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cscott | 03-12-2009 12:46 PM
Electrical question... To any electricians in the house... Does a 30 amp double pole breaker fault (open) each leg at 15 amps or is each leg switched at 30 amps? The output of my portable generator is 30 amps though a L14-30 receptacle, and I think it has a built in breaker on each of the 120v circuits at 15 amps, and if I were to run it to a double pole breaker would I need a 15 or 30 amp breaker? ---- C. Scott

Fullpower | 03-12-2009 12:48 PM
Re: Electrical question... 30 amps EACH leg. The specific breaker in your panel ( house) protects the WIRING in the house from overcurrent/ overheating. 12 gauge wire--- 20 amp breaker 10 gauge wire--- 30 amp breaker 8 gauge wire --- 40 amps 6 gauge ---- 50 amps 2 gauge --- 100 amps . . As you noted, the Generator set has its own overload protection device, which has NO bearing on your house wiring protection device (s).

cscott | 03-12-2009 12:51 PM
Re: Electrical question... Ok, thanks. :coolbeer:

BellevueBully | 03-12-2009 01:28 PM
Re: Electrical question... i assume you are trying to back feed your panel buses from a genset. If you do this, make damn sure you isolate the utility with the main.

cscott | 03-12-2009 01:38 PM
Re: Electrical question... Yes I am, I have a main disconnect to turn off incoming power from the utility company. I'm building a poor man's transfer switch. Based on the breaker being 15 amps each leg I can get a 12/4 extension wire instead of the 10/4 I was planning on. I guess I could get the 10/4 just to be safe but it is a little more expensive.

Tn...Andy | 03-12-2009 02:40 PM
Re: Electrical question... IF you're coming off the 30 amp plug on the generator ( I assume it's 220-240v ), you'd need 10/3......two hots and a ground.....if feeding something that uses 220v, AND has no 120v use inside it ( for example, a dryer has a 220v feed, but by code, you must run 10/4 to it....2 hot, 1 neutral, 1 ground.....because inside the dryer, one leg of the 220 is tapped to run a 120v motor to turn the drum and it must have a neutral that is NOT the ground ) IF you're running the generator to a panel, you'd most likely also want 10/4 so you can feed the two bus bars in the panel with a hot each, have a neutral so you could run 120v breakers off either bus bar, and a ground. You would come out of the generator plug and feed into a 30amp double pole breaker in the subpanel.....and as BB said.....make DANG SURE you can isolate that panel from your normal entry service so you don't back feed the main line.

BellevueBully | 03-12-2009 02:43 PM
Re: Electrical question... I'm a little confused. You say you have 240Vac and 30A output from the gen. The extension cable which you plug in to the gen to provide feedback power to your panel must be able to handle 30A. That will allow 30 amps to flow to your panel through a 10gua. wire (30 A rating) and will be protected from a 2 pole 30 A breaker mounted on the panel. This only protects the wire between the gen and panel. (You might even have a 30A o/p breaker on the gen, dunno) Once the power is in the panel the individual cct breakers will protect the individual ccts, as they are in normal service. How are you building the switch??

BellevueBully | 03-12-2009 02:47 PM
Re: Electrical question... Quote:
Originally Posted by Tn...Andy (Post 1621759) and as BB said.....make DANG SURE you can isolate that panel from your normal entry service so you don't back feed the main line.
TnA, Theoretically, what do you think would happen if no transfer took place and utility and local generation went p to p? -gen fry? -gen blow up? -gen start spin like a motor? The utility is grounded and the gen is not so I suppose a lot would have to do with how high or low the ungrounded supply was. Do you know how many cylces is suggested for switching?

