Can someone check my calcs?

[Gib57]

Hi all,

This is my first post here, so a little bit of background: I just purchased and moved into a small (900 sf) old (1943) beach bungalow about 6 weeks ago. I decided that the detached garage will be top priority before any inside work in the house, and replacing the old knob-and-tube wiring is top of that list. The house has a 100A main, and the garage was fed by a single pair of 12AWG in 1/2" conduit, feeding one lightbulb and a socket for the door opener

My plan is to install a sub-panel in the garage, fed from a new 50A breaker at the house, in new conduit. If I'm reading my charts (as well as the numerous similar posts here) correctly, I should be able to pull (3) #8AWG and (1) #6AWG ground, all THWN, in 1" pipe to do this. Overall pipe run will be about 60', with no more than (2) 90° bends between any two pulling elbows.

Does this sound reasonable, or should I be considering sizing up my conductors and/or pipe?

Thanks,
--Mike

 

[pattenp]

The equipment ground can be a #10. You will need ground rods at the garage and you can use a bare #6 for the electrode conductor.

 

[ddawg16]

Sounds pretty much like my setup....wire sizes are about right....though, I could see some inspectors giving you grief on the #8,....they may have NM cable in their mind and want #6 (which is what I have)

You might want to consider going to 6awg wire because of the distance....and just in case you get carried away with your man cave....(we all do)

The ground (#10) can be a bare wire....and will actually make the pull just a little easier.

You can't have more than 6 breakers in your sub panel unless you have a main breaker in the sub panel.

Like Pattenp said....2 ground rods.....confirm the distance...NEC I believe is 6'....some cities require 8'....

Do not bond the ground to neutral bus....

If you have the garage I think you do...it's almost as big as your house....

Make sure you post pics.....

And....welcome to GJ

 

[Gib57]

Wow, thanks for the quick reply! Just out of interest, why is more than one ground rod required? The cold water supply runs through the garage before heading underground toward the house; could I use that in lieu of one of the ground rods? The main is already grounded to that pipe at the other end.

 

[pattenp]

The #8 in THWN is fine for 50A at 60ft. If an inspector has NM on their mind for a run in conduit under ground then that inspector should get a different job. I'd stick with all the conductors being THHN/THWN. You can get them as 2 black,1 white, and 1 green.

Edit: You may be able to use the pipe for the earth ground. I can't remember the actual requirements when using the pipe.

 

[Charles (in GA)]

do yourself a favor and pull three #6 (red/black/white) and a #10 green. Air compressor startup, even a small portable one will draw a lot of current and this will help prevent issues.

Charles

 

[Gib57]

Yeah, that's where I was kinda on the fence- the 1" pipe size will be OK, but I'm leaning toward the #6 so there just won't be a problem.

 

[Charles (in GA)]

Buy a decent size panel, at least 16 or so breaker slots. Sounds like a lot but you will use two for a backfed main breaker, and then you have an door opener (I like them on their own circuit) and then two or three receptacle circuits, possibly a 240v for a compressor or small welder, a circuit for outdoor lights and a couple of circuits for inside lights....... Thats eleven slots right there. Breakers are cheap, panels are really fairly cheap, it just don't make sense to go small, even if it is only a one bay garage.

Charles

 

[Norcal]

Buy a decent size panel, at least 16 or so breaker slots. Sounds like a lot but you will use two for a backfed main breaker, and then you have an door opener (I like them on their own circuit) and then two or three receptacle circuits, possibly a 240v for a compressor or small welder, a circuit for outdoor lights and a couple of circuits for inside lights....... Thats eleven slots right there. Breakers are cheap, panels are really fairly cheap, it just don't make sense to go small, even if it is only a one bay garage.

Charles

A backfed breaker requires a holddown / retainer kit. Just a reminder. Putting in a 2/4/6/8 space loadcenter is a waste in most cases, IMHO, a 16-24 space panel is a great idea, but I hate twin breakers so a bigger panel is a must.

 

[Charles (in GA)]

A backfed breaker requires a holddown / retainer kit. Just a reminder. Putting in a 2/4/6/8 space loadcenter is a waste in most cases, IMHO, a 16-24 space panel is a great idea, but I hate twin breakers so a bigger panel is a must.

I generally try to mention the hold down kit, forgot this time. I too hate small panels, they are hard to work in, and usually end up not having enough space. 16 is a minimum for a small single bay for me, I agree, 24 is even better. I installed a full 40 space in my shop and don't regret having space to work in the panel.

