Hi guys (and occassional gal),
Its been wayyyyyyyy toooooo long since I’ve been here so thought I’d pop in to say Hi and of course ask something of which my knowledge is limited. Here is the scenario:
Homeowner has a detached garage he wants to supply electrical service. Service to home is 200 amps. Detached garage is about 25 feet away and is to have a 200 amp panel as well. Local power company will not allow a separate meter and service for the garage so it must be run through the service to the house. He will need 200v in the garage in order to power a compressor and very limited use arc welder.
Does it get run right off the meter box? Off an open breaker in the main panel and if so will that handle the welder and compressor? Does a separate panel get put next to the house panel and then the second 200 amp one in the garage itself?
Been talking to the electrician and also referencing Rex’s book and both have differing methods…So I turn to you guys for some ideas…
Thanks.
Mike Rimoldi
Replies
First of all, Mike, why does the garage need 200A service? A welder, a compressor, a few 20A receptacles, 500 - 1000 watts of lights, definitely not 200A territory. If the customer really needs 200A in the garage AND the house, start by upgrading to a 400A service. When the cost of that shocks the owner back into reality suggest the following: a 100A sub-panel(w/maybe a 125A panel if they want a lot of spaces) fed by a 100A breaker, three (3) #2 copper conductors and a # 4 for ground. I would run it in 1-1/2" flexible watertight conduit, buried at least 24" underground, preferably 36".
Second, the garage supply will have to go through the main service panel and circuit breaker to the garage sub-panel. The sub-panel will have to have a separate equipment ground bus installed. Sub-panel ground and neutrals cannot be connected. I would run (1) set of feeders to a single sub-panel located in the garage to make the underground work simpler.
If the customer has a 5 hp, 240 vac compressor, it will take 28 amps at full load. A typical arc welder uses a 50A, 240 vac circuit, but seldom draws more than half that. 2000 watts of light will be about 17 amps at 120 v. Throw in a small AC unit and some power tools all being used at the same time, you could trip a 70 A breaker with help from two friends,maybe, but not buy yourself. 100A would be more than adequate.
I power my spa (240v, 50a), my arc welder (240v, 50a) and my ACCU (air conditioner condesing unit, 240v, 50a) all though a sub-panel fed from the main panel thru a 70 amp breaker. Never tripped it and I never will (I NEVER weld in the spa with th AC on).
Tim,
Very good info. Thanks! Yeah, welding in the spa with the AC on is usually not an accepted OSHA practice...
Mike
I did a 60AMP subpanel in my garage. Just fine for my needs including 240V compressor and all my shop tools. Usually work alone so no chance of tripping the breaker either.
bit
i have a 100 amp service in my garage i run my welder tablesaw and compressor plus lights and about 15 receptacles
just for the record the wire i used was aromex type that i buried in 2"pvc conduit about 3 ft underground i used the romex so that i could run the romex along the cieling in the basement i believe it has three #2 wires and a ground inside
Edited 12/16/2002 3:28:53 PM ET by kdinger
Ditto.
Never been in the spa either when I'm welding, nothing else except lights when welding. Have only 50A to barn, never a problem, wood heat in barn only though.
First of all, what you will have is a main panel and a sub-panel. You cannot have an unfused conductor over 10 ft long in the building. What that means in english is, that after the meter, everything goes through a breaker in the main panel first.
The feeders for the sub-panels have to include a ground that goes back to the main panel ground. Where the main panel has the ground and neutral bus bars tied together, the sub-panel(s) cannot. They must be kept separate. So the sub-panel has a hot bus, a neutral bus and a ground bus.
also there is a code regulation, On a "DETTACHED" bldg , a ground rod must be installed also at the subpanel box. Nothing to do with the netrual, still must be separted.
dettached, a bldg that has no common feature with the main house as in fdn, slabs, roofs connected each together.
What does the electrician say? There are a lot of variables and if he's the guy on the job I'd be inclined to go with his plan.........Ed
By feeders, plural, I was refering to all of the conductors that are not there to serve as a grounding conductor. You can buy a multi-conductor cable that will have all this included in a single jacket. In the case of feeding a subpanel that will have 240 volts, that means two "hots" (red and black), a neutral (white) and a ground. The hots and neutral are sized for the service to be fed and the distance to be covered. People mentioned specific requirements for a ground ina detatched building. I would follow their lead and look this up in the code.
Tim,
Updated question for ya.
Would this have to be #2 for the conductors or would #4 be acceptable. I ask because all the 100 amp breakers I've ever seen won't accept #2 size. But then again, I'm not an electrician so I don't see them everyday.
Thanks again.
Mike
Mike,
A Square-D, QO 2-pole 100 amp breaker (part # QO2100) will accept from #4 to 2/0 wire size (per their catalog data, though I can't imagine 2/0 being connected to a 100A breaker).
In the National Electrical Code 1996, Table B-310-3 "Ampacities of Multiconductor Cables With Not More Three Insulated Conductor...in Free Air" #2 copper rated at 75 degC is good for 118 amps, rated at 60 degC its good for up to 92 amps. Regular old NM-B (Romex) is typically rated at 60 deg C. However, there is a little leeway in the sizing of the conductors and without going into the details, you can use #2 for a 100a circuit.
For direct buried cable at 60 degC the ampacitiy of #2 is 137 amps, #4 - 107amps.
I suggested #2 because it is big enough under most circumstances and there will be less voltage drop if you are running the cable any distance.
Something to consider, a 60A 2p breaker costs about $20, a 70A 2p breaker costs about $50 and 80A, 90A & 100A 2p breakers cost about $70. A 110A 2-pole breaker costs about $155. You could easily do this garage with a 60A circuit. A 100A main lug panel (Square-D QO) comes with 6 or 8 spaces. If you need more than 8 spaces you would have to use 125A main lug panel. For my sub-panel, I bought a 125A main lug panel, feed it with a 70A 2-pole breaker and #6-3 NM-B.
Tim,
Excellent info! Thank you very much. I appreciate the notation from the NEC as well as now I can have a look at mine and use it and your input as a guideline.
Mike