Generator wiring question

[Terry Browne]

I have a Generac 4000EXL generator that has a 120/240 volt AC, 20 amp locking receptacle. The receptacle accepts a NEMA L14-20 plug (included with the generator). Instructions state to use a 4-wire cord set rated for 250 volts at 20 amps and that you can use the same 4-wire cord if you plan to run a 120 volt load. I need this 20 amp souce to run my 120 volt compressor and I'm lost as to how to wire this. It seems counter intuitive to me that one receptacle can run both 120 and 240 volt loads using the same cord. All help is appreciated.

The operator manual can be viewed from the following link.

http://www.generac-portables.com/service_support/owner_manuals/search_results.cfm

[Greg Sznajdruk]

Terry:

The site comes up with an error when attempting to access.

Greg

[Ken Fitzgerald]

Terry,

I went to the Genrac site and looked at the plug and their statement. I'd give Genrac's customer service line a call later today.

I could see 2 ways you could wire that plug to get 120 but you would only need 3 wires of the 4. Maybe it's senses the load?

Contact Genrac to verify their statement and meaning.

Good luck!

[Terry Browne]

You might have to go to this link and click on Service/Support tab, then select Operators Manuals and enter 1645-0 for the model number. The site apparently doesn't allow you to go directly to that page. Sorry!

http://www.generac-portables.com/

[Rod Sheridan]

Hi Terry, the generator supplies 120-0-120 just like your home service.

If you want 240 volts you use both lines, and ground.

If you want 120 volts, either line and neutral, plus ground are what you want.

You could make a cord up with the cap on one end, and a 4 inch square box on the other end. That way you could have a 240 volt receptacle, and a split 120 volt receptacle that would provide two 120 volt circuits.

Note that you have to monitor the load to make sure that you don't overload either line.

Regards, Rod.

P.S. Don't forget to ground the generator when you are using it, instructions should be in your manual.

[Terry Browne]

Thank you gentlemen. I have a better idea now on how to make this happen.
:)

[Gene O. Carpenter]

Terry,
Check this site out for anything you might need to know about your generator. If the link violates TOS then just add the www. to "smokestak.com" ..These guy's can tell you the where's, whens and hows about just about any power plant made from the earliest up to today's unit's.. They're a great bunch of guy's and even a few gal's in there also! If you plan on using the genny for home backup power you would be better informed if you ask them about "back feeding"..
Gene
PS: If you like old gas or steam engines, old farm machinery, just any rusty old iron this site will be beneficial for you..

www.smokstak.com (http://www.smokstak.com)

[Charles Lent]

Be careful about trying to do this. An air compressor draws an exceptionally large amount of current when it is starting up. Even though the generator may have enough current capability to run your compressor by the nameplate ratings it may not have enough reserve capacity to start and run your compressor properly. My son and I just ran into this when trying to run his 2 hp 240 volt compressor from our 2 KW 120/240 generator (both are almost new and in perfect condition). The nameplate ratings implied that it could do it, but when we actually tried we found that the compressor never came up to full speed, and it's motor got really hot in only about 30 seconds. I think a generator with about 2X the rated capacity of the compressor's full load rating will be necessary for it to work properly. The only way that the compressor would run from our generator was if there was no pressure in the air tank when it was started. Once it had shut off via the pressure switch and then tried to re-start by itself there was still pressure in the tank and the motor couldn't attain full speed.

I wasn't able to view your generator's website so I'm just telling you about the problem that we had. Make sure that your generator has enough reserve capacity to handle the long start-up current load of your compressor and watch it carefully in the beginning or you may burn up your compressor motor.

Charley

[Tom Veatch]

Charles Lent's caution about the starting surge is on point. Your generator has a 6600 watt surge rating (4000 watt continuous) but you didn't mention anything about the compressor load requirements. If your compressor motor's data plate has an LRA (Locked Rotor Amperage) value, that's an order-of-magnitude indication of the instantaneous current the motor will draw on startup.

Page 14 of your operator's manual shows 5000 watts as an example requirement for starting a 1 1/2 HP compressor. That approximates 30 amps per HP (at 120v) for the start surge, but YMMV.

Going strictly be the surge wattage of your generator, it should be able to handle a 120v start surge that doesn't significantly exceed 50-55 amps - assuming there's no other load on the generator at the time. Extrapolating from the example in your manual, you should be OK if your compressor is less than about 2HP. More than that, and you're likely to run into problems.

[Josiah Bartlett]

I also wouldn't run a single, big 120V load on a generator for long periods of time- You are only using half of the windings in the generator that way, and it will result in thermal stresses that aren't balanced. It would be best to rewire your compressor for 240 V if you can or convert it to gasoline power if it is belt driven.

[Terry Browne]

I'm really glad I asked this question, because I had no idea there was so much to learn. The compressor has a 1.5 HP motor and it can be wired for 240 volts, so I'll give serious consideration to making that modification. I really do appreciate the information - and thanks again!

[Tom Veatch]

...it can be wired for 240 volts, so I'll give serious consideration to making that modification. ...

If you intend to run your compressor from the generator exclusively, or if you have a 240v outlet convenient to where you normally run the compressor, OR if you're going to use a long extension cord get power to the compressor then I'd switch the leads around to run it on 240. Higher voltage equates to lower amperage and less voltage drop/power loss/heating at a given wattage.

From page 15 of your owner/operator manual:



Rated Maximum Continuous Load Current
At 120 Volts . . . . . . . . . . . . . . . . . . . . . . . 33.3 Amps
At 240 Volts . . . . . . . . . . . . . . . . . . . . . . . 16.7 Amps


That implies to me, perhaps incorrectly, that the generator head feeds a center tapped transformer that provides the 120/240 line voltage. If that is the case, the generator head itself won't know whether the load is drawing 120 or 240 volts, it's simply feeding the transformer.

In either case, running the compressor alone, you're going to be well below the maximum continuous rating of the generator unit. For operation off the generator, if you don't have a convenient 240v outlet and there's no long extension cord involved, then I don't think I would bother converting the motor.