Sub-panel questions

[AUTX]

I'm working through a design to add some stuff to my garage. I want to add circuits for these loads:

• Air Compressor (220V, 30A breaker planned)
• Welder (220V, 50A breaker planned)
• RV recep (110V, 30A breaker planned)
• Garage lighting (110V, 15A breaker planned)
• Garage receps (110V, 15A breaker planned)
• Master bathroom dedicated GFCI recep (110V, 15A breaker planned)

The master bathroom recep is required by SWMBO to be allowed to do this project.

I have a 200A Challenger SB20 (125A max per stab) load center that's partially filled, and if I jockey everything around, I can just fit in all the new breakers with no slots to spare. That doesn't sound like a good idea, so I want to add a sub-panel. The main load center is in the garage, so the sub-panel would be right next to the load center.

The load center currently has 240A worth of breakers on one leg, and 270 on the other. I'm thinking I need at least a 80A sub-panel, if not a 100A?

The garage has an attic above it, but the top plate for that wall is wedged way out into the corner of the roof. Also, the top plate directly over the load center looks like it's slap full. I don't think I can get in to drill the top plate for any more circuits, so I want to surface mount the sub-panel and run conduit/THHN on the walls for everything in the garage. Also, I really hate doing drywall. I plan to put a j-box on the ceiling to split out to all the fluorescent lights, and I'll use that box to get out to romex in the attic for the master bathroom circuit. That romex will have to go back into the top plate over the bathroom, but I think that's doable.

Questions:

1. Can I buy a flush-mount sub-panel (they already have lots of knockouts, which seems easier) or does it have to be surface-mount specific?
2. I've read that I need to put a plywood plate between the sub-panel and the drywall. Is this really necessary? I can't use drywall anchors or some kind of brackets to mount the sub-panel? If it's spaced off the thickness of the plywood, am I going to have to bend joggles into all the conduit if I use conventional conduit hangers?
3. What size breaker should I be using to feed the sub-panel? I'm aware that it needs to be less than or equal to the sub-panel rating, but is it personal preference after that? I'm probably going to leave the compressor breaker off most of the time because I don't want to listen to it and I don't want it to open if I want to cook and run the dryer at the same time (gas heat and water heater in the house though, so I got that going for me).
4. What do I need to be feeding the sub-panel with? I don't want to mess with aluminum. Can I run some jacketed cable with 3 conductors and a ground out of the load center, through a stud, and into the back of the sub-panel if I use clamps at both panels? That's it?
5. Any advice on conduit sizing, fill, derating, etc? I'm grinding through the NEC calcs but they're mind-bending.
6. Any advice on safely and inexpensively putting a receptacle in a drawer for a hair dryer?

I think you can tell by now that I'm not an electrician. I think if I had to get this done by a pro, I couldn't afford it, and I'd give up. I am reading the hell out of the NEC, and I plan to pull a permit but I don't know how paranoid I should be about inspectors for homeowner jobs. I am building a PowerPoint of the whole project and plan to run it by the inspector before I start.

TIA!

 

[wyliesdiesels]

Way too much to read.

Im guessing several of your questions have already been answered in other threads.

Have you tried the search feature?

Motor circuits are sized based on motor HP. Do you have a conpressor?

 

[Bert_]

Questions:

1. Can I buy a flush-mount sub-panel (they already have lots of knockouts, which seems easier) or does it have to be surface-mount specific?
2. I've read that I need to put a plywood plate between the sub-panel and the drywall. Is this really necessary? I can't use drywall anchors or some kind of brackets to mount the sub-panel? If it's spaced off the thickness of the plywood, am I going to have to bend joggles into all the conduit if I use conventional conduit hangers?
3. What size breaker should I be using to feed the sub-panel? I'm aware that it needs to be less than or equal to the sub-panel rating, but is it personal preference after that? I'm probably going to leave the compressor breaker off most of the time because I don't want to listen to it and I don't want it to open if I want to cook and run the dryer at the same time (gas heat and water heater in the house though, so I got that going for me).
4. What do I need to be feeding the sub-panel with? I don't want to mess with aluminum. Can I run some jacketed cable with 3 conductors and a ground out of the load center, through a stud, and into the back of the sub-panel if I use clamps at both panels? That's it?
5. Any advice on conduit sizing, fill, derating, etc? I'm grinding through the NEC calcs but they're mind-bending.
6. Any advice on safely and inexpensively putting a receptacle in a drawer for a hair dryer?

