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Old 11-04-2012, 03:21 PM   #1
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Generator Size


I want to purchase a generator to run 2 sump pumps in my house. I am choosing between two sizes:

Smaller: 3500w continuous / 4000w starting
http://www.amazon.com/Champion-Equip...r+remote+start

Larger: 5500w continuous / 6875 starting
http://www.homedepot.com/buy/generac...l#.UJbK4HXonEQ

If you look on most generator sizing guides, they say that a sump pump pulls about 800w when running, and about 2000w when starting. Even running both sump pumps at the same time (that would be 1600w), the 3500w generator shouldn't break a sweat. Starting one while the other is running (800w + 2000w = 2800w) should also be OK.

But you never know when sump pumps are going to turn on, so my concern is what will happen if both sump pumps try to start at the exact same time? Both generators have overload protection, so would that stop the generator from breaking? Also, how long would a sump pull that starting wattage. If it pulls 2000w for a second or two, then it's possible that both sump pumps could overlap when they try to start. If it's only for a fraction of a second, then it would be less likely for them to start at the same time (since that overlap window would be smaller). Also, if it happened, I wonder if one sump pump is starting, and the other would try to pull the wattage that isn't available, will it just strain for a second and then start a second later?

My concern isn't the cost difference between the two generators...it is the gas usage. In NJ, this past week, getting gas during the outage has been a nightmare. The 5500w generator listed above runs (at 50% load) for 10 hours on 7 gallons of gas. The 3500w generator runs (at 50% load) for 12 hours on 4 gallons. That means that the 3500w generator consumes gas at about half the rate, which is a big deal when gas availability is sparse.

Since I really don't need to run anything other than these sump pumps, I'd rather go with the more gas efficient model if possible. Thanks for any comments that you have!

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Old 11-04-2012, 04:07 PM   #2
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Generator Size


I have the 5500 watt generator and have ran 2, pumps, my well pump, ref, TV and cable box, all the lights (not all on at the same time) and had no issues with it.

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Old 11-04-2012, 04:26 PM   #3
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Generator Size


the Smaller genset shouldnt have a problem.
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Old 11-04-2012, 04:27 PM   #4
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Generator Size


The actual starting and running load of every motor is based on the HP and type of motor.

Sump pumps are typically fairly low HP (1/4 - 1/2) so running both isn't an issue.

Starting, on the other hand, is a different story.

Sizing a generator to start a motor is more of an art than a science.

A 1/2HP split-phase motor (common for sump pumps) will draw about 60 - 80 amps when starting at full voltage. This is about 7000 - 9000 watts.

But sump pumps, by their nature, don't need full voltage to start. They start easy, the load increases with the speed of the motor.

In my experience, a 3500 watt gen will very likely start a 1/2HP sump pump, and maybe even two at the same time.

Since the starting surge of a typical sump pump is of very short duration, the size of pumps a gen can start is more dependent on the rotating mass of the engine and gen rotor than it is on the brute force output of the engine.

In other words, a 3500 watt gen can very likely come up with 10,000 watts for a half-second or so. 7000 watts for a second, and 5000 watts for a couple seconds.

The starting rating of a typical gen is the maximum it can produce when it's new, has fresh gas, is operated at sea level, and its governor is fully open. It will produce this amount continuously, but the engine won't last very long. The continuous rating is the same conditions, but the engine governor is not fully open, and the engine will last much longer.

I don't know your exact situation, but if both pumps are 1/2HP or less, and the distance from the gen to the pumps is 50' or less, and you use #12 cords, you'll be fine with the 3500 watt unit. If there are other loads ( less than 1000 watts) on the gen and one pump starts, the voltage and frequency will dip momentarily, but it'll start. With both pumps starting at the same time plus other loads, they might not start. Also, if the egn is not up to operating temperature, its output (especially its stsrting surge output) will be greatly diminished.

As a sort of loose rule of thumb, for small gens like the ones you're describing, figure about 4000 watts per HP to start an unloaded motor.

Rob
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Old 11-04-2012, 04:31 PM   #5
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Quote:
Originally Posted by ed3120 View Post
Both generators have overload protection, so would that stop the generator from breaking? Also, how long would a sump pull that starting wattage.
The "overload protection" in small, consumer-grade generators is pretty unsophisticated -- just circuit breakers. Starting a big motor can bog them down momentarily. It's not so much a concern about damaging the genset as it is loads -- don't hook your TV, computer, or microwave to a small generator if you are also using it to run sump pumps! It might not break your TV the first time but it is possible.

How long the pump draws "starting wattage" varies not only by pump but also the installation. If your water outlet is only 1 foot above the water level in the sump when the pump comes on, it can build head pressure really quickly. If you are pumping water up several feet it will take longer.
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Old 11-05-2012, 09:43 AM   #6
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Quote:
Originally Posted by micromind View Post
But sump pumps, by their nature, don't need full voltage to start. They start easy, the load increases with the speed of the motor.
Thanks. I didn't realize that. I guess that works in my favor.

Quote:
Originally Posted by micromind View Post

I don't know your exact situation, but if both pumps are 1/2HP or less, and the distance from the gen to the pumps is 50' or less, and you use #12 cords, you'll be fine with the 3500 watt unit. If there are other loads ( less than 1000 watts) on the gen and one pump starts, the voltage and frequency will dip momentarily, but it'll start. With both pumps starting at the same time plus other loads, they might not start. Also, if the egn is not up to operating temperature, its output (especially its starting surge output) will be greatly diminished.

Rob
They are either 1/3 HP or 1/2 HP, but not bigger. I ran an inline amp meter on them once and I think one drew 10A and the other drew 12A. One sump pump will be 50 ft away and the other will be 100 ft from the generator.
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Old 11-05-2012, 09:54 AM   #7
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The smaller generator should work for the sump pumps. I would imagine that you might be tempted to plug in the fridge when faced with a long power outage. If that is the case then I would go with the bigger unit.

If you have natural gas service, you might consider a kit to allow your generator to run on natural gas as well as gasoline. That way you would basically never have to stand in line for gasoline and since natrual gas is much cheaper now than gasoline, the kit would probably pay for itself in about a week.

http://www.propane-generators.com/
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Old 11-05-2012, 10:13 AM   #8
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Generator Size


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Originally Posted by Auger01 View Post
The smaller generator should work for the sump pumps. I would imagine that you might be tempted to plug in the fridge when faced with a long power outage. If that is the case then I would go with the bigger unit.

http://www.propane-generators.com/
My plan is to plug in both sump pumps (and nothing else) while it is raining heavy. When the rain stops, I would unplug the sump pumps and plug my 2 refrigerators in for about 2 hours per day, just to keep them cold. I may also plug my TV and cable box in.

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