4 KW Generator with 3 prong 250V output question

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  #1  
Old 12-27-20, 07:31 PM
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4 KW Generator with 3 prong 250V output question

I was recently given a good working 4 KW portable generator that has two duplex 120V outlets and one 20 amp/250V 3 prong outlet. I would like to use this to power some basics in my house in case of an extended power outage. I am aware that this must be done in a manner that will not risk back feeding power into the grid and causing harm to anyone. I've noticed the portable generator world has moved on to 4 prong outlets for the 240V and I would prefer to wire the inlet box with a 4 wire connector in case I ever decide to upgrade to a new generator. I would also prefer to use a 4 wire umbilical from generator to the inlet for the same reason which brings me to my question. I see there are 3 prong to 4 prong 240V adapters available but I'd like to be sure that simply using one these to connect the 240V, 3 prong generator output to the 4 wire umbilical is a safe way to go. I haven't had any luck finding a schematic that shows how these are wired. Do they simply not use the ground wire prong on the 4 prong side?
 
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Old 12-27-20, 07:55 PM
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Does that 20A outlet say 250V and no other voltage marking?
4 prong outlet supplies both 120V and 250V. I have never seen a generator with 3 prong 120V/240V outlet. Most likely it is 240V only or just another 120V outlet with twist lock.

Please post a picture of the generator or post generator model number.

3 prong to 4 prong adapter is probably not a good idea without knowing how your generator is wired.
 
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Old 12-27-20, 08:35 PM
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Most three prong receptacles are 30A 120v only.
 
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Old 12-28-20, 08:09 AM
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To connect to an inlet to a transfer switch or an inlet to your panel protected by an interlock, you only need three prongs for 240v if you are not switching the neutral - 240v plus ground
 
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Old 12-28-20, 08:21 AM
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A 20 amp 250 outlet can't be used to power your house. It does not have a neutral.
Post a make and model of the generator so we can see the specs and confirm exactly what you have.
 
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Old 12-28-20, 09:07 AM
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The inlet needs 4 prongs to support 120/240 volts.

An inlet or receptacle with two flat prongs in a straight line and one round prong is for 240 volts only. It is improper to use the round prong for neutral.

Whether or not the neutral is being switched, it is not proper to run a separate neutral conductor because the plug or receptacle does not have a prong for neutral.
 
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Old 12-28-20, 11:33 AM
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Here are some photos. I haven't had much luck finding an owners manual on line for this. I believe Coleman sold the Powermate brand to someone else which I'm sure doesn't help. I did measure the output with a DVM on both legs of the 250 outlet and did have 125 on each leg.




 
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Old 12-28-20, 12:22 PM
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This is an example of an adapter that would go from the L6-20 to L14-30. Do they simply not use the W ?

https://www.amazon.com/NEMA-L6-20P-L.../dp/B0091UMN1E
 
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Old 12-28-20, 02:34 PM
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The 240 volt receptacle is labeled (using nearby engraving). 120/240 volts. Ergo it will serve a 120/240 load including a not to heavily loaded home electrical system isolated from utility power..


With that in mind I would go ahead and use (or make) an adapter (comparable to an extension cord) that connects the two almost straight prongs to the two hot prong holes of a 4 prong female receptacle and connects the bent prong to the neutral hole of the latter receptacle. This latter receptacle fits on the wall inlet to your home electrical system.


The question will arise, what to do when ground and neutral and frame are bonded (connected together) within the generator and ground and neutral are also bonded in the main panel in the house (which panel may or may not be the panel into which the generator feed is introduced). No attempt should be made to unbond neutral and ground at the generator unless it is sure that the bent prong hole of the generator 240 volt receptacle will retain connection (bonding) to neutral.
 

Last edited by AllanJ; 12-28-20 at 03:12 PM.
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Old 12-28-20, 04:45 PM
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That receptacle should be 240V only. 120V you are getting is using ground prong, which is not really intended for carrying current. However, since the generator probably has neutral wire connected to the ground, it would handle full 20A load.
So, that adapter cable will work, however, you cannot separate ground and neutral.
You are required to separate neutral and ground when you are transferring power to the panel with neutral bonded to ground (which is first means of disconnect. aka main panel).

