Generator Amp Question

[reid9439]

Can someone help me with generator amperage?

I have a 7,500 watt Generac diesel generator, with a 30amp 2-pole breaker. My understanding of how to figure amps, is to divide the watts by volts. If that is correct, this would equate to approximately 62 amps.

To a novice, you would think that the generator is producing more amperage than you would get from 50amp shore power. Wrong. I found this out while we had an 8 day power outage after hurricane Irene. This is when I found out that the generator is belt driven when it squealed during an overload. Didn't know.

Why do you need a 62amp generator that has a 30amp breaker. Isn't this overkill?

[wolfe10]

First, welcome to the FMCA Forum.

Let's look at the 50 amp shore power first: You have TWO 50 amp hots, so you have 100 amps total @ 120 VAC available. If you look closely at the breakers in both the CG outlet box and your coach main 120 VAC breaker panel, you will find two 50 amp breakers pinned together.

A generator that has two 30 amp breakers can supply 30 amps PER LEG or 60 amps total.

But 60 (generator) is indeed less than 100 (50 amp shore power).

Brett

[reid9439]

Thanks. That answered my question.

Again thanks.

[hanko]

Wolfe, you are totally incorrect on your explanation. A 50 amp service is actually a 240 volt 4 wire system, common grounded conductor (neutral)which is isolated from the grounding conductor. A common trip pin on the breaker is required because its is infact a 220,230 volt service. The hots as you refer to are properly called ungrounded conductors. you do not combine the individual poles, two 50 amp breakers equal 100 amps.

[wolfe10]

Hanko,

Not sure of the difference in our explanations. Are there different ways to say the same thing-- of course. Been wiring these things for 30 years and it sounds like you have "been there and done that" too.

We agree there are two hots, a neutral and a ground. A voltmeter between the two hots shows 240 VAC. A voltmeter between either hot and neutral (or either hot and ground) shows 120 VAC. Very similar to that in most residential boxes. As you say, the two hots have a common trip pin (called a duplex breaker). And indeed you DO have 100 amps @ 120 VAC. And with few exceptions, RV's are wired with appliances using only one hot, the neutral and ground (just like the lights, outlets, etc in your stick home). Could you have a 240 VAC appliance-- yes, just as in your stick house. In fact, I just pulled a new 40 amp service to a new stove in the house. 4 wire duplex 40 amp breaker, etc.

Looking at the 50 amp RV outlet:

Both outer straights are the HOTS (generally one BLACK and one RED).

Center straight is the NEUTRAL (WHITE).

Center round is the GROUND (GREEN).

Outer straight to outer straight= 240 VAC

Either outer straight to center straight= 120 VAC

Either outer straight to center round= 120 VAC

Center straight to center round- 0 VAC

[hanko]

Yes your correct, i guess i explained it the complicated way. I was trying to explain to my neighbor that his new coach came with a 5.5 KV generator. He was upset because his old one was 7KV. I tried to explain that the the new one 5.5 at 230 volts versus his old one 7 at 120 volts. didnt make sense to him.

[Guest BillAdams]

For those confused, Brett was NOT totally incorrect on his explanation. Quite the opposite was true.

[wolfe10]

Yes your correct, i guess i explained it the complicated way. I was trying to explain to my neighbor that his new coach came with a 5.5 KV generator. He was upset because his old one was 7KV. I tried to explain that the the new one 5.5 at 230 volts versus his old one 7 at 120 volts. didn't make sense to him.

Curious, what RV generator is rated at 5.5 KV @ 230 VAC? Or even 5.5 KW @ 230 VAC? Unusual ratings for an RV generator.

[hanko]

If you have a 50 amp service on your coach, we have already agreed that it is in fact a 230 volt 4 wire system. The generator has to supply the coach with the same voltage as shore power. so the gen is rated at 5.5 kv at 230 volts. 5500 watts is 5.5 kv, but I'm sure you know that.

[wolfe10]

Looking at Onan's website for 5.5kW gasoline generators :http://www.cumminsonan.com/rv/products/gasoline#tab1

I could find none that are rated for 230 VAC. All are 120 VAC.

Even with larger generators, being rated at 230/240 (i.e. two hots) raises a real issue-- the voltage regulators can control voltage very well on ONE leg of the generator output. So, the other leg is either high (if load is less on the second leg than that on the regulated leg) or low (if load on that second leg is higher than that on the regulated leg).

In fact there are quite a few discussions on 10kW and larger generators on the advisability of changing from the motorhome factory setup of 120 VAC to 240 which the generator is capable of putting out (again with the problem that the "second leg" far less voltage stable than the one that is regulated).

