Electrical question ... capacity

[jayc]

Hello,

Long time reader, first time poster.

I see alot of people are putting in 100A subpanels in their garage. 100A seems like a lot to me. Why so much? Maybe I could see it if multiple people were going to be working in the garage at any given time, but like most I'm sure I will be the only one operating any power tools in the garage. Obviously there are things like lighting that operate independantly, but for the most part large load items would be used one at a time. So why all the need for additional power?

I'm planing on a 40A subpanel, unless I can be convinced that it is a bad idea. I'm looking at typical hand power tools, largish compressor (5hp 220V), 110V Mig Welder, flouresent lighting and stereo ... nothing really out of the ordinary.

Jay

 

[cj7jeep81]

i had a 30 amp panel in my old garage (previous owners built it), and really wished for something bigger. i never blew the main breaker, but would never weld while the air compressor ran, and just hated worrying about it.

another reason is space. if you want to put in a bunch of 220 circuits, they'll chew up a lot of space really quick, and typically higher capacity panels have more spaces in them.

one big reason to go bigger is cost. if you look at the costs, it really isn't that much more expensive to put in a bigger service now. if you decide to upgrade down the road, it will cost a lot more.

 

[Steve in Mi]

Put in what you need, now and in the foreseeable future. I wanted to have lights when the 90 Amp input TIG was being used plus some spare Amps for the furnace blower etc. - 150 Amp works fine. You might like to have lights on when your compressor starts up - do you think 40 Amps will do it?

 

[PAToyota]

Hmmm... The breaker to my compressor alone is 30amps... If you are looking at a 5hp compressor you will be too..

I have 200amp service to the shop. The TIG welder alone is a 100amp breaker. Add in lighting, the compressor possibly cycling, having my father or a friend out there doing his thing... Then there is the woodshop. Again, perhaps with someone out there with me. Main tools are 220V, dust collector, lighting, compressor for pneumatic nailers...

If it is a garage and you're talking mainly lighting and possibly some hand tools, you definitely don't need to consider a big panel with lots of capacity. But I there are quite a few people on here who have much closer to a commercial shop with equipment much more than what the weekend warrior typically picks up from Lowes and Home Depot.

 

[jayc]

Thanks, this is the information I was looking for.

Looks like 100A subpanel it is. My house has 200A service, will the 100A subpanel be too much? This is a detached garage about 60ft from the house panel.

 

[locoman]

I have the same situation but opted to go with a 200 amp in the garage and under ground sub feed the house existing 100 amp load center. That way I get ride of the drop to the house and get a new upgraded drop to the garage that is out of view from the house. It will give me plenty of room to expand. The house will be fine at 50 amp with the power use now.

 

[timgr]

I just upgraded my garage service, and it's 100A. This is for a 21x17 detached single-door garage. I have two overhead electric heaters - that's 30A x 2 right there. If I add a compressor and a welder, that's a 30A and 50A breaker. Each of the 110V outlets is a 20A breaker, and I need lights and the door opener.

It adds up pretty fast. I thought 100A was going to be way more than I needed when I upgraded the house service.

<edit> I bought the house last winter, and it had a 100A service. The home inspector said that a 100A service was plenty for a house this size, 1200 ft^2 bungalow. I'd imagine a bigger house might need more, but the only way to tell is to add up the loads. Watts divided by volts equals amps.

 

[Junkman]

I have a 200 amp panel in the cellar and a 100 amp sub panel also next to that one, since I had run out of room in the original panel. I also installed a 100 amp sub panel in the garage when I added onto the existing garage. When I added on an additional Master Bath, I put a 60 amp sub panel in a closet in that room. I have never tripped any of the breakers, but I do feel comfortable knowing that everything is properly protected. Every room in the home is broken down to at least 2 breakers serving it, and in some cases, there are more. That way, you are never in the dark if one breaker should trip. It cost a little more to do it the way that I chose, but to me it was well worth the extra cost. If I were to do it again, the only change that I would make would be to put a sub panel on each floor instead of running wire back to the cellar from the second floor.

 

[PoorOwner]

Not sure but on a 100 amp breaker box or subpanel is usually rated for 200 amp "worth" of breakers.. so I would think on a 40 amp breaker box you can only put 80amp of breakers. Compressor 30, light 15, outlets 20, there goes most of it. 40 Amp subpanel won't do no matter what.

Edit: 240V * 40Amp = approx. 10k watts. Decide if that is ok for your scenario.

 

[Charles (in GA)]

A large panel is not very expensive, what is the issue? If you can buy a 200 amp contractor pack panel with 40 spaces and a bunch of breakers, for a little over a $100, and a 40 or 60 amp panel costs $50 bucks and you still have to buy breakers, why do you want the little panel? Panels are cheap, its the work to install them thats the hassle. Put in a couple of dedicated 240 circuits and that 40 amp panel you are talking about is out of space. Turn on the lights, start welding and have the compressor kick on, and something is going to blow. A dedicated motor circuit for a compressor should have a breaker sized at 250% of Full Load Current (FLC). Thats a whole lot more than a 40 amp panel can handle. (I'll admit I have a 50 amp breaker on my 7.5 hp compressor that draws 32 amps running, so I'm undersized and never have had a problem, but that is not what is called out in the codes.

Charles.