cscott | 03-12-2009 02:57 PM
Re: Electrical question... Quote:
Originally Posted by BellevueBully (Post 1621764) I'm a little confused. You say you have 240Vac and 30A output from the gen. The extension cable which you plug in to the gen to provide feedback power to your panel must be able to handle 30A. That will allow 30 amps to flow to your panel through a 10gua. wire (30 A rating) and will be protected from a 2 pole 30 A breaker mounted on the panel. This only protects the wire between the gen and panel. (You might even have a 30A o/p breaker on the gen, dunno) Once the power is in the panel the individual cct breakers will protect the individual ccts, as they are in normal service. How are you building the switch??
I know this is pretty confusing, let me try and explain the best I can. I have a small 3500W generator that has a L14-30 receptacle on it, and based on my memory (I'm at work so I can't look this up right away) I think each hot wire has a built in 15 amp reset-able breaker. Which is why I was asking the question about the a 15 or 30 amp breaker for my house breaker box. If the generator switches each leg at 15 amps, then I would think I would use a 15 amp breaker in my box, as long as my wire size is big enough to handle the 15 amps. So what I'm planning on doing is connecting the two hot wires to my newly installed 15 amp double pole breaker (or 30 double pole if that is what is required), and connect the ground and the neutral to their respective bars i my breaker box. I'm not actually building a "transfer switch" per say, what I'm planning on doing is switching off the main disconnect to my panel, turn off all the CBs to the individual house circuits, turn on my new generator powered breaker, and then turn on the individual circuits that I want to power. Do you guys think I'm wasting my time with this project? edit... The question about the wire size is, since each leg can only run 15 amps each then I think I would get away with 12/4 wire instead of 10/4.

TTAZZMAN | 03-12-2009 03:10 PM
Re: Electrical question... I cringe to state this.....but in the olden days before it was illegal...we back fed the pannel with a double ended male cord through the dryer plug in emergencys but...yah gotta gotta gotta disconnect the mains so you dont kill a lineman......and anytime you handle a hot double ended male cord its dangerous so you must NOT handle it hot. I am only stating this because it is essentially what your attemting to do the hard way and also illegal The RIGHT Legal way is to buy a transfer switch setup and plug so you have no possibility of killing a lineman, cause even if you do things right and your neighbor kills the lineman if they find you hooked up wrong chit will hit the fan.

Tn...Andy | 03-12-2009 03:45 PM
Re: Electrical question... You're not wasting your time....I've done it.....but pay attention to what Tazz said. And you need a 30amp DP breaker on the panel with #10 wire. BB: If the local utility and your generator went P to P, I suspect nothing would happen....you'd simply have plenty of power. :biggrin:

Dave | 03-12-2009 03:57 PM
Re: Electrical question... Quote:
Originally Posted by Tn...Andy (Post 1621856) BB: If the local utility and your generator went P to P, I suspect nothing would happen....you'd simply have plenty of power. :biggrin:
Besides, BB, why would you want to contribute to the grid and not get paid for it? ;) BTW: OP - I have done this and it is pretty easy to do. I have a 10/4 running to my box on a 30A double pole breaker which I leave off for safety reasons and have labeled. The 10/4 runs to an outside rated receptacle which I have mounted near the roof line (just in case I left it on, my kids couldn't reach it). I take a heavy duty extension cord designed for my generator and hook it up to my outside receptacle. This way I can easily hook it up and unhook it without much fuss. I even have a diagram for the wife if she needs to do it on her own. Just remember your steps to first turn off the main before anything. Dave

maddyn99 | 03-12-2009 09:07 PM
Re: Electrical question... Please make sure you turn off the main breaker at the very least if your not using a manual transfer switch. http://www.generatorjoe.net/html/whataboutsws.html Transmission lines use a step down transformer, if you dont and backfeed current the transformer steps the the power UP...and makes it very dangerous and deadly for linemen that are working on the line. Having said that. Like its been said before use a #10 wire to be safe and turn off the main breaker.

TechGuy | 03-12-2009 09:23 PM
Re: Electrical question... Quote:
Originally Posted by maddyn99 (Post 1622392) Please make sure you turn off the main breaker at the very least if your not using a manual transfer switch.
Or just pull the meter to end all doubt.

G.Gordon | 03-12-2009 10:08 PM
Re: Electrical question... I feed in my generator using a 50 amp stove outlet, running back to the box with 8-3 wire. The generator to outlet cord is also 8 gauge. This allows for corrosion or other gremlins. Thus the 30 amp breaker is the weakest link and will pop well before I have any problems elsewhere. I don't want to burn down my garage by turning my generator cord into a toaster wire. Going to a larger wire is relatively cheap for a short length and you can still use it if you upgrade to a larger generator in the future.

TTAZZMAN | 03-12-2009 10:24 PM
Re: Electrical question... generator cord should be sized for amps and length i use a 6/3 seow wire for 50ft 30amp 240 (i figure it is oversized but safe)(6/3 is hard to wire into the plugs so shorter and 8/3 would be more practical)

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