Do I need 50Amp Generator?

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Old 01-11-07, 12:26 PM
J
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Do I need 50Amp Generator?

During a power outage in December (11 days), I had a borrowed Briggs & Stratton 5550 watt generator (8,550 Starting Watts). I would plug the 30amp 220V outlet into the home's transfer box. Worked fine. But our well house is on it's own branch about 250' away so we had no water. So as a TEMPORARY solution I picked up a 30 amp 220 breaker and put it into the well house's panel and wheeled the generator down there and plugged it into the panel's new 30 amp breaker (after throwing the main off). It would run the well pump fine. BUT... on the well house's panel is also the breakers for the septic and sand filter pumps (20 amp 120 V each). It would run the septic but NOT the sand filter pump (I ran each alone). When trying only the sand filter pump, the generator would lug down a bit for about 5 seconds and then throw it's breaker. I'm assuming the pump's starting current is above 30amps for a bit and would therefore throw the generators breaker (but NOT the 20amp breaker in the panel which I assume is more forgiving). I've checked with others who use the same generator (a common Home Depot generator around here) and they also could not run septic pumps (all on 20amp 120 breakers!). Most were surprised since 20 amps at 120 is not even close to the rating of this generator.

So anyway, does this mean that I require a generator with a 50amp plug? Thoughts would be appreciated as I'm now looking into a new generator (and will hook it up to the well house properly!).

Thank you!
 
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Old 01-11-07, 03:04 PM
I
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Do you know what the HP of the pump motors are? The breaker size for pump motors is only part of the story, as breakers for motors are usually oversized.

The generator definitely can't handle the startup current of the sand pump motor as you found out. The only solution to that is to get a bigger generator or a smaller pump. I suspect that part of the problem may be that the motor runs on 120V instead of 240V which places a heavy unbalanced load on the generator making it difficult for the generator to maintain stable voltage and frequency.
 
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Old 01-11-07, 03:54 PM
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No, I don't know what the HP of the pump is. And without pulling it up I have no way to find out. I never thought about it simply being a heavy off-balanced load. I've seen some generators that have a switch to provide "full power to 120". Maybe this is to address that? It always seemed odd to me... why wouldn't you ever want "full power". The literature never really says much about it to understand what it is doing.

When you say "get a bigger generator"... I guess this is where I'm confused. If you are still using only a 30A 220 connection out of the generator, aren't you still going to be limited to 30A no matter how big the generator? Or would a larger generator tolerate more than 30A on the 30A plug for startup? You would think that would be the case now. Since I know the 20A 120V service breaker does not trip, even if the pump drew twice that for start up... 40A at 120 is not even close to the generator's peak rating. That is why I was thinking that it was a limitation of the 30A plug on the generator and that a 50A was needed. Of course, my thinking could be off base!
 
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Old 01-11-07, 07:36 PM
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> switch to provide "full power to 120". Maybe this is to address that?

That's what it's for. Running only 120V load is like carrying a 5 gallon bucket of water with just one hand; it's not that much weight, but trying to maintain balance makes the job harder. It's actually easier to carry one in each hand even though the weight is doubled. That full power switch reconfigures the windings in the generator to optimize for 120V load instead of 240V load.

> If you are still using only a 30A 220 connection out of the generator,
> aren't you still going to be limited to 30A no matter how big the generator?

No -- the size of the plug and breaker is only important for running current; any standard plug can carry huge startup current for a short time. When it comes to starting motors, the important part is the inertia of the generator and its ability to quickly regain voltage control. Both of these are functions of the generator's physical size and HP of the engine.

> even if the pump drew twice that for start up... 40A at 120 is not even
> close to the generator's peak rating.

Some motors draw startup current that is several times their running current; especially if the pump is starting under head pressure.

40A actually exceeds the generator rating. 8,550W @ 240V means that each 120V leg can provide 4,275W, which at an optimistic 120V is 35A of startup current to a 120V motor.
 
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