So, you cannot meet the code if using your generator with an adapter cable.

I would rather replace receptacle on the generator than using an adapter.
Replace receptacle with NEMA L14-20R. Connect neutral to neutral and ground to ground and rewire the generator to have floating neutral (just disconnect neutral wire from ground/frame).
 
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Old 12-28-20, 05:43 PM
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Thank you Gents. I do like the idea of replacing the receptacle with an NEMA L14-20R. Hopefully it will be obvious that they have the neutral and ground connected and I can separate them. I have also been reading about the subject of grounding the generator to it's frame vs. floating neutral. From what I gather, as long as I don't use it as a stand alone unit to power equipment, changing it to floating neutral is correct for powering the main panel.

I was thinking of using an interlock on the main panel. It appears these are legal in CT and I do have the means to measure the power consumed by my appliances (and mark the appropriate breakers that can be on when generator is in use).
 
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Old 12-29-20, 07:08 AM
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OK so I got the output end of the generator removed and it appears pretty straightforward. The two hot legs (black wires) go to a breaker and then to one side of the duplex outlet and then to the 250V outlet. The white neutral only goes to the duplex confirming that the 250V is using ground for the return path current. I also see that the duplex has the jumper tab installed connecting neutral to ground. I seems all I need to do is get a jumper wire of appropriate gage to bring neutral to the 4 prong L14-20R. I was also thinking it might be a good idea to install a switch allowing one to easily float neutral or connect it to ground. If I were to do this, could an ordinary household SPST switch be used? (would it need to be 20 amp rated?). Out of curiosity, what is the function of the semi-conductor type devices on the yellow wires?





 
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Old 12-29-20, 02:40 PM
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Disconnect green wire from 120V duplex receptacle, then install a jumper wire from this terminal to neutral of new 120V/240V receptacle. That will isolate ground and neutral.

I also see that the duplex has the jumper tab installed connecting neutral to ground.
Do not remove this tab. It is what connects neutral of bottom and upper receptacles together and it is in all duplex receptacles.

I was also thinking it might be a good idea to install a switch allowing one to easily float neutral or connect it to ground. If I were to do this, could an ordinary household SPST switch be used?
Simple switch will work, but not sure if it is acceptable per code. I would use something that cannot be switched by accident. Installing a terminal that can be easily disconnected probably is better option.
I never understood why generator manufacturers don't provide easy method of disconnecting ground bonding.


Out of curiosity, what is the function of the semi-conductor type devices on the yellow wires?
I'm not quiet sure, but it looks like rectifier diodes. Probably used for generator field excitation.
 
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Old 12-29-20, 04:35 PM
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Thanks L. I think you mean to remove the ground jumper wire (arrow) and run a new wire from the duplex to the new 4 prong. Makes sense. As for the switch, I was thinking of getting one of those outdoor plastic boxes and a waterproof switch cover to prevent accidentally moving the switch. https://www.amazon.com/Weatherproof-.../dp/B009A8AIQY

 

Last edited by Mike C5; 12-29-20 at 06:42 PM.
  #15  
Old 12-29-20, 04:46 PM
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I think you mean to remove the ground jumper wire (arrow) and run a new wire from the duplex to the new 4 prong.
Yes. That is what I meant.

Do you really have enough room of that switch cover?

What I would do is just extend neutral and ground wire outside and install insulated male and female connector on each ends. Any type will work.
Then you just connect them together when you need ground bonding and disconnect when you don't.
 
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Old 12-30-20, 05:02 PM
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Thanks L. I'm not quite sure what to look for that would be adequate for this amperage range with respect to bonding/unbonding the neutral/ground (insulated male and female connector). I was think of a grounding lug but it's not insulated.
 
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Old 12-30-20, 05:38 PM
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Insulated flag or bullet terminal will work. Only neutral wire needs to be completely insulated since ground side is just frame of the generator anyway.

https://www.homedepot.com/p/Gardner-...205861878-_-N&

https://www.homedepot.com/p/Gardner-...155F/205846616

https://www.homedepot.com/p/Gardner-...205846605-_-N&
 
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Old 12-30-20, 06:08 PM
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Got it. Thanks again for the help.
 
 

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