Running a subpanel to new garage

[nola mike]

Just putting the finishing touches on a new garage, and need to wire it. It's 50' from the main panel. Ran conduit about 5" below the new fill dirt that was placed in between. Right now I don't need much power at all (basically garage door opener and a few lights). However, I'd like to future proof it in case I want to run a few 220v tools, or put a TV/stereo/mini fridge on the rooftop deck
I'm planning on running a 100amp subpanel out there, which is overkill.
1. What gauge/type wire do I need? Was thinking AWG 2 THWN, though I'm not sure it needs "W" if running through conduit?
2. I need GFCI protection, but don't think that I need AFCI. Do they make 100 amp GFCI or combo AFCI/GFCI breakers? If so, I haven't seen them.
Anything else I need to consider?

 

[DougA]

I just finished doing one for my daughter. 60 amp sub panel should be more than enough for what you are describing. You need to make sure there are 4 open slots on your existing panel for the new 60 amp breaker to go in. Then, you need to remember that if you have a 200 amp service, 200 amp is all you have including your sub panel. So, if you add 60 on the sub, you only can have 140 on the main breaker. Add the numbers to make sure.

There are lots of on-line calculators for determining wire gauge. I used 6 gauge and I think that should be fine for 60 amp/50' In my case ( I think yours too) that means four 6 gauge wires (hot/hot/neutral/ground). Not sure IF you might be able to install a ground rod at the garage and have that legal. Not sure it will save much though.

I would buy GFCI's for the outlets as that is much cheaper than using GFCI breakers - at least in Canada. You can string multiple outlets AFTER the GFCI and they become GFCI but in a garage, you likely will be running tools that will need most of a full 15 amp. Better bet: I had to go with 20 amp GFCI's because code here dictates that a kitchen must have split 15's or a 20 amp GFCI for a wall outlet. They don't sell split 15 GFCI's here. the 20amp solution was far cheaper in the end but that dictates you must run 12/2 to the outlets instead of 14. If it's exposed, you'll need armourclad wire or steel conduit.

Last point, in our area, running a conduit 5" below surface in not allowed - just so you know. I'm sure your codes will be different and you should look them up on line.

 

[semipro]

There are quite a few "subpanel" threads in this forum so you may want to seek them our and read if haven't already.
Unless I'm mistaken your subpanel will require a floating ground by code. This is something that many DIYers miss.

 

[jharkin]

I just finished doing one for my daughter. 60 amp sub panel should be more than enough for what you are describing. You need to make sure there are 4 open slots on your existing panel for the new 60 amp breaker to go in. Then, you need to remember that if you have a 200 amp service, 200 amp is all you have including your sub panel. So, if you add 60 on the sub, you only can have 140 on the main breaker. Add the numbers to make sure.

NO, the total amount of all the breakers in the panel has nothing to do with the size of the main. The breaker for each circuit needs to be sized to the wiring and equipment on that circuit. The total combined rating of all the breakers in the box can be , and often is, a lot more than the main breaker rating . Unless you have every circuit loaded to capacity and all devices on at the same time that wont be a problem. If somehow you do, the main will trip and protect the panel.

FYI, unless you have electric heat and electric hot water (or some other big load) very few houses ever get anywhere near 200 amps (48,000 watts) continuous load.

I did a 60 amp sub install in my garage a few years ago to upgrade the 4 socket fuse box that was there before.

• First off, you need 2 open slots in the main breaker (not 4) for the 60amp double pole breaker. (2 hots to the breaker, neutral and ground to the ground bus of the main box)
  
• I ran underground conduit from the house to the detached garage. I ran #6 THWN in the conduit. You are correct you need THWN as underground conduit is a wet location by code.
• Inside the house I spliced the THWN into 6/3wG NM-B. This is legal but know what you are doing if you are going this route as you need bolt splices insulated with rubber tape... wire nuts are not enough.
• Code usually allows the ground on such a run to be one size smaller (typically #8 in a 6/3) But check your local code.
• The ground and neutral bars must be separated in the subpanel. Only tie to ground in the main panel.
• The sub panel will typically require a dedicated ground rod if in a detached building. Again check local code.
• Check the code on GFCI... I dont know about putting an entire sub panel behind one, its usually done circuit by circuit. (Even if its legal; if you had one on the whole panel it will be very hard to find the offending device if it trips)

It goes without saying that you should not take my word for it and check your local codes. Get any permits required. And if you are not very comfortable working in the breaker box then call a pro.