You will find the backfed main breaker hold down kits for GE, Siemens, and Square D in Home Depot and Lowes hanging on the pegs just above the breakers, in small bags or clear packs, along with the breaker knockout filler plugs, the small ground bars, the add on lugs for oversized ground and neutrals, etc.

Charles

 

[pattenp]

Don't mess with a backfed breaker as a main. You should just get a 20 space 100A main breaker panel. The 100A breaker just serves as the main disconnect. The panel is about $55. Also the #6 THHN/THWN gives you up to 70A.

 

[madosta]

This forum is awesome. I am now considering upgrading my 8 slot 50amp sub panel (60 amp breaker in the house with #6 copper). I am happy so far, but yes, run #6 if you are a man.

 

[wyliesdiesels]

Sounds pretty much like my setup....wire sizes are about right....though, I could see some inspectors giving you grief on the #8,....they may have NM cable in their mind and want #6 (which is what I have)

You might want to consider going to 6awg wire because of the distance....and just in case you get carried away with your man cave....(we all do)

The ground (#10) can be a bare wire....and will actually make the pull just a little easier.

You can't have more than 6 breakers in your sub panel unless you have a main breaker in the sub panel.

Like Pattenp said....2 ground rods.....confirm the distance...NEC I believe is 6'....some cities require 8'....

Do not bond the ground to neutral bus....

If you have the garage I think you do...it's almost as big as your house....

Make sure you post pics.....

And....welcome to GJ

NM is NOT rated for use in conduit that's in wet locations. And #8 90C deg conductors would be fine @ 60', accounting for voltage drop, but if you wanted more headroom, then u could go with #6!

 

[pattenp]

NM is NOT rated for use in conduit. And #8 would be fine @ 60' but if you wanted more headroom, then u could go with #6!

To clarify, NM can be used in conduit, just not in wet locations as in outside and or underground. Not sure which #8 you meant, but #8 NM is good to 40A because of the requirement to be sized based on 60C degrees. Using singles of #8 THHN/THWN in conduit is good for 50A because it can be sized at 75C degrees.

 

[ddawg16]

NM is NOT rated for use in conduit. And #8 would be fine @ 60' but if you wanted more headroom, then u could go with #6!

I used THWN.....while it 'might' be ok to pull NM throgh conduit....it's a ***** to do on long runs....

I was just saying that the inspector may not know that the THWN has a higher current capability than NM...since most of them are used to NM cable...

 

[wyliesdiesels]

To clarify, NM can be used in conduit, just not in wet locations as in outside and or underground. Not sure which #8 you meant, but #8 NM is good to 40A because of the requirement to be sized based on 60C degrees. Using singles of #8 THHN/THWN in conduit is good for 50A because it can be sized at 75C degrees.

Yes, that's what I meant- no NM in conduit in "wet locations". And yes I was referring to 90C deg. conductors(THHN) sized @ 75C deg. ampacity!

I was in a hurry and didn't have time to type the details! Next time I will go into detail to cut down on confusion(I will edit my previous post, too).

 

[Gib57]

Thanks for all the replies and the subsequent discussion that it spawned, guys. Since I originally posted a few days ago, I've made some progress. I picked up #6 THHN/THWN from the local supply house (.50/ft vs north of a buck @ H Depot!), picked up conduit (but not all the fittings), and worst of all, dug about 20' of trench between the house and garage. My professional opinion is that trenching is a good job for somebody else to do. Oh well, too late now.

As for my subpanel, I've already installed an 8-slot sub with isolated neutral bar. However, based on the advice given a few posts back, I'm re-thinking that. A back-fed main and a single 240V circuit will consume 4 of those positions, leaving me a total of 8 more circuits if I use tandem breakers. 2 plug circuits, 1 lights, 1 garage door could be done, but I guess it's cutting it close. It's a simple 2-car garage, I do some woodworking but no heavy automotive work, and as someone pointed out in another thread somewhere, you can have all the tools in the world, but one guy can only use one at a time. It's all open studs right now, and my intent is to wire it up, insulate and drywall, and be done with it. On the other hand, as an engineer, I've learned that about an hour after you commit to "that should be all we'll ever need" something new comes up. I think I already know what I need to do, but I'm always open to opinions. (Also, I do have photos, but I have to resize them for upload)