 

[W-Cummins]

A dedicated motor circuit for a compressor should have a breaker sized at 250% of Full Load Current (FLC). Thats a whole lot more than a 40 amp panel can handle. (I'll admit I have a 50 amp breaker on my 7.5 hp compressor that draws 32 amps running, so I'm undersized and never have had a problem, but that is not what is called out in the codes.

Charles.

No matter how many times you say that, it's still is not true! It says you CAN use up to 250%, not that you HAVE too. In fact it is better if you DO NOT use the MAX value of 250% unless you NEED too. Your own example is not undersized, and in fact is better than using the whole 250% that you could use. You have a larger margin of protection using a smaller breaker, and like you say it works fine! As long as your not getting nuisance trips of the current limiting device your fine with less than the MAX value you can use.

William....

 

[Charles (in GA)]

No matter how many times you say that, it's still is not true! It says you CAN use up to 250%, not that you HAVE too. In fact it is better if you DO NOT use the MAX value of 250% unless you NEED too. Your own example is not undersized, and in fact is better than using the whole 250% that you could use. You have a larger margin of protection using a smaller breaker, and like you say it works fine! As long as your not getting nuisance trips of the current limiting device your fine with less than the MAX value you can use.

William....

Indeed, the code does say in 430.52 that the 250% is a max. However, it is a very real possibility, and to expect to run a shop on a 40 amp panel with a 5 hp compressor, lights and other items is not realistic, without being limited to the point of impractical.

Charles

 

[Greenjeans]

I think everybody is missing a big element in this....what size wire is being used? The whole point is to protect circuits from drawing more current than the wire will carry, overheating and causing a fire....I can just see some poor soul putting in a 200 amp box because it is not that much more expensive and gives him lots of room for extra breakers.....and feeding it with 8 gauge wire!

 

[SteveU]

I have a 60 amp in my barn/shop & want to go bigger. My 5 hp Eaton compressor draws 45 amps starting but only runs at 19 or 19.5 ( forget which) and if I run all 7 8' HO lights that's another 9 amps so for the instant the compressor kicks on it is drawing 54 of 60 amps & causes the lights to flicker. Do yourself a favor & get at least 100 amps preferably 200 & you should be set for anything you want to do now or in the future.

 

[Franz©]

These discussions are so pointless till somebody slaps an Amprobe & a voltmeter onto the circuit.

250% is a suggestion, not a hard & fast rule so pursuing that figure is a complete waste of time.

Wire size tables re amperage are also not relevent till you read far enough to see the ampacity is different for conduit, conduit in the ground and overhead wiring.

 

[john56h]

Not to highjack the thread...but I have a related question.

I have a 100 amp service in my house and have built a new detached garage. I've installed a 100 amp panel in the garage, but I don't want to have a separate service dropped to the garage. The garage panel will be a "sub-panel".

Can a sub-panel have a amp rating equal to the main panel? Is it okay to run a 50 amp circuit from the main panel to the sub-panel even though the sub-panel has a "main" breaker rated at 100 amps?

My reason for doing it this way is because I have temporarily run overhead wire to the detached garage from the house. In the future, I am planning to install buried electric to feed the garage and I intend to upgrade the house to 200 amp service and the garage circuit to 100 amps. I want to wait until I excavate for a planned house expansion before I install the permanent buried garage electric feed.

I don't currently have a heavy electrical demand in the garage, but want the electrical system to be able to handle it in the future when the service is upgraded. I have been assuming that any overloaded circuit in the garage would trip its own circuit breaker and a total overload would not trip the sub-panel's 100 amp main breaker, but instead trip the 50 amp breker in the house panel for the garage circuit. Is my reasoning valid?

I've seen branch circuits with subpanels that had no "main" breaker, so I'm figuring that the 100 amp breaker in my garage sub-panel is just serving the purpose of a convenient way to shut-off power manually without walking back into the house. That is, until I upgrade the garage circuit to the full 100 amps which may be a few years off.

 

[Charles (in GA)]

I don't currently have a heavy electrical demand in the garage, but want the electrical system to be able to handle it in the future when the service is upgraded. I have been assuming that any overloaded circuit in the garage would trip its own circuit breaker and a total overload would not trip the sub-panel's 100 amp main breaker, but instead trip the 50 amp breker in the house panel for the garage circuit. Is my reasoning valid?

I think so. If the feed to the garage is protected by a properly sized breaker, it really, practically, doesn't matter how big the panel in the garage is, the 50 amp breaker, protecting the wire you have run to the garage should do its job, so long as the wire to the garage is not too small. I assume you used #6 most likely?

Charles

 

[eisinger]

I had a 200 amp main service...I'm at the moment building a 36x36 garage/shop and have replaced the house main with a 400 amp and am going with a 200 amp subpanel in the new shop. As stated, panels are relatively cheap in the large scale and you will always want more breakers rather than fewer...in my case I have separate 30 amp 220 v circuits for compressor, lift, three 5 kw electric wall heaters, plus 2 additional wall outlets for welder, powder coating oven, etc , plus a whole bunch of 20 amp 120 v circuits.

Your total individual breaker capacity can exceeed that of your main breaker as load diversity is assumed in the design.

Keep in mind also, that home electrical services are generally sized for volatge drop and flicker and not when the wire melts. Most utilities use just a couple or so wire sizes for residential service...the particular size is typically determined by voltage drop due to the length of the run from the transformer to the house service.