Sizing Garage Sub Panel

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Old 01-28-13, 04:58 PM
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Sizing Garage Sub Panel

Currently I have a detached garage with a single 20A branch circuit.

I plan to do a lot of homebrewing (electric heating), but also be sure I accomadate for general garage use. I took a screen shot of everything I can think of that I would need. I included comments to clarify some items and questions on specific items.

I think I'll need GFCI protection for everything since it's a garage... would it be the most cost effective to have the main breaker for the subpanel be GFCI (or the breaker in the main panel that serves the sub)?

Any feedback is appreciated.

Thanks!

Nic

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Old 01-28-13, 06:38 PM
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I think that a second main breaker would be required by code. Do you intend to file plans with the building department? It might be a good idea, in case you sell the house some day. If so, I would have plans drawn.
 
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Old 01-28-13, 07:08 PM
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would it be the most cost effective to have the main breaker for the subpanel be GFC
Technically it is not used as a breaker*. It is only there as the code required disconnect. The breaker for the subpanel is the one in your main panel. Having a remote GFCI could be a nuance it you have trips. Probably better to have the 120 volt receptacles protected by GFCI receptacles at the first position of the circuit. Dedicated 240 volt circuits will not need GFCI protection.

*You will probably be using a 100 amp panel that comes with a 100 amp main breaker on a 60 amp circuit but no need to use a smaller breaker because as I said it is only there as a disconnect.
 
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Old 01-29-13, 12:28 PM
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How'd you figure 60 amp circuit?
Also, should I have a dedicated circuit for each of the following:
EOHD
Refrigerator 1
Refrigerator 2
Vent Hood or Exhaust Fan
Computer
 
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Old 01-29-13, 01:48 PM
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Assuming that EOHD stands for electric overhead door operator that can be powered by a general purpose 20 ampere 120 volt circuit.

I would suggest TWO general lighting circuits, only because if there is only one and its circuit breaker trips for any reason it will leave you in the dark. Using fluorescent lighting you will not need anything close to 15 amperes at 120 volts.

For the air compressor I would definitely use a 240 volt dedicated circuit and wire it for 30 amperes (use #10 conductors) but use the smallest circuit breaker that will reliably start the machine.

Power hand tools can run off of a general purpose 20 ampere, 120 volt circuit. I would have at least two such circuits. Depending on the size of the table saw it might be better as a 240 volt circuit sized for 20 amperes.

The heating/cooling unit at 5kw requires a 30 ampere 240 volt circuit.

The "cooker" using three 5.5 kW heating units would require either three 30 ampere circuits or a single 90 ampere circuit with supplemental overcurrent protection (fuses or circuit breakers) to divide the load. If the third element is never actually used but is only a back-up if one of the other elements fail you might be able to get away with only two circuits with a selector switch for switching in the third element while at the same time switching out one of the other elements.

I see a need for at least a 125 ampere feeder to the garage with the appropriate circuit breaker panel. You do not want, nor need, to GFCI protect the panel or the 240 volt circuits. Using GFCI receptacles will be far less expensive than GFCI circuit breakers and you only need one such receptacle per circuit. While you certainly could run both refrigerators on a single 20 ampere general purpose circuit I would suggest that they be on two different circuits.

Recap: I recommend two 120 volt, 15 ampere general lighting circuits; three 120 volt, 20 ampere general purpose circuits; three (or two) 240 volt, 30 ampere circuits for the brewing vat; one 240 volt, 30 ampere circuit for the space heating/cooling unit and a circuit wired with #10 conductors for the future air compressor which could also be used for a large table saw.
 
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Old 01-30-13, 08:39 AM
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Thanks. By design, I should only have a maximum of two of the three 5.5KW elements operating at any given time, so I'll just do two 30A circuits.

Follow-up question on GFCI. I can wrap my head around just using GFCI outlets for the 120V circuits. But why not put GFCI protection on the 5.5KW heating elements? They are submursible elements and will be heating water in an open stainless steel container that may be contacted by humans (and stirring the water at times), I would think it would be smart to use GFCI protection on these. It will probalby be expensive, though. I've never seen 30A/240V GFCI receptacles but I think I've seen 30A 2 pole brekaers with GFCI.

Thanks for the response.