 

[jeffesonm]

I just did the same and ran 2/2/2/4 aluminum SER. It is good for 90 amps I think, maybe 75 after derating through the crawl space, and was like half the price of 6/3 copper. Mine was all indoors though and you might have to use 2/2/2/4 USE instead.

 

[DougA]

First off, you need 2 open slots in the main breaker (not 4) for the 60amp double pole breaker. (2 hots to the breaker, neutral and ground to the ground bus of the main box)

You are correct. Sorry, I was not thunkin right.

You are accurate that you can exceed the 200 amp on your panel main breaker. I have 2 sub panels and I do exceed it but it still is not something that should be done by design IMHO. I live in a house with electric heat, electric hot water, 6 ton a/c plus a fair bit of big machinery equip. If I turned everything on, I'd be screwed but that hasn't happened but when giving advice, I prefer to be more cautious. Codes vary quite widely between jurisdictions and you need to check as we both posted.

 

[jharkin]

You are correct. Sorry, I was not thunkin right.

You are accurate that you can exceed the 200 amp on your panel main breaker. I have 2 sub panels and I do exceed it but it still is not something that should be done by design IMHO. I live in a house with electric heat, electric hot water, 6 ton a/c plus a fair bit of big machinery equip. If I turned everything on, I'd be screwed but that hasn't happened but when giving advice, I prefer to be more cautious. Codes vary quite widely between jurisdictions and you need to check as we both posted.

Doug,

I dont mean to criticize, but we need to be careful giving advice. A lot of forum members will read this, go look at there box add up the numbers on the breakers and get something like 500 then freak out and worry for no reason...

In my 200 box for example I have around 300 amps of breaker per leg and Ive never tripped the main... Never tripped it when that same load was shoehorned into a 100amp main when I bought the place either ( and back then I had more real load, Ive since removed the electric range). My whole house energy monitor in fact shows Ive never exceed more than 50amps max load - typically far less. I have a lot of breakers because I have things split up a lot of circuits, not because I really have huge loads.


The thing is - electricians do NOT size panels by the sum of the circuits to be connected. What they do is a formal load calculation for the house. The formula is stipulated by the U.S. NEC and it can vary a bit locally but goes roughly like so:

• Square footage of house x 3w/ft2 for indoor lighting and outlet load
• 1500 watts per kitchen and bath circuit
• 180 watts per outdoor light and outlet
  
• Take the wattage rating for the larger of the heat or AC
• Add the wattage rating for any other major hardwired appliances (dryer, dishwasher, power tool etc)

The sum of all this is gross load. To get the true load they take the first 10,000 watts then add 40% of the remainder to account for the fact that nobody ever uses it all at once. Sum that then divide by 240v to get the amp draw. Whatever the number is you choose the next larger sized panel (say this figure comes up as 85 amps you would pick a 100 panel). Houses without electric heat and HW typically dont even need 200 however 200 has become the de facto minimum standard for new construction.

Now using this formula with your big loads might mean you do need more than a 200amp main panel (They do 250s, 300s even 200x2 double services for large all electric houses now)... but the sum of your breaker ratings , again, is meaningless. (You could put every single outlet on the house on its own breaker if you want and get some crazy big number that has no relation to the actual load)

 

[nola mike]

• First off, you need 2 open slots in the main breaker (not 4) for the 60amp double pole breaker. (2 hots to the breaker, neutral and ground to the ground bus of the main box)
  
• I ran underground conduit from the house to the detached garage. I ran #6 THWN in the conduit. You are correct you need THWN as underground conduit is a wet location by code.
• Inside the house I spliced the THWN into 6/3wG NM-B. This is legal but know what you are doing if you are going this route as you need bolt splices insulated with rubber tape... wire nuts are not enough.

I don't need to do this, the main panel is a straight shot to the near wall.

• Code usually allows the ground on such a run to be one size smaller (typically #8 in a 6/3) But check your local code.
• The ground and neutral bars must be separated in the subpanel. Only tie to ground in the main panel.
• The sub panel will typically require a dedicated ground rod if in a detached building. Again check local code.

