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Old 06-21-2011, 10:53 AM
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Default How to calculate generator size watt capacity



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Is there an alternate way to calculate the watt - size capacity needed for a generator besides reading the label on the device/equipment? Are the labels exactly accurate for the start up draw?

I suppose for devices with an electrical plug I could use a Kill A Watt meter.

But I need to know the start up amperage draw for my well pump at my BOL and the start up amperage draw for my circulating pumps for my hot water zones for my gas furnace at my year round home. These are hard wired into the house electrical circuits.

Is there a type of VOM the leads to which I could place around the wiring to read how much amperage they draw?
Old 06-21-2011, 12:51 PM
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You should not need to calculate for starting current. Just add up your loads in Kw add 20% and that will be good enough for your use...+ 50% is of course better..

Bigger is better unless you're talking diesel.. then it is not to your advantage to greatly oversize.
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Old 06-21-2011, 01:15 PM
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The company I work for manufactures automation and industrial robotics. I need to check but I think fuses and overloads for AC Motors are rated at 250%. There is a brief momentary inrush current on startup.

Look at the motor nameplate then multiply by 2.5 for this momentary draw.


I have a 5KW Generator, I think it has 6KW Momentary Surge capability.

There are AC amp clamps for monitoring draw. Also simply look at at what amperage your breaker panel has for that given device, it should have been rated appropriately.
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Old 06-21-2011, 01:50 PM
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If you want to measure inrush, get yourself a clamp on ammeter. Make sure it's analog (D'Arsenval movement) since the sample rate of a digital can be misleading for your application, and clamp around one and only one conductor at a time.

Here's one from Granger.
http://www.grainger.com/Grainger/EXT...4&ci_sku=1YPF4

Since you're looking for a spike, start your pump/heater up a bunch of times and take the average. Add 10% for margin/error and you're numbers should be okay.

Understand that the spike can be tall/large/big, but what matters here is the width of the spike. How long the current spike exists is how long the generator over-exerts itself to supply the current your inductive device demands.

-----

How I'd choose a generator:

House max power + Pump&Heater Peak (ie, startup) < 90% of generator peak output

AND

House max power + Pump&Heater sustained < 90% of generator sustained output

Satisfy both of the above relationships for a minimum of 10% margin and you should be okay.

-----

Hope this answers your question.
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Old 06-21-2011, 01:56 PM
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I could just get a 10,000 watt generator and be done with it.

The problem is the more wattage, the more expensive the generator is. Also, they use more fuel.

I have lived at my home for 25 years and have never needed a generator. If i did, I could probably go to a relatively nearby relatives or friends.

My BOL has been there for 110 years and granted it has only had electricity for half of that time, but it too has never needed a generator. I have a hand dug well that I could use for water but plumbed water would be better.

My area just has blizzards or hurricanes so my typical SHTF is relatively short and for a TEOWAWKI, I don't plan to store that much gas - I'll make other arrangement if and when..

I was/am very tempted with the current 3500 watt Champion generator deal for $211 with free shipping, but don't know whether it is big enough. I would prefer a multi-fuel one but for 200 bucks I might go for something rather than the best or nothing. but not if it is not big enough to power my well pump or my hot water circulators.
Old 06-21-2011, 02:01 PM
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BlackHawkFan,

Thanks. The clamp on ammeter is exactly what I was needing.
Old 06-21-2011, 02:26 PM
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Well pump in rush current can be up to 3 times normal running current. You would need a peak reading ammeter to measure it as it is very short in duration. A hard motor start can be rough on the pump motor and generator. On my set up I removed the dinky little pressure tank and replaced it with a 60 gallon tank. If I need more water I and can always turn off other accessories if I have to. The big power users are microwave ovens, toasters,coffee makers,blow dryers and the like. I replaced all my old style light bulbs with CFL's. Not crazy about them but they do reduce my electric power requirements. I would not go overboard on a big watt generator they use significantly more fuel than a smaller unit. A 5KW generator is more than enough for most houses if you use wise power management.
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Old 06-21-2011, 02:39 PM
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So, let's say I get a peak reading ammeter and it records that my well pump does briefly peak at 3 times the constant use amperage.