To address pulpo's question, I plan to talk to the city building service office and file whatever is required. I've already had one conversation and at a minimum will need a permit for trenching to the garage for conduit.
 
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Old 01-30-13, 09:40 AM
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But why not put GFCI protection on the 5.5KW heating elements? They are submursible elements and will be heating water in an open stainless steel container
When I wrote that I was generalizing. You always use GFCI if recommended by the manufacturer. Perhaps the pros will know if there is a specific NEC reg in regard to commercial equipment that would apply here. There is though no reason to not use a GFCI breaker in the subpanel for the circuit. That would probably be cheaper then a GFCI at the main because the larger the GFCI the more it usually costs and perhaps more importantly you don't loose all power to the garage because of a problem with one circuit. Reason #3: if you ever have a GFCI problem troubleshooting will be a lot easier if the problem is limited to one circuit.
 
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Old 01-30-13, 10:10 AM
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Yeah I think from a troubleshooting standpoint, individual circuit protection would be easiest so I'll put GFCI protection on each circuit required. This is probably also safer in terms of lighting... if my main is where the GFCI is and has to trip, I'll be stuck in the dark.

I'm guessing the manufacturer does NOT recommend or require GFCI since these are just typical water heater elements, and from all I've read, my understanding is that NEC doesn't require it...at least on residential water heaters. And 2-pole breakers with GFCI seem to be pretty pricey. Whether required or not, I think it would be on the safe side to include GFCI protection on my water heating circuits.

Are there, perhaps, other GFCI devices out there other than breakers with GFCI and receptacles with GFCI, that could be placed in-line with the heating element circuit (Not on the branch circuit itself, but maybe on the wiring from the plug to the element)?

Time for a google search.
 
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Old 01-30-13, 10:53 AM
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This is probably also safer in terms of lighting... if my main is where the GFCI is and has to trip, I'll be stuck in the dark.
If you're saying that you are planning to not install GFCI protection on your lighting circuits, that's the correct way to go. No GFCI on lighting.
 
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Old 01-30-13, 03:18 PM
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Two-pole (240 volt) GFCI circuit breakers are available but they have a fairly high price, something like $60 and up.

Also remember that any circuit that supplies a "continuous" load, defined as one being energized for more than three hours, needs to be designed to 125% of the applied load. That is why you need 30 ampere circuits for the brew vat heaters and the space heating/cooling system even though their full load draw is only a bit more than 20 amperes. This means at the absolute minimum you will need a 100 ampere feeder from your house panel and a 125 ampere feeder is definitely preferable.

I haven't read your post on the house but keep this in mind when figuring the requirements of the house panel.
 
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Old 01-30-13, 04:29 PM
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Thanks, I remembered the 3 hour rule = size the entire circuit for a minimum of 125% of whatever that constant load is.

Is sizing the service entrance for the house and account for this as easy as adding my constant (3+hour continuous load) to whatever my previously calculated load on the house service entrance? FOr example, let's say the house load calc came out to 90 Amps, so I would have gone with 100 Amp service, but add the 100 amps continous load for the garage would be 190, so I should shoot for 200? Or is it not quite that simpe?
 
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Old 01-30-13, 04:45 PM
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If the demand load calculation for the house came out at 90 amperes and you add the known load (while brewing and using the space heater) of 90 amperes you come up with 180 amperes so a 200 ampere service is required. Remember that a demand load calculation takes into account that not everything will be in use at the same time, but adds up all the probable loads that will be on at one time. Granted, the brewer will cycle between 23 and 46 amperes as the second element cycles to maintain the temperature (or is it that the second element is only used when coming up to temperature and then the first element will cycle to maintain?) so the full load on the sub-panel will be less than 90 amperes.

The alternative would be to have two separate services, one to the garage and a separate one to the house. This would, of course, require two separate accounts with the power company and it is possible the power company would either flat-out refuse this arrangement or would bill the garage at a commercial rate. Not knowing the size of your family or their habits when using electricity I cannot state absolutely that a single 200 ampere service would be adequate but I think it would be okay. While the cost of a 200 ampere service IS more than for a 100 ampere service the increase is not that much, mostly just the cost of the wire between the service point (overhead connection at the mast) and the main circuit breaker. On the other hand, the cost for any service greater than 200 amperes goes up astronomically.
 
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