Generator

Don in LA · Feb 25, 2009

I have a gasoline powered 7.5kw, 13.5kw peak, generator with 110V and 220V outlets. The 220V outlet is rated for 20 amps. Can I rewire it to accomadate a 30 or 40 amp outlet? I want to use it to wire into my service panel to backfeed to power my central AC unit (20 running amps, 220V) or my camper which uses a 30 amp, 110v outlet. I've used the generator to backfeed into the panel and feed the camper but if I use anything, even a small TV, it trips the 20 amp breaker on the generator. The breaker that I used to backfeed through is a 220V, 30 amp breaker.

Thanks,

Don

Travelover · Feb 25, 2009

Does the generator have a GFCI? Some of them have to be modified to attach to a whole house circuit without tripping the GFCI.

Also what load does your AC pull during start up? It may be too much for this generator.

George2615 · Feb 25, 2009

I'm sure the 220V 20A generator breaker could be changed to a 30A but I doubt the generator would handle very long it internally.
The generator was apparently factory designed to have a 20A breaker. Unless the generator and all internal wiring to the breaker and then to the plug is rated for at least 30A then changing it is looking for trouble.

My 10KW welder/generator has a 50A plug with 50A breakers that I use to backfeed my house panel but it is internally wired for that rating.

Dcara · Feb 26, 2009

George2615 said:
I'm sure the 220V 20A generator breaker could be changed to a 30A but I doubt the generator would handle very long it internally.
The generator was apparently factory designed to have a 20A breaker. Unless the generator and all internal wiring to the breaker and then to the plug is rated for at least 30A then changing it is looking for trouble.

My 10KW welder/generator has a 50A plug with 50A breakers that I use to backfeed my house panel but it is internally wired for that rating.

Do you have any 220/240in your house? I've been thinking of back feeding my house but the utility wiring to the house (3 wire) is actually two wires of 120V and a ground. However, since the two 120V wires are 180 degrees out of phase you measure 240V across them, but 120V from each wire to the ground wire. Generators on the other hand may have a single phase at 220/240V (not sure). To back feed 220/240 to a house I think you may have to feed 220/240 to the 220/240 ckts a little differently.

Don in LA · Feb 26, 2009

Travelover,George, Dcara: Thanks for the replies. The Gen. itself does not have a GFCI. There are GFCI circuits in the house. My AC draws 18 running amps but I don't know what the startup amps are and I'm not sure how to determine that. From what I've heard in the past, startup amps can be from 1.5 to 2 times the running amps. Dividing my running watts (7,500) by volts
(220) gives me 34 amps. Do you think that would overtax the generator? The generator has the 20 amp/220 circuit, with a 4 prong twist lock receptacle. I was using this to backfeed into my service panel using a 220v/30 amp breaker in the panel. Before connecting to the panel I disconnected the house wires feeding into the breaker and I disconnected the panel main breaker. The Gen. also has two 15 amp/110v receptacles and seperate breakers. I would like to find out the possibilities but I sure don't want to mess up my AC or generator. During the last power outage, I just powered a couple of window AC units. Maybe I should just stick with that.

Thanks again,

Don

Travelover · Feb 26, 2009

Don, you might be able to gauge the start up amperage of the AC unit with a clamp on AC ammeter. I'm a mechanical engineer, so I know just enough about electricity to get the nickname "Sparky". I'm betting the generator is too small for the task.

An example of a clamp on ammeter: Clamp-On AC Ammeter - RadioShack.com

The fact that the breaker trips with a very small load in the trailer is puzzling. Unless there is a short or another large load that is also coming on, it doesn't make sense.

I'd suggest that you do a forum search here and learn about how others have safely attached generators with transfer switches or breaker box lockouts.

George2615 · Feb 26, 2009

Don in LA said:
Travelover,George, Dcara: Thanks for the replies. The Gen. itself does not have a GFCI. There are GFCI circuits in the house. My AC draws 18 running amps but I don't know what the startup amps are and I'm not sure how to determine that. From what I've heard in the past, startup amps can be from 1.5 to 2 times the running amps. Dividing my running watts (7,500) by volts
(220) gives me 34 amps. Do you think that would overtax the generator? The generator has the 20 amp/220 circuit, with a 4 prong twist lock receptacle. I was using this to backfeed into my service panel using a 220v/30 amp breaker in the panel. Before connecting to the panel I disconnected the house wires feeding into the breaker and I disconnected the panel main breaker. The Gen. also has two 15 amp/110v receptacles and seperate breakers. I would like to find out the possibilities but I sure don't want to mess up my AC or generator. During the last power outage, I just powered a couple of window AC units. Maybe I should just stick with that.