[hanko]

OK, then what your saying is when the generator fires up its only feeding one side of the line in the circuit panel? You stated before that there are two hots feeding the panel from let say the pedestal at the camp ground. That is if its a 50 amp 230 volt service, correct? So how does when you disconnect shore power and start the gen if only producing 120 volts, does it energize both sides of the circuit panel?

I'm sure we'll get the stright sooner or later. Not to throw my hat in the ring, but I am a 35 year master electrician, and an electrical contractor. I just didnt want you to think I didnt know whjat I was talking about. **** maybe I dont. You have Motor home if Im correct? take your panel cover off, start your generator and measure your voltage at the main lugs in the panel or at the main whatever it might be. Possibly at the transfer switch it combines both ungrounded conductors (hots as Wolfe call em) sort of like a 50amp to 30 adapter?

[hanko]

I assume you have a 50 amp service in your coach

[wolfe10]

Hanko,

The 120 VAC generator provides power to both sides of the 50 amp coach breaker box just exactly as does the 30 amp male to 50 amp female adapter when using 30 amp shore power.

Said another way, the single 120 VAC hot it tied to BOTH hots on the 50 amp side.

Brett

[hanko]

Oh, see I didn't know that. That's funny, why don't they put 230 generators in motor homes equal to what shore power would b?

How do they get the 120 generator to feed both hot conductors and then isolate them when your using shore power?

[wolfe10]

OK, for those who want a "blow by blow" description of a 50 amp RV with 120 VAC output generator.

The 50 amp shore power is wired/works as discussed above. EXCEPT, insert an ATS (Automatic Transfer Switch) between the shore power cord inlet to the RV and the 120 VAC RV breaker box. Most but not all ATS's are wires so that in the absence of power from the generator, the ATS connects the two hots and neutral of the 50 amp shore power to the 120 VAC RV breaker box. Ground is always connected.

If 30 amp shore power, in the 30 amp male to 50 amp female adapter, the single hot of the shore power end connects to both hots in the male (coach) end of the 50 amp (4 prong) shore power cord.

When the 120 VAC generator starts, most ATS's are wired so that it takes precedence over shore power. Said another way, the ATS ONLY allows one source of 120 VAC. If the generator is started, most coaches switch the full load from shore to generator. Yes, it could be wired the other way, but NEVER are both shore power and generator allowed to provide power to an RV (no way to synchronize frequency).

With a 120 VAC generator, it works exactly the same as the 30 amp male to 50 amp female-- the single hot from the generator connects to both hot leads on the ATS generator "IN" side.

Really pretty simple. Been working for decades like that.

Brett

[hanko]

Well Brett, I'm not an expert on Rv's. although I've had a few, they all had 30 amp 120 volt systems. Your still referring to the breaker panel in a coach with a 50 amp service panel as a 120 volt system when its not. Its a 230 volt system with a common grounded conductor. With a perfectly balanced system meaning exactly the same current on both ungrounded conductors (sorry I cant say hots as you refer to them) you have 0 current on the neutral. but the ungrounded wires must be on opposite sides of the line meaning a 230 volt potential between them. If not then the neutral will be over loaded. I can tell you one thing for sure, Im going to General RV ASAP to find out myself.

[wolfe10]

Hankso,

I think you are letting your particular terms get in the way of what we are both agreeing on. If you like, let's continue this conversation as Private Messages-- not sure we are adding anything to the general group knowledge here.

Go back to my first post (or anyone of them for that matter). We BOTH agree that a 50 amp has two hots, neutral and ground-- call them what you want.

And with the two hots reading 240 between them, indeed the neutral will only carry the DIFFERENCE in load on L1 vs L2.

Terms may be different than the typical house service, but the electrical set up is virtually the same.

[hanko]

At least we are giving everyone else a good education.

[hermanmullins]

I'm so confused!!!!

My coach is wired for 50 amp service. It will operate on 30 amp service also. But I can operate only 1 A/C at a time.

I beleive each of you are saying the same thing but putting it in different words.

[wolfe10]

I'm so confused!!!!

My coach is wired for 50 amp service. It will operate on 30 amp service also. But I can operate only 1 A/C at a time.

I believe each of you are saying the same thing but putting it in different words.

Herman,

Yes, first, I agree that there is not 1% difference in what is being said (the exception being the generator output). Electrically we are all in agreement. But coming from a residential or commercial wiring background, hanko could well use different terms to say the same thing. Not better or worse, but certainly confusing to those not really familiar with discussing 120 VAC wiring in an RV application.

I know and apologize for the confusion it has caused others. That is why, in my post last night, I requested that further discussions by hanko on this subject be done by private message to me. I don't mind further discussions and will be glad to invest my time. BUT, I don't want everyone else to leave more confused than clarified.

In fact, I am going to close this thread. Please feel free to start a new thread if there are other electrical questions.

Brett