Feeder to garage

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jpd24

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Can you guys tell me what wire size you use for a 100 amp feed to a garage from a house. I believe it should be #3 cu or #1 al. Which is not commonly carried in our area. Is there a cable available or pipe and wire? Thanks.
 
Assuming a relatively short distance I would run 3 AWG CU THWN or even better XHHW-2 but its pricey. in 1.25" PVC.
 
That is a common mistake. Table 310.15(B)(6) cannot be applied to a feeder supplying a garage, only dwelling units.

If push comes to shove, you could simply protect it on a 90A breaker.
 
Bob has it right but if you want to use copper and your supplier doesn't carry #3 you can use #2 TW. More expensive but will work even thought it is rated 95 amps. You can put it on a 100 amp breaker.

Hope I am right this time Bob-- thanks for the correction on the GFI thread. I was in lala land I guess.
 
Thats what I told the customer but he wants the full 100 amps, which I no he will not even come close to using. Also does anuone no what the terminal ratings are on an Sq D QO 100 amp MB panel.
 
jpd24 said:
Thats what I told the customer but he wants the full 100 amps, which I no he will not even come close to using. Also does anuone no what the terminal ratings are on an Sq D QO 100 amp MB panel.
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75 deg.

Dennis, I believe Bob was talking about using #3 XHHW-2

Roger
 
Also does anuone no what the terminal ratings are on an Sq D QO 100 amp MB panel.
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As in, what size conductor they can handle?

Dennis Alwon said:
...you can use #2 TW. More expensive but will work even thought it is rated 95 amps. You can put it on a 100 amp breaker...
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According to 240.4(B), correct - as long as the connected load does not exceed 95 amps. :)
 
iwire said:
Assuming a relatively short distance I would run 3 AWG CU THWN or even better XHHW-2 but its pricey. in 1.25" PVC.
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If I read Bobs quote correctly he is talking about #3 Copper thwn-- I concur with that but jpd24 says those sizes are not available readily in his area. That is why I suggested #2 copper TW.. Am I nuts?
 
If the 100 Amps is a result of "Pick a round number" rather than a load calculation, then you can probably use a 90 Amp breaker for the feeder and meet the code requirement for 2 AWG aluminum. You could still put a 100 amp disconnect breaker in the sub-panel.

A 90 amp service will supply a lot of load, including a good size welder, for a non-commercial garage.
 
Dennis Alwon said:
If I read Bobs quote correctly he is talking about #3 Copper thwn-- I concur with that but jpd24 says those sizes are not available readily in his area. That is why I suggested #2 copper TW.. Am I nuts?
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No your not nuts, I just misread your meaning.
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Roger
 
Thanks for the input guys, it all helps. Starting to do more residential work gets me more into the code and sometimes more confused.
 
Bob NH said:
If the 100 Amps is a result of "Pick a round number" rather than a load calculation, then you can probably use a 90 Amp breaker for the feeder and meet the code requirement for 2 AWG aluminum.
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It's highly likely that upsizing the cable will be cheaper than downsizing the breaker. I learned that one the hard way a long time ago, in a galaxy far, far away.

I wired a 15Kw heat-pump furnace with #2 AL, and had to use a 90a breaker. It would have been cheaper to use larger wire and a more-readily-available 100a breaker, had I known.
 
Are you going to have any three-way switches between the house and garage? I just finished a garage where the HO wanted that done in addition to the feeder, and I got to derate the feeder since there were more than three current-carrying conductors in the raceway. The original plan was to put in a 100A subpanel, but I wound up putting in a 60A breaker in the panel and feeding it with #3 Al XHHW due to the required derating.
 
jeff43222 said:
Are you going to have any three-way switches between the house and garage? I just finished a garage where the HO wanted that done in addition to the feeder, and I got to derate the feeder since there were more than three current-carrying conductors in the raceway. The original plan was to put in a 100A subpanel, but I wound up putting in a 60A breaker in the panel and feeding it with #3 Al XHHW due to the required derating.
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Could have used Leviton's Homecontrol....?
 
Dennis Alwon said:
That is why I suggested #2 copper TW.. Am I nuts?
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Are you nuts? I don't know, well maybe if you're still using type TW conductors. We haven't used them in over 15 years, maybe longer. If #3 copper conductors aren't readily available I don't know where he would get TW insulated conductors.
 
infinity said:
If #3 copper conductors aren't readily available.
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I am having a hard time with that part of this.

Where is 3 AWG CU not available?

I can not think of a time where I could not get the wire I needed.

Sometimes it may take a day or two, I needed to wait a couple of days to get a few hundred feet of 4/0-4 MC.

I suggest trying a larger supply house.
 
Why is the customer insisting on a full 100-amps to the garage? Has anyone done a load calculation to determine the actual load for the garage? If not, how has it been determined that there is enough capacity in the existing service to supply a 100-amp feeder to the garage? Guessing is not an NEC recognized activity! Also does the customer realize that a 90-amp, 240-volt feeder may supply 180-amps of non-continuous 120-volt loads? I once had an "electrical engineer" insist that a 90-amp 3-phase feeder was needed for an equipment shed, when I had calculated that a 30-amp feeder was sufficient. When we tried to compare calculations I found that she had simply added the amperage ratings of all of the breakers that were to be installed to come up with a "load calculation". I could not convince her that, since everthing in the building operated at 120-volts with the exception of a 1500-watt electric heater, we could put 90-amps on each phase of her proposed feeder for a total of 270-amps (3X90)! Sometimes a little knowledge is a dangerous thing. The only way to do this job correctly is to do a load calculation on the existing service and then a calculation of the additional load for the garage to determine if the present service is adequate. Then determine what size feeder is actually needed for the garage. You will also need to determine if a 3-wire feeder is acceptable or if a 4-wire feeder will be needed (see 250.32). Then determine what wire size is available in your area that will provide the ampacity you require and determne if you are going to use direct burial wire or install a conduit system. You will also need to determine the type of "grounding electrode system" that will be installed at the garage. There are no shortcuts. Nobody ever said that doing electrical work correctly would be easy. Don't try to skip steps. Do the calculations and then you can discuss facts, not opinions.
When I was a young electrician I bought a house that had a 150-amp electrical service. All of my "electrician friends" advised me that the "service was too small" and should be immediately replaced with a minimum of a 200-amp service. I was not having any problems, but did want to add a couple of circuits for which no spaces were available in the panel. We sat down and did a load calculation on the house and they were amazed to learn that the calculated load for the house was about 90-amps which meant the 150-amp service had plenty of additional capacity. We added a small sub-panel to give some additional circuits and 20+ years later the electrical system is still trouble free!
 
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