Garage Feeder

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jeff43222

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Minneapolis, Minnesota
I have a customer who wants me to wire up his new detached garage, 43' from the house. The plan is to install a 100A subpanel in the garage, then code minimum for lights/receptacles for now.

Before copper prices spiked, my standard practice for a job like this was to lay PVC in a trench and run individual dual-rated THHN/THWN conductors for the feeder. But when I checked prices at the supply house, I discovered that the price of #2 XHHW Al is half that of #4 THHN/THWN Cu.

Is there any reason I should go with the copper rather than the aluminum?
 
Jeff, the real problem is, neither of the conductors you propose to use are large enough for a 100 amp feed to this garage. You'll have to protect it with a 90 amp breaker.

Roger
 
roger said:
Jeff, the real problem is, neither of the conductors you propose to use are large enough for a 100 amp feed to this garage.
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Good point.

As far as the AL I will just remind you of 250.120(B) which does not prevent using AL but suggests insulated is the way to go for an underground feeder.
 
I agree with Roger you'll need to adjust either your OCPD size or conductor size. For 100 amps with AL you would need a 1.25" Sch. 40 PVC conduit with #1 AL compact conductors.
 
Conduit size won't be a problem. I'm planning on using 2", mainly for the ease of pulling. I generally upsize conduit for that reason. I've had more than enough experience trying to pull wire through conduit that was barely legal.

As for the size of the wires, the first line of Table 310.15(B)(6) seems to indicate #4 Cu or #2 Al are legal for a 100A feeder. Or am I misinterpreting that since 310.15 mentions feeders to dwelling units?
 
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jeff43222 said:
As for the size of the wires, the first line of Table 310.15(B)(6) seems to indicate #4 Cu or #2 Al are legal for a 100A feeder.
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Yes it sure does....if your supplying a dwelling unit.:)

A detached garage is not a dwelling unit.
 
iwire said:
jeff43222 said:
As for the size of the wires, the first line of Table 310.15(B)(6) seems to indicate #4 Cu or #2 Al are legal for a 100A feeder.
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Yes it sure does....if your supplying a dwelling unit. :)

A detached garage is not a dwelling unit.
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Well, one of the potential uses envisioned by the HO is using the upstairs part of the garage for breeding/housing championship bulldogs, so maybe an argument could be made that the garage will be a dwelling unit... :D

Good catch, though. It seems my understanding of the "feeder" part of Table 310.15(B)(6) has been wrong all this time. Glad all the previous garages I've done have been under 100A, so I was consulting Table 310.16 for those.

Looks like #3 Cu or #1 Al is what I'd need.
 
iwire said:
Good point.

As far as the AL I will just remind you of 250.120(B) which does not prevent using AL but suggests insulated is the way to go for an underground feeder.
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I probably would have gone with an insulated EGC anyway, but thanks for the code reference.
 
jeff43222 said:
I probably would have gone with an insulated EGC anyway, but thanks for the code reference.
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I figured as much but it was the only semi-relevant code section I could think of for your installation.
 
jeff43222 said:
It seems my understanding of the "feeder" part of Table 310.15(B)(6) has been wrong all this time.
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If you had a meter main ahead of a panel T310.15(B)(6) would definitely apply, after that IMO it gets a bit murky as to what feeders can use that table.
 
In this case, I'm going to protect the feeder with a 100A breaker in the main panel. Not sure yet if I'm going to put a main in the subpanel (I'm leaning against it), as current plans only call for two circuits. I suppose a backfed main breaker could be installed at some point in the future if the garage winds up with more than six circuits.
 
you will need to install a main of some kind as the Garage this a seperate structure and requires a disconnecting means at the Garage. you will also have to drive a ground rod and establish a new grounding system.
 
Riograndeelectric said:
you will need to install a main of some kind as the Garage this a seperate structure and requires a disconnecting means at the Garage. you will also have to drive a ground rod and establish a new grounding system.
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There has been some debate as to whether or not 6 throws of the hand or less would require a main at a subpanel. He said that he's only using 2 breakers.
 
I'm pretty sure the "six-throw" rule would apply here. Initially the panel (most likely a Siemens 8/16) will only have two circuits in it, but it could conceivably have more than six someday, as there is talk of putting in an A/C, 240V receptacles for shop tools, probably some kind of electric heat, and whatever is needed to breed championship bulldogs (maybe a 16/24 panel would be a better idea). At that point, a backfed main could be installed. Personally, I prefer a main breaker in any panel (my "one-throw" policy), but I try to look at things from the customer's perspective, especially when it comes to spending their money. In cases like this, I tell the customer that a main breaker in the subpanel is optional, but it adds to the cost. The customer then makes the call.

I'm on top of the grounding. I'll be running an EGC to the subpanel and driving a ground rod.
 
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jeff43222 said:
I'm pretty sure the "six-throw" rule would apply here. Initially the panel (most likely a Siemens 8/16) will only have two circuits in it, but it could conceivably have more than six someday,
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The only issue is that the panel you chose must be listed as service equipment without a main breaker installed. 225.36

Some panels are some are not.
 
I suppose I could just put in a main breaker panel and be done with it.

I doubt cost will make much difference to this customer, as he has already spent over $2500 for me to upgrade his service and rough in a new bathroom. I'm also going to be doing a full-gut kitchen upgrade for him next month. The garage is just the latest project he's got me doing for him. By the time all is said and done, all the wiring in his house will probably have my fingerprints on it.

And I've already gotten to play with the new bulldog puppies. :D
 
Here is a sample of a panel label from Dave Nix.

408-14.jpg


If the label says this you will have to put in a main as it would be a lighting and appliance branch circuit panelboard.
 
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