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I have a four-pin plug from my generator (240v/30A) that I would like to connect to an outlet, which is only serviced by three wires - two hot wires and a bare copper ground or neutral wire. The outlet was used previously to power (240v/40A) a welder and is serviced thru a 40A breaker. What is the proper and safe way to ground that fourth pin from the generator plug? Would it work to run a ground wire back to the breaker panel or connect to a grounded metal rod adjacent to the outlet?

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  • Please edit to include the brand/model of generator - that's always helpful in figuring out proper answers.
    – FreeMan
    Sep 11, 2023 at 18:58
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    If it was for a welder the third pin is ground and no neutral. Really you cannot use that setup. You need an inlet plug, four wire cable or wires in conduit, and an interlock for your main breaker.
    – crip659
    Sep 11, 2023 at 18:59

2 Answers 2

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STOP BEFORE YOU KILL YOURSELF AND THE UTILITY LINEMEN

In order to use a portable generator to power your house in a blackout you need two key things:

Inlet

You are trying to connect to an outlet. That is for sending electricity from your panel to your appliances.

You are sending electricity in the other direction, from your generator to your panel. In order to do that safely, you need an inlet. An inlet has male pins instead of female sockets. A standard extension cord will work from an outlet (on your generator) to an inlet (on your house). If you have to chop up a cord to get two male ends then you know you've got it wrong. Those cords are called suