Electrical Service for Detached Garage (?)

[65cayne]

When I built my house th electrician spec'd a 200 amp service panel. I asked if that would support a future garage and he then added a 100A panel adjacent to it. Circuits for the entire house are fed from both (200A and 100A) panels, and both panels have several unused slots on the bus bar.

My question is, how does this additional panel provide a means of getting service to a detached garage? The overall capacity is there, but I am sure I would not run all the garage circuits individually from these two panels right?

How would a sub panel (in the garage) be fed from this configuration?

 

[mrb]

you would put a breaker in the 200 amp panel to supply a feeder to the garage and you would have a panel in the garage. Are you doing this or are you having an electrician do it?

 

[65cayne]

you would put a breaker in the 200 amp panel to supply a feeder to the garage and you would have a panel in the garage. Are you doing this or are you having an electrician do it?

So you just drop a 100A 2-phase breaker in there and run it to a sub panel? That is kind of what I figured but I am confused as to why the electrician installed the extra 100A panel in my house? Was that an unecessary expense?

And no, not doing this myself. I am curious more than anything.

 

[Stuart in MN]

I'm guessing he put in the extra panel so you would have enough available slots for all the branch circuits in the house. It sounds like if he had not done it and you just had the 200 amp panel, all the slots may have been filled up by now and then you wouldn't have room to add the garage.

 

[Charles (in GA)]

If the 200amp panel would not have been full if installed by itself, then your electrician did not need to install the 100 amp panel. If the 200 amp would have been full, then installing the 100 amp panel was good, it gave you enough space for all of the house breakers and the double pole 240v feed you will need for the garage.

You can only have one circuit between buildings, so no, you cannot run multiple cables out of your house panel to a detached building.

Charles

 

[dankicksass]

Having separate service for the outbuilding with its own meter and its own bill makes things easy if you ever want to do business there or lease it out. That's how I have my shop setup. The power company would be happy to make accommodations, I'm sure.

 

[Falcon67]

Having separate service for the outbuilding with its own meter and its own bill makes things easy if you ever want to do business there or lease it out. That's how I have my shop setup. The power company would be happy to make accommodations, I'm sure.

And if you are not zoned as a business (like - residential) then you will experience other legal issues. Texas PUC allows a second meter at an address to be charged at commercial rates, so that is also not always a good idea. Been there, been billed that. As a local example, if I drop $500 to run my 100A feed 100', the payback on the wire at local min meter billing rates is 13 months. That says "don't do it". If the OP would like to meter for a business deduction and keep the profile low, buy a meter off eBay and install it at the shop under cover. You don't have to be the PoCo to own a meter LOL.

To the OP - depends on how your installer did the hookup. If the 100A panel comes off the meter base like your 200A, then you put a 100A breaker in there and run to a panel in your detached building. This is how my shop feed is wired.

If the 100A is a sub panel off the 200A, then the wiring is different and you'd probably need to look at the loads involved.

 

[65cayne]

I'm guessing he put in the extra panel so you would have enough available slots for all the branch circuits in the house. It sounds like if he had not done it and you just had the 200 amp panel, all the slots may have been filled up by now and then you wouldn't have room to add the garage.

Nope, plenty of slots available. That's what got me wondering...

If the 200amp panel would not have been full if installed by itself, then your electrician did not need to install the 100 amp panel. I....Charles

That what I was thinking....did I get a 300A service?

... if you ever want to do business there or lease it out. ...

Neither are allowed (HOA)

To the OP - depends on how your installer did the hookup. If the 100A panel comes off the meter base like your 200A, then you put a 100A breaker in there and run to a panel in your detached building. This is how my shop feed is wired.

If the 100A is a sub panel off the 200A, then the wiring is different and you'd probably need to look at the loads involved.

Falcon, my color coded reply...
I believe the 100A panel is fed directly from the main lugs on the 200A panel, essentially, from the meter. I'll have to check. This would mean that the main feed from the meter would have to be upsized to account for the additional load capacity, giving me a 300A service?
I can't install a 100A breaker in the 100A panel if there are already a dozen circuits in there can I?
Thanks for the input guys.

 

[I void warranties]

separate building with separate neutral bus and grounding rod. at least i hope so. your panel may not be rated for more amps per bus bar. never ever overload a breaker bus bar. overall rating. sounds like he did the rite thing to me..

 

[Falcon67]

I believe the 100A panel is fed directly from the main lugs on the 200A panel, essentially, from the meter. I'll have to check. This would mean that the main feed from the meter would have to be upsized to account for the additional load capacity, giving me a 300A service? [/COLOR]
I can't install a 100A breaker in the 100A panel if there are already a dozen circuits in there can I?
Thanks for the input guys.

In my case, the 100A box will hold 6 breakers but I will use only one in there. If there are already circuits pulled from the 100A panel, then no - you can't really put a 100A in there. If it's close to the 200A box and there is space, those could be re-routed to the 200A box. That would be ideal IMHO, because then you could use the 100A box to be your garage overcurrent limit. Any box with a main breaker installed in the garage becomes more of a convenient local disconnect.

My PoCo did the install (for free) and they did not seem to think that a 200A house panel with a 100A shop panel on the same 4/0 aluminum feed was any big deal. So technically - yes, I can pull 300A through the meter. It's their wire, so what-me-worry.

Practically - I'd have to work at hitting 150A with the house and shop running full song on most days. I could see maybe 200A+ in the winter since our house is all electric and the house heater pulls 60A easy when all the elements are running. We'd have to have the heater, dryer, water heater and stove all running at the same time plus the shop compressor, mill, lathe, lights, well pump (drained in winter - yard only), drill press, shop heater, etc to even get close to that number.