Thanks again,

Don

Don, As I stated previously my 10KW generator handles my entire house. You state your generator has a continuous duty cycle of 7500 watts and a surge of 13,500. You may get by with one window A/C but maybe not two. If A/C running amps are 18 then A/C startup amp surge will be greater than your 20A breaker. If you figure surge wattage of 13,500 divided by 220V then your up to 56.25A. The weak point is your generator's 20A 220V breaker.

Dcara said:
Do you have any 220/240in your house? I've been thinking of back feeding my house but the utility wiring to the house (3 wire) is actually two wires of 120V and a ground. However, since the two 120V wires are 180 degrees out of phase you measure 240V across them, but 120V from each wire to the ground wire. Generators on the other hand may have a single phase at 220/240V (not sure). To back feed 220/240 to a house I think you may have to feed 220/240 to the 220/240 ckts a little differently.

Dcara, Yes I have 220V in my house. I have a main disconnect (Large fused knife switch) where the power comes in to totally disconnect me from the utility co. My generator then backfeeds into a 220V 50A breaker. Two hot leads plus a ground and a neutral total of 4 wires. Each hot lead of the 50A house breaker is on a separate phase in the panel.

Don in LA · Feb 26, 2009

Travelover,

Thanks again. I'll do some more research. I know just enough about electricity to know that I don't know too much.

Don

BobRip · Feb 26, 2009

There is very little chance that your generator will start an AC unit with 18 or 20 amps running current. The starting currents can be six times the running current. Plus the generator just does not have good surge capability. It is not just the breaker, but engine, generator head, wiring, ect. It might run a small window unit. I just forget about AC with the power out. Get a window fan. AC is not critical. Run you refrigators, freezer, lights, TV, Radio and cook on your grill.

RonMar · Feb 26, 2009

That rateing of 7.5KW with a surge of 13.5KW dosn't sound right to me, especially with a 20A breaker in the generator 240 circuit. 20A X 240VAC is only 4.4KW. A generator manufacturer selling a genset with a 100% surge rateing is probably using some fiction in it's marketing numbers. A more realistic surge number for a 7.5KW would be in the area of 9KW. What size and HP engine does it have on it? What make/model is it, perhaps I can find some more specific information about it online that may help answer your questions.

Dcara
120/240 generators are usually a center tapped winding. Each end of the winding to the center tap is 120VAC. From one end of the winding to the other is the full 240VAC. Because each 120V leg only encompases half of the generator stator windings, each 120V leg can only provide for 1/2 of the rated generator capacity. IE: A 5KW generator could power 2.5KW worth of load on each 120V leg. It could power a single 5KW 240V load as the 240V winding encompasses all of the stator windings. This center tapped winding is the same configuration as the power reaching most US homes. The transformer out on the power pole steps the mainline voltage down to a center tapped transformer winding. Each end of the winding measured to the center tap is 120V. The end to end voltage is 240V. The center tap of the winding is the neutral. Code requires that this neutral be bonded to ground in many places, particularly the main panel, but it is not ground. Ground is a safety and a backup for the neutral because bad things can happen when the neutral becomes disconnected and you loose that 120V path back to the center tap of the winding. A generator with a 240 output, 4 wires, 2hot, 1neutral, 1ground) can be setup to backfeed into a main panel. The two hots go to the two poles of a 240V breaker. the neutral goes to the neutral bar in the panel and the ground goes to the ground bar in the panel(same connection bar as the neutral). To do this safely and legally, the backfeed breaker and the panel main breaker should be physically interlocked so that it is physically impossible for both these breakers to be turned on at the same time.

Don
That 18A running A/C load probably peaks close to 50A during startup. How large of a circuit breaker is this A?C unit on in your main panel?

I am a little confused about the description of your camper hookup. You said the camper has a 120V 30A plug. Yet you said you hooked it up to the 20A 240V generator output? How exactly did you accomplish this?