I have a 15amp circuit breaker from my main panel going to my garage
panel. The wire going to the panel to the garage is 10awg. I was
wondering if i could change that circuit breaker to a higher one. I
have a 220V heater that is 20 amps that keeps tripping the 15 amp
breaker on the main panel. Could I go to a 30amp breaker on the main
panel to the garage? On the garage panel there is 2 15amp circuits and
the 2pole 20amp circuit for the heater.
Probably. Naturally you will want to be certain there is nothing on the
circuit that can't handle 30a; though I can't think of what there could
You also have to figure out how long the cable is, what the load is, and the
voltage drop. Do a search on VD calculators.
If the only load on it is the heater and some lights, VD isn't all that
important. If you have motors or electronics, much more so.
In short, there could have been a good reason for the 15a breaker; make darn
sure there wasn't before changing it.
The reason of the 15 amp breaker on the main panel was that when they
put in the garage panel it only had the 2 15amp circuits (one for the
lights and one for the outlets). I got the electrican to put in the
240V 20amp circuit a few years later for the heater. So would i need a
30amp circuit breaker going to the garage or could i go with a 20amp
You have to consider what I said above. If you have a very long cable run,
you could get bad voltage drop on big loads. A 20a breaker will stop you
from having big loads, and thus prevent VD. If your run is short, or you
don't care about VD, then a 30a breaker will be fine.
Now, I am saying that without seeing the circuit! I can't think of any
reasons that would prevent you from using a 30a breaker, but that doesn't
mean there aren't any that someone actually looking at it would see.
Your electrician was pretty dim to put a 20a heater on a 15a circuit.
Perhaps he did other dim things. (mostly covering my ass here, but it is
A #10 wire is good for 30 Amps, though you would need #10/3 to get your 220v
for the heater. The breaker would be a 2 Pole Breaker (either a Double, or a
Quad with the inner/outer connected.
Like Toller mentioned. Make sure that the #10 from Breaker to Panel is the
only thing that connects the two panels together. Does the Garage have a
Main or is it wired into lugs at the top of the panel? Should be wired into
Lugs and the panel should be rated for the 30amps or better.
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.
Thanks Again for your help
Scott Townsend wrote:
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
from the 15A breaker is a feeder.
If you put this subpanel on a 30A DP breaker, your worst case for
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
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.
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