That was another question I had. A dedicated ground would probably be easier than running 70' of #8.

• Check the code on GFCI... I dont know about putting an entire sub panel behind one, its usually done circuit by circuit. (Even if its legal; if you had one on the whole panel it will be very hard to find the offending device if it trips)

Upon further review, I can't imagine any situation where I'd need 100amp to my garage. The #2 that I'd need is >twice the price of the #6, and waaay thicker/harder to work with. 60 should be plenty. And yeah, I thought it would be easier to put in one GFCI on the main. Looks like it would be cheaper/easier just to protect the outlets, esp since I'll only have 2 or 3 circuits max.

 

[semipro]

A dedicated ground would probably be easier than running 70' of #8.

I believe you still have to run a ground to the main panel (i.e. a floating ground).

 

[nola mike]

Just curious, what's the point of that? Especially if both were tied to the ground panel?

 

[DougA]

Just curious, what's the point of that?

That's why you need to check the code for your area. Mine stipulated ground wire through the conduit. There are some code regs that I disagree with but it's the law and you do what it says.

 

[jharkin]

Just curious, what's the point of that? Especially if both were tied to the ground panel?

National code requires both, however some local codes and inspectors dont enforce it. The ground back to the main panel AND a separate ground rod if its a detached building. A sub within the main building does not require a dedicated ground. the neutral MUST float in the sub.

Reasoning is as follows:

• The ground connection between sub and main panel ensures that in case of a ground fault that the fault current has a path back to the main service entrance and transformer on the street so that the breaker will trip
  
• The ground is allowed to be undersized because its should never have to handle the full load current of the panel rating. All it needs to do is take the instantaneous current spike of a fault and cause a breaker to trip and kill power.
• The ground and neutral must be isolated in the sub panel, otherwise neutral current has two possible paths back to the main panel (and will follow whichever has lower resistance) - you can get live neutral current on the ground wire if you bond them in two places
• The separate ground rod is required for panels in outbuildings I believe to give a more direct path to ground in case of a lightning strike, so that massive current would not try to follow the small gauge ground all the way back to the main panel.

I always recommend that if you are going to play with electricity its worthwhile understanding the logic behind the code and not just following the letter of the law. It could save your life.

 

[nola mike]

The code will tell me if it's required, but not the logic behind it. Is it just to make sure that both panels have the same ground reference (in which case, why am I grounding it outside? In case of failure of the ground to the main?)

 

[nola mike]

Reasoning is as follows:

• The ground connection between sub and main panel ensures that in case of a ground fault that the fault current has a path back to the main service entrance and transformer on the street so that the breaker will trip
  
• The ground is allowed to be undersized because its should never have to handle the full load current of the panel rating. All it needs to do is take the instantaneous current spike of a fault and cause a breaker to trip and kill power.
• The ground and neutral must be isolated in the sub panel, otherwise neutral current has two possible paths back to the main panel (and will follow whichever has lower resistance) - you can get live neutral current on the ground wire if you bond them in two places
• The separate ground rod is required for panels in outbuildings I believe to give a more direct path to ground in case of a lightning strike, so that massive current would not try to follow the small gauge ground all the way back to the main panel.

I always recommend that if you are going to play with electricity its worthwhile understanding the logic behind the code and not just following the letter of the law. It could save your life.

You beat me to my last response. That all makes sense. Although a ground fault should cause the breaker in the sub to trip, it's reasonable to want to cut the power to the sub from the main panel as well.

 

[BlueRidgeMark]

Ran conduit about 5" below the new fill dirt that was placed in between.

So what is the final bury depth after the fill dirt was added?


BTW, GCFIs are a royal pain if you have them serving a lot of loads. The more loads (outlets), the more trouble they are. Code permitting, I would always prefer to have them at the outlet rather than the breaker for the sake of troubleshooting when one decides to keep tripping for no apparent reason. The closer to the individual load, the better in my book. Again, code permitting. I know that's the more expensive approach, but it will save you some headaches.

I always recommend that if you are going to play with electricity its worthwhile understanding the logic behind the code and not just following the letter of the law. It could save your life.

Dems good advice.