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[email protected]

wrote:
The wire from the main panel to teh garage is less than 100' ,
approx.75'. The wire from the main panel goes only to the garage
nothing else. With this length the voltage drop is is about 5 volts.
The electrician that put the 20amp circuit in the garage panel did not
know that the breaker in the house coming to the garage was only 15amps
I guess he just assumed that it would have been higher.

Stu:

First, I am not an electrician, just a homeowner who isn't afraid to
read the NEC manual when I need to. Take what I tell you with a
grain of salt, and confirm it with an experienced electrician.

Okay...the NEC does not *restrict* us to a maximum voltage drop (at
least for residential wiring) but does recommend a maximum of 3% on a
branch or feeder and 5% on branch & feeder combined, and leaves it up
to the AHJ, but let's go by their recommendation for fun. Your 10awg
run
from the 15A breaker is a feeder.

If you put this subpanel on a 30A DP breaker, your worst case for
voltage
drop would be a 30A 120V load, because 3% of 120V is less than 3% of
240V. All current would then be flowing out one hot conductor, and
back
down the neutral.

Voltage drop allowed:

F = Length of feeder ((hot length + neutral length)/2)
E = IR (Ohm's Law)

E = 3.6V (3 of 120V)
R = .0012 ohm/ft (resistance of 10AWG = 1.2 ohm/ 1000ft)
I = 30A (max current)

E = 30 * .0012 * (2*F) = 3.6
1/(30*.0012*2/3.6) = F = 50 feet

Your 10AWG 30A feeder should then be no more than 50 feet long
"for acceptable efficiency" when operating completely unbalanced
at its maximum load. Calculating the voltage drop for a 30A completely
balanced load (both breakers loaded to 30A) allows you to use 3% of
240V over the two *hot* conductors, ignoring the neutral (because
neutral currents would then cancel) and giving you a 7.2V acceptable
drop...you would then be allowed a 100 foot feeder.

So what about your 75' feeder? Well, since this is a garage, you can
probably cope with a bit of reduced efficiency. Furthermore, the garage
branches aren't going to be long unless this is Jay Leno's garage, so
there shouldn't be much voltage drop on them. If everything between
the main panel and garage subpanel is able to handle 30A (#10 or
larger) and no cowboy has hacked on a 15A circuit somewhere along
the feeder, and the garage panel is a proper subpanel as mentioned
before by other people, then you can safely put a 30A double-pole
breaker in to replace that 15A double-pole. But remember that the
electrician might have had a very good reason for doing what he did,
and make sure of what you have before changing anything.

Cordially yours:
G P