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Author Topic: What size generator would I need to run something that needed 220v/30A?  (Read 2553 times)
pickpaul
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« on: October 18, 2011, 11:35:37 PM »

And what if I wanted to run A/C too?
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robertglines1
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« Reply #1 on: October 19, 2011, 04:22:14 AM »

Volts X Amps = Watts     That said a 15000 roof top will reqiuire up to 20 amps to start and 15 amps to run at 120 volts.   so what ever you want to run find out highest amperage need (peak @ startup) X voltage= wattage
220x30=



« Last Edit: October 19, 2011, 05:12:51 AM by robertglines1 » Logged

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Oonrahnjay
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« Reply #2 on: October 19, 2011, 05:22:44 AM »

    Yeah, Paul.  You're looking at some detailed figuring here.  When you say "needed 220v/30A", do you mean 30A of actual calculated current draw or is that a spec for a circuit?  If you have, a 30A circuit, you probably shouldn't have more than about 22-24A actual current draw on it (it will vary a bit depending on type of load).  And do you have a number of small loads that make up that current draw?  If so, will any of the loads have "startup" power draws?  Will you have any arrangements where one load will be locked out while you're using another?  Also, will your set up "split the legs" of the 220v supply for two 110v circuits?  If so, you'll need to arrange them to provide a balanced current draw and calculate those loads into your total.

    What you need to do is figure exactly what your real loads are (including those startup loads) and arrive at a total figure in watts. Once you're there, add approx. 20-25% for your circuit size and breaker specification.  Once you're there, look at the generator that you're considering using.  You don't want to load a generator to the max all the time (i.e. if you have lights, refrigeration, battery charging, air conditioning, and water heater that add up to 6.8 KWatts, you wouldn't want to specify a 7.0 generator.  On the other hand, if your total load is about 4 KWatts, you wouldn't want a 12 KWatt generator (many people have experienced problems like carboning up, rapid ring wear, speed control issues and other problems on a generator that's never run on more than "lightly loaded" conditions). 

    To go back to your original question, if you're saying that someone has previously installed a 30A service in your bus but the real max/ usual load is about 22A, that you're probably appropriately sized in terms of that circuit (i.e. about a 4400 Watt load) for a 6.5 or 7 Kwatt generator.

    As Bob said, you have to specify the total real current loads, figure them into wattage requirements, and then calculate the then select a generator using the "not too heavily loaded but not too lightly loaded" rules of thumb.

    (BTW a real load of 30A at 220V would be 6600 Watts but, as explained above, a 7 KWatt (i.e a 7000 Watt) generator would probably be marginal for this use.)

     But the real answer here is to not use any guesses that we may make here -- get some actual calculations and figure from there.
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Bruce H; Wallace (near Wilmington) NC
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« Reply #3 on: October 20, 2011, 04:19:34 PM »

I was hoping that someone would discuss the other issue with running a generator set up for 220V -- load balance.  In a 220V system, you have to make sure the two legs are in somewhat close balance.  I found the following in a forum that explains it pretty well:

Quote
In a perfect world with ideal settings your neutral conductor should carry no current. That is to say if one of your phases is carrying 20 amps and the other phase 20 amps as well, the neutral would carry "0" amps. However, if one phase is carrying 10 and the other 20, the neutral would carry the difference (the imbalance) of 10 amps.

Avoiding the imbalance as things turn on and off is of no concern so long as the imbalance will never create a scenario whereby the neutral is asked to carry more current than it is rated for.

The big issue is that we don't know the rating for the neutral in our generators.

At every one of Dick Wright's seminars he was clear that he did not think that any generator less than 12K should be wired for 220V.  He and I had a lot of discussions about the subject, since I have a 10KW and need the 220V for our dryer.  I have ammeters for each leg and am pretty careful to balance loads.  In my case, that is a bit of a challenge as one leg has a large squirrel cage blower for the generator remote radiator and that does not show up on the ammeter due to the wiring layout.

Jim
« Last Edit: October 20, 2011, 07:00:52 PM by rv_safetyman » Logged

Jim Shepherd
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Oonrahnjay
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« Reply #4 on: October 20, 2011, 09:36:48 PM »

  I was hoping that someone would discuss the other issue with running a generator set up for 220V -- load balance.  In a 220V system, you have to make sure the two legs are in somewhat close balance.  (snip) 

    That's what I meant when i said "Also, will your set up "split the legs" of the 220v supply for two 110v circuits?  If so, you'll need to arrange them to provide a balanced current draw and calculate those loads into your total."   Thanks for making that really clear and emphasizing it (I'm sorry that I didn't).

    And thanks for pointing out that there is a neutral limitation *within* the generator.  No matter how perfectly you pick wire sizes and breakers in the bus, if your two legs are off-balance, you're putting all that load into the neutral inside the generator.   It's obvious when you state it, but it was something I didn't know - thanks.
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Bruce H; Wallace (near Wilmington) NC
1976 Daimler (British) Double-Decker Bus; 34' long
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« Reply #5 on: October 20, 2011, 10:23:23 PM »

It looks like you need a 10 KW plant. As the others said, wire it for 120, if possible. And it should start your AC without difficulty.

Good luck!

Tom Caffrey
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Tom Caffrey PD4106-2576
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robertglines1
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« Reply #6 on: October 21, 2011, 05:31:56 AM »

Just helped a member out (not Phil) who was on 3rd gen set. Apparently original  installer only took the 120 off one side of gen set letting the other side idle. Result off balance destroyed gen sets. Other line (leg) was taped off.  Just food for thought. If you wish 120 volt service panel only do your home work. It is really simple once you follow it thru and you will use both sides of gen set.(if so equipped); as most larger gen set are.  As a rule of thumb if you have 2 breakers or a 220 plug your generator has 2 lines producing 120 volts each.  General observations only.   Bob 
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« Reply #7 on: October 21, 2011, 05:51:30 AM »

I think you guys are undersizing it.  220v 30A load for the unknown main load, 120v 20A load on one side for the AC, various other loads mostly balanced on the side the AC is not on, probably a second  AC to be honest, I see a 60A load per side with headroom, and that is 14.4 KW.  Power management will pull that down, plus knowing what the loads are, but if you ever wanted to run both AC's while you were drying your clothes with that home style dryer, watch TV, have a few lights on, cook dinner in the microwave, keep the drinks cold in the fridge, you'd need every bit of it.

It's not knowing what the loads are, or the anticipated usage, that makes it hard.  But the AC plus the 30 amp 220 volt load is the killer.

Brian
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« Reply #8 on: October 21, 2011, 02:40:41 PM »

One other factor that folks forget about is de-rating for altitude.  Both the engine and generator are de-rated for altitude.  If you are marginal, and try camping at something like 5K feet, you could be in trouble.

Jim
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Jim Shepherd
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« Reply #9 on: October 21, 2011, 05:00:26 PM »

The way Powertech on the 8KW model does 120 volt is it still uses the same two hot wires, but they are tied together in the generator instead of separate like when it is configured for 240 volt.  When I converted my generator to 120 volt I left the two hot wires still going to the 50 amp transfer switch and the transfer switch still works fine.
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Brian Elfert - 1995 Dina Viaggio 1000 Series 60/B500 - 75% done but usable - Minneapolis, MN
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