What happens - to either the pump or the generator (if its peak power is not rated at 3 times its constant use rating)?

Does (just) a circuit breaker trip? Does the pump just not start? Does it damage the pump or the generator? How is it hard on the pump motor and generator? Do the coils melt?
Old 06-21-2011, 02:44 PM
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While Freeholder is correct about the inrush spike, I wouldn't pay the price of a peak ammeter BECAUSE it's of such short duration.

$$$ vs accuracy...where to draw the line....

Freholder makes a valid observation concerning restricting which appliances/circuits you should consider not using while on generator power.
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Old 06-21-2011, 02:50 PM
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Quote:
Originally Posted by TANSTAF1 View Post
So, let's say I get a peak reading ammeter and it records that my well pump does briefly peak at 3 times the constant use amperage.

What happens - to either the pump or the generator (if its peak power is not rated at 3 times its constant use rating)?

Does (just) a circuit breaker trip? Does the pump just not start? Does it damage the pump or the generator? How is it hard on the pump motor and generator? Do the coils melt?
Peak vs constant load is a tradeoff.

The prime mover (gasoline engine) must have the power to turn the generator for peak load.

The generator's windings and insulation must be able to sustain the additional heat and current draw.

Worst case scenario is the prime mover will stop or the generator will overheat, breaking down the insulation, shorting windings.

Don't overthink this one. Use the generator a few times to get used to it. Plan which loads you'll use in an emergency. Listen to the generator. You'll learn to hear when it's loading too much.

Go have a beer. :-)
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Old 06-21-2011, 02:55 PM
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Doublepost.
Old 06-21-2011, 03:06 PM
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I am only wanting to run the two items I mentioned. These do not need to run constantly. When they are not, I would power my refrigerator which also does not need to run constantly.

At my BOL I do not even have AC and rarely need it at my house.

If the pumps just peak briefly for a short time and then settle down to a constant (but lower) draw, how would they (have time to) overheat?

As for the price of a peak clamp on ammeter, here's a refurb one for $60. Granted it's digital but it has some method of recording peak or "in rush" current. I do see others at $500.

http://www.icraveit.com/cgi-bin/cate...em=10016964-22
Old 06-21-2011, 03:33 PM
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Before retiring, I had access to 'scopes and amprobes etc'. I used those and compared it to readings I took using digital and analog meters. The surge current was not much of an issue... lasted maybe 3-5 cycles typically. It was acceptable, and predictable using the current ratings on the appliances and motors I planned on powering with the smallest practical generators. Refrigeration, minimal lighting, radio/tv, internet... I decided to buy two smaller generators instead of one large one.. saves fuel. Oh, and some lo-o-ong heavy power cords. Works for us. I do not plan on running the AC, 220 appliances, etc. Minimal power usage.
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Old 06-21-2011, 03:38 PM
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Quote:
Originally Posted by TANSTAF1 View Post
I am only wanting to run the two items I mentioned. These do not need to run constantly. When they are not, I would power my refrigerator which also does not need to run constantly.

At my BOL I do not even have AC and rarely need it at my house.

If the pumps just peak briefly for a short time and then settle down to a constant (but lower) draw, how would they (have time to) overheat?

As for the price of a peak clamp on ammeter, here's a refurb one for $60. Granted it's digital but it has some method of recording peak or "in rush" current. I do see others at $500.

http://www.icraveit.com/cgi-bin/cate...em=10016964-22
This is precisely the point.

Do as I suggested for your calculations and you'll save yourself some money, have a nifty meter, and have plenty of margin with your power set up.
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Old 06-21-2011, 05:50 PM
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In cold weather states it makes sense to have an LP capable generator because LP is a reasonable price and it stores safely/does not go bad.

I have had a gas generator for decades and at 5,000 watts it will run the
220v well pump no problem. I am looking at having a 110v backup well pump and purchasing either a multi fuel or LP generator soon to have a backup well pump and generator.

After years of thought I am of the same mind of WSierra that smaller is better with backups. When the rubber meets the road using the least amount of fuel to do what is needed is best. Hell even now using less fuel is better.
I am seriously looking to add a second 1000 gal LP tank ASAP just for backup and generator use before the price goes up any higher as it would be an investment that will pay off no matter what.
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