I think you can tell by now that I'm not an electrician. I think if I had to get this done by a pro, I couldn't afford it, and I'd give up. I am reading the hell out of the NEC, and I plan to pull a permit but I don't know how paranoid I should be about inspectors for homeowner jobs. I am building a PowerPoint of the whole project and plan to run it by the inspector before I start.

TIA!

1- many residentail panels are the same for both flush and surface

2-Nothing says you have to use plywood, but I would not use just sheetrock anchors for a panel. I would want both panel and conduit straps to hit something solid.

3-We need more info to tell you a feeder size. You listed breaker sizes but that is mostly meaningless, tell us your actual loads. Ex. 5hp air compressor, (10) 100w lights, ect.

4-NM (romex) or SER cable would both be options. Can't recommend a size because we don't know your load.

5-Not really sure what to tell you here. You can run multiple circuits in one conduit but 9 current carrying conductors is usually the magic number with THHN before you have use a bigger wire for a given breaker. Conduit fill is pretty easy if you look back in chapter 9.

6-Personally I wouldn't put an outlet in a drawer, I'm sure it's been done though. I also would want that hairdryer unplugged after use.

 

[TRWham]

I just installed a sub next to my main and have done this before in another house, not to mention we have done many for clients (I manage projects for a GC), but those were installed by our electricians. I think you'll find most main lug panels can be mounted flush or surface but you might need a different cover. Different brand panels require modification to isolate the ground for a sub, while others come isolated (my Cutler Hammer CH panel was already isolated).

I would run 12 AWG for the garage circuits and put them on one or more 20A circuits. Power tools sometimes need more grunt. We moved enough existing circuits out of the main to allow the 240V compressor and welder circuits to come from the main rather than the sub. I have not run additional 120V outlet or lighting circuits just yet, but they will come from the sub.

Forget the copper feeder. At the size you will need the cable will cost $5 a foot or more versus a buck or so and it is hard to find (unless you are running THHN in conduit as the feeder) because electricians don't use it very regularly. There is nothing wrong with aluminum conductors in heavier gages. I ran 2224 AL SER for mine with a 90A breaker. It is probably good for 100A but there is disagreement about that so I stuck a 90 on it and it will be fine.

I am not sure about feeding through the back of the sub. That might work, but you need to be sure you can make the bend into the panel with heavy cable. I flush mounted and bottom fed mine (flipped upside down so the lugs were on the bottom), but that worked better for my application because it allowed me to intercept some circuits in that bay I wanted to move into the sub anyway. I'll just pay one of our crews to repair the drywall.

 

[mike93lx]

Adding up your breakers does not tell you what size subpanel you need other than number of slots.

Can you use tandems in your panel? That would save you a ton of time, effort and money.

 

[pattenp]

Being that the subpanel is right next to the main just use a 100A main lug panel and run a short piece of 3-3-3-5 copper SER cable. That will give you a 100A, running any size smaller isn't going to save you much money at all.

 

[TRWham]

...
Can you use tandems in your panel? That would save you a ton of time, effort and money.

Until you run out of circuits. That is one reason I did our sub-panel. My main has 30 spaces, with the option of 12 tandems in the bottom, but you can run out of lugs for neutrals. The way that panel is laid out there really is no room to add bars and it was hard to work with it so crowded anyway. The installing electrician had stacked each neutral with its ground (common in 1972 but I'm not sure it was allowed even then). I singled up all the neutrals by stacking grounds to make space (CH allows up to 3 grounds on one lug depending on wire size) and moving 14 circuits to the new panel.

 

[mike93lx]

Until you run out of circuits. That is one reason I did our sub-panel. My main has 30 spaces, with the option of 12 tandems in the bottom, but you can run out of lugs for neutrals. The way that panel is laid out there really is no room to add bars and it was hard to work with it so crowded anyway. The installing electrician had stacked each neutral with its ground (common in 1972 but I'm not sure it was allowed even then). I singled up all the neutrals by stacking grounds to make space (CH allows up to 3 grounds on one lug depending on wire size) and moving 14 circuits to the new panel.

Sure, but expansion will naturally slow or stop at some point...there are only so many circuits needed. I would run tandems if it makes sense.

 

[TRWham]

Sure, but expansion will naturally slow or stop at some point...there are only so many circuits needed. I would run tandems if it makes sense.

I agree completely, just pointing out that they don't always solve the problem. I relocated my tandems to the new panel. It won't be anywhere near full, but there's no reason to replace good breakers.