Generator sizing


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Old 12-30-05, 03:19 PM
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Generator sizing

I am about to start framing a house but I have to get a generator for power on the jobsite, I normally dont have to deal with this because power is normally already in. How many watts do I need to run a 2 hp compressor and a chop saw at the same time?? Sugestions
 
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Old 12-30-05, 05:04 PM
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The compressor will draw significant power since it will typically start under load, air already in the tank, though it will probably not be running continuously.

Start with 6500-8000 watts with a fuel tank that will provide at least a 6-8 hour runtime. You will need at least 1-2 5-gallon gas cans as well.
 
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Old 12-30-05, 05:12 PM
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Easiest way is to figure out what the tools use in watts- P=I x E, where P= watts, I= current in amps, E= voltage. That will tell you the minimum.

One HP= 735 watts (if I remember correctly).

The kicker is the surge wattage of the motors- the power used at startup. I don't remember the general rule for that one.

I have a 3kW Honda inverter and it will run a couple of saws and a 500W halogen no problem. Very quiet and easy on gas.

Fuel consumption and noise should also be considered.

One thing to keep in mind is that the standard 110v outlets are usually tied to the same circuit in the generator and limited to 15A total- even if the generator delivers more. The only place to get the full rated output is from the 110v twist lock socket.
 
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Old 12-30-05, 08:20 PM
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8 kw would be in the ballpark depending on which compressor you get.

Considering the air compressor would be the biggest current draw you could think about getting a 2500 watt gen for the chop saw and a gas powered air compressor.
The gas compressor and the 2500 watt gen might be cheaper than an 8 kw and a 2500 watt gen would be handier to have after the job is finished than the big one.
 
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Old 12-30-05, 08:57 PM
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A gasoline powered air compressor might get a bit more expensive than needed and an electric compressor would be the tool to have after the house is built. The tank on a gas-engine compressor is fairly small.

Consider a welding machine with 8KW aux power capability to go with the electric compressor and the chop saw. It will be a bit more noisy, but you will not likely run out of gas or power. Most have 220V capability as well.
 
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Old 12-31-05, 03:57 AM
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My 3kW Honda will run my Emglo two tank compressor- IF I plug it directly into the generator and turn off the low idle feature. It will not start on a 100" 14ga extension cord.

One other thing- cold compressors are more difficult to start.

My generator woupd probably run both of the tools mentioned at the same time, but it will not start both at the same time. A 5kw would have a difficult time starting both tools at once.

A 5kW entry level Honda or a Coleman Powermate will use about 5 gallons of gas in an 8 hour day at partial load if you turn it off for break and lunch. It uses much more gas as the percentage of rated load used goes up. I have plenty of on the job experience with both. The Honda gensets have always started quickly compared to the Coleman rigs.

If you are buying the generator for yourself, how often will you need the compressor and chop saw at the same time? A smaller generator is lighter and more efficient. My 3kW inverter is almost unnoticeable at 25' away, while either of the regular generators mentioned above are at a dull roar.

If you are going to run a crew, it is probably best to get a bigger genset so you don't have to worry about a couple of simultaneous tool starts. Larger carpenter crews in my experience use either an 8kW or the 10kW water cooled Honda.
 
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Old 12-31-05, 05:37 PM
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I agree with the larger-crew thinking. If you were renting the unit and had need of more power, just swap out the rental or rent an additional unit. Construction is a power-hungry enterprise - nearly every power tool runs an electric motor or is fed air from an air compressor. Better grade generators will have more resale value if you don't need it when the job is done.
 
 

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