Circuit Breaker

• posted on November 8, 2006, 12:16 am

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.
Thanks Stuart
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• posted on November 8, 2006, 12:24 am

circuit that can't handle 30a; though I can't think of what there could be... 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.
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• posted on November 8, 2006, 3:36 am
Toller wrote:

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 breaker
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• posted on November 8, 2006, 4:07 am

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 possible...)
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• posted on November 8, 2006, 4:15 am
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.
Scott<-

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• posted on November 8, 2006, 4:38 am
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 Stuart Scott Townsend wrote:

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• posted on November 8, 2006, 3:24 pm
snipped-for-privacy@msn.com wrote:

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