I tested my old shop one Saturday by switching on all lights, DP, lathe, mill, heater, 3.5HP compressor and fans and I just barely got to 50A draw. I think it was 49.6 or some such - got it written down somewhere.

 

[volleyball]

You can test how your panels are wired by shutting off the 200a main and see if you have power anywhere.

How big are your panels? and how many slots empty? Could one panel have sufficed and had 6 spaces left?
I have no spaces left on my 40 space panel. And I could use another dozen spaces. Luckily I have FPE so it makes it easier to want to change.

 

[jvitez]

How many slots total are in the 200 amp panel and the 100 amp panel? Are there any tandem breakers? These are 2 single pole breakers that fit in one regular slot. You need to figure out if the 100 amp panel is separately fed from the meter or is functioning as a subpanel. Does the 100 amp panel have a 100 amp breaker, or no main breaker at all (main lugs subpanel)?

 

[65cayne]

...Nope, plenty of slots available. That's what got me wondering....

Okay...I recounted and and there are actually about 4 more branch circuits (total) than what would have fit in the 200A panel. That said, I dont understand why he added the 100A panel when I mentioned a future garage if all the circuits would not have fit anyways. I also find it odd that I have no 15 amp circuits. Everything is 20A or more...

 

[65cayne]

... If there are already circuits pulled from the 100A panel, then no - you can't really put a 100A in there. If it's close to the 200A box and there is space, those could be re-routed to the 200A box...

They are adjacent but it would require tearing the sheetrock down. All the wiring is behind the wall.

You can test how your panels are wired by shutting off the 200a main and see if you have power anywhere...

I could think of an easier test without having to reset all my clocks...haha!

How many slots total are in the 200 amp panel and the 100 amp panel? Are there any tandem breakers? These are 2 single pole breakers that fit in one regular slot. You need to figure out if the 100 amp panel is separately fed from the meter or is functioning as a subpanel. Does the 100 amp panel have a 100 amp breaker, or no main breaker at all (main lugs subpanel)?

I'd have to count the slots but I have several empty in both panels. Both have tandem breakers (A/C units, Dryer, Oven, Air compressor, etc...). I am pretty certain the last time I had the cover off, the 100A panel was piggy backed on the main lugs (basically fed from the meter). Both panels have a main breaker.

I am not sure why they did not just fill up the 200A panel and spill the rest into the 100A panel. That kind of hoses things up.

 

[jvitez]

I'd have to count the slots but I have several empty in both panels. Both have tandem breakers (A/C units, Dryer, Oven, Air compressor, etc...). I am pretty certain the last time I had the cover off, the 100A panel was piggy backed on the main lugs (basically fed from the meter). Both panels have a main breaker.

I am not sure why they did not just fill up the 200A panel and spill the rest into the 100A panel. That kind of hoses things up.

Aha! Now we're getting somewhere. If both panels are fed from the meter, then it's just a bit of a screwy way of increasing the number of spaces in your main panel. Not sure why the he didn't just install a 40 circuit panel. Maybe he had both in his truck and wanted to use them up, who knows.

Anyway, you need to figure out the maximum wattage, not amperage, of everything you could ever foresee running at the same time the garage, plus anything you might consider in the future. Now divide this by 240. This is the amperage you must deliver to the garage. You also need to see which panel has the most load, not number of circuits, but load. For example, if you come up with 9600 watts or less load for the garage, you can run a 40 amp circuit. Unless the 100 amp panel has all the AC/heater/stove/etc loads on it, you could add another 40 amp load to either panel.

BTW, 240V breakers that take up two spaces are called dual pole. Tandem breakers are 2 single pole breakers that take up only regular space in a panel, so two circuits only need one panel space. The reason I mention this is that if you physically need 2 spaces side by side for a 240v dual pole breaker to feed your garage and you don't have it, you can switch a few circuits to tandem single pole breakers if you panel allows, which would free up the space you need.

 

[65cayne]

...BTW, 240V breakers that take up two spaces are called dual pole. Tandem breakers are 2 single pole breakers that take up only regular space in a panel, so two circuits only need one panel space. The reason I mention this is that if you physically need 2 spaces side by side for a 240v dual pole breaker to feed your garage and you don't have it, you can switch a few circuits to tandem single pole breakers if you panel allows, which would free up the space you need.

Good paragraph and (****) yes you are correct. I had dual pole on the brain when I read that post. I do not have any tandem of "mini breakers" which which is what I know them as. All dual pole or single pole and nothing smaller than 20A.

40 amps for a garage?....surely not...although my last house had a 50A main breaker. It was a small house...

Let me check the panels again tonight and get an accurate picture.

 

[jvitez]

40 amps for a garage?....surely not...

I used 40 amps as an example only. Humble appologies, as this is GJ after all. I should have said 200 amps fed from your 400 amp industrial panelboard. Whew. My man-card was whimpering there for a bit.

Another thing to check: do your panels accept tandem breakers? Not all do. And local codes may have different regulations. I'm in Canada, so we're under the Canadian Electrical Code not the the National Electrical code. The two are similar, but quite different in certain aspects. In my own house, I've got 400 amp service (really I do...) which feeds two Siemens 200 amp main breaker service panels, which are 40/80. So if I had no dual pole breakers, theoretically I could fill them up with tandem single pole breakers, and have 80 circuits in each panel and still be code compliant. Yikes! The NEC doesn't allow that.