wire in each. The outer sheathing is black. I think that makes this 8/3 w/ground right? I don't want to buy and fish in new wire. Should I keep the 50A main breaker and just put 30a & 20a breakers in the subpanel? My air compressor wont be over 5hp (haven't bought it yet), and my drill press motor is 1.5hp with its own capacitors and can run on 125v if needed.

 

[Tom in Seattle]

That wire looks exactly like the 6/3 wire I ran for my 50A double ovens. The black sheathing should specify the gauge and type... it may have faded... mine was pretty light. If it is 6/3 it is good for a 50A breaker like you have installed, if it is 8/3, it is good for a 40A breaker.

That doesn't mean you are limited to a 30A + 20A circuit in the remote panel. You can have more circuits or larger circuits that add up to more than the breaker size because the neutral currents won't always be additive, and that is what trips the breaker. For example, add up all the circuits in your main panel and the total will be much higher than the main breaker size... yet it still all works because not every circuit is loaded at once.

 

[Highbeam]

6/3 is good for a 60 amp breaker.

 

[browntown]

OK, i searched about 10 feet of the black wire and was finally able to read the markings. It is definitely copper Romex 8/3 with ground NM-B.

So it sounds like that 50A breaker at the main panel is pushing it. Should I step down to a 40A breaker at the main (or is the 50 there because of the 125% rule) ? It's a 25 foot run or there abouts. If I have a 40a breaker on the main, can I have two 30a circuits on the sub-panel as long as I don't pull the trigger on the mig welder at the same time the compressor starts up?

 

[Aceman]

I don't like it.

A 5HP compressor by the book draws 28 amps at 240v, just by itself. With #8 Romex feeding a sub you're looking at a 40 amp breaker to feed the whole panel.

That's too tight by my book, you need bigger wire. Saying you won't pull the trigger on the MIG while the compressor is running doesn't cut it. Legally, you need a 35 amp breaker minimum just to feed a 5HP compressor, so you can see there is hardly any room left over for other equipment.

 

[pattenp]

Yep.. 40A breaker on #8 cu. That's it.

 

[pattenp]

If you up it to #6CU and want to reuse the 3/4" conduit don't pull NM or SE cable, use single stranded THHN/THWN wire. Four #6's is the max you can put in 3/4" conduit

 

[Tom in Seattle]

6/3 is good for a 60 amp breaker.

Could someone confirm this please. I thought #8 NM-B was good for 40A and #6 for 55A, which means a 50A breaker.

 

[browntown]

Well OK, I want to be safe, and am not prepared to replace the wire at this time, so I'll replace the breaker at the main with a 40A instead of the 50a someone put in before me.

Then i'll terminate the conduit with a twist lock receptacle in a box. That way I can keep the compressor plugged in, and if I want to weld, I can unplug it and use the feed.

Any recommendations for a outlet that can handle a 5hp motor and my planned purchase of a mig machine (miller 211 I think), and can terminate those fat 8 gauge wires directly?

Should I get a 30A or a 50A outlet?

edit, I think I'll still do the sub-panel. That way I can have a 30A plug in 240v, and a 15A power strip in 120.

 

[pattenp]

#6 CU is rated @ 55A. It's my understanding it's acceptable to push the breaker to the next common breaker size which is 60A, but common practice is to use a 50A breaker.

Could someone confirm this please. I thought #8 NM-B was good for 40A and #6 for 55A, which means a 50A breaker.

 

[browntown]

So, I cashed in some HomeDepot gift cards I had lying around.

Since my main panel is Siemens, and there are multiple threads around here espousing the professionalism of same branded sub panels, I bought a 125A, 4 space Siemens Hecho in Mexico sub-panel. I couldn't find any copper bus panels at my HD that weren't full size boxes. Al is second choice, but it'll do at $15.

I also got the twist lock 30a 250v outlet and a box/plate that fit it. I'll breaker that 250v outlet at 30A, and that will size both my air compressor and welder purchases in the future.

I'll also put a couple 120v circuits in. One dedicated for the drill press at 15A, and one 15A for an outlet strip.

I'll post up pics once I get it all mounted.

Two quick questions:
- should I just use a pocket knife to strip 8 gauge wire, or is there a tool that'll handle big wire that I need?

- how do you run 3/4" conduit into a knockout on the box? With flexi-cable you use those clamps to offer strain relief, but with conduit do you need strain relief? Or do you just put a male adapter at the end of the plastic pipe and a knockout nut to tighten it inside the box?

- where's norcal to tell me I'm doing it wrong?

 

[pattenp]

Use this to connect conduit to panel:

 

[browntown]

That's what I thought, just wasn't sure if the cable needed clamping when using conduit. Sounds like that means no. Cool, that makes life easier. I should have things put together by the weekend and I'll post a picture or two. Thanks for all the help.

 

[pattenp]

Use a nut like this on the male ****** to fasten to panel.

And add a PVC bushing on the end:

 

[browntown]

OK, no one said I was quick at getting my projects done. But I finally got her done today. I'm sure it looks bush-league to anyone that does this kind of thing professionally, but there was already a 3/4" plastic conduit run, so I bought a box and put in 3 circuits.

Main box to sub panel is 8/3+G stranded
Sub panel to plugmold hard-wired wire strip 12/2+G solid (yellow cerrowire) 15A 125v
Sub panel to duplex receptacle, switched by industrial switch, 12/2+G solid (yellow cerrowire) 15A 125v
Sub panel to air compressor plug 10/2+G solid (orange cerrowire), receptacle is l6-30r twist lock. I plan on mainly using this for a compressor, but have the ability to unplug the compressor and plug in a smallish mig welder.

 

[Mmfh]

Just taking in consideration the way everything looks, I'd say you did a pretty good job!
I'm sure it will be nice to have that power over the workbench. Now go buy a compressor and show us what you bought

 

[Tom in Seattle]

The Plugmold is probably only rated for 15A, but since you used 12/2 cable how come you didn't install a 20A breaker and receptacle for the switched outlet?

A minor point but you may want to add some black tape to the white wire for the 30A load to indicate it's hot and not a neutral.

 

[browntown]

Thanks Tom, I'll mark that wire, I remember reading that somewhere. I kept the two smaller circuits at 15 amps because the main feed is 8 gauge wire going to a 40amp breaker at the main panel. So the sub panel is at 60amp max (15+15+30) and thought it would be a good idea to keep things low rated as I was pushing that 8 gauge wire.

 

[Speedy Petey]

I kept the two smaller circuits at 15 amps because the main feed is 8 gauge wire going to a 40amp breaker at the main panel. So the sub panel is at 60amp max (15+15+30) and thought it would be a good idea to keep things low rated as I was pushing that 8 gauge wire.

Just so you know, that's not nearly how it works. The rating of the breakers is completely meaningless. Besides, the two 15's a 120v and the 30 and 60A feeder are both 240v. So in this case 15+15+30 does NOT = 60.

I would have also used 20A breaker just so you have the full 20A circuit available on each if you need it.
Remember, regardless of what a circuit is rated, the draw is only the load you place on it.
It's like having tires rated for 150 mph. Typically you're only doing 70 or less. 150 is just the rating.

 

[browntown]

OK good to know I can upgrade it if I need to. That receptacle is really just for the craftsman drill press I'm restoring. The original motor was toast, and I have a 1.5 horse one that I plan on hooking up that is rated to draw less than 15 amps. Even on startup (it has big capacitors hanging off the side which I think help), when I bench tested it, it didn't throw a 15a breaker. Breakers are cheap enough though, I'll grab a 20 next time I'm at the hardware store.

As for the math, I'm very upfront about my less than complete knowledge of electricity. Since the feeder line goes to a 40A breaker in the main panel, I guess the dangers of overloading the feeder wire are protected by that breaker.

Thanks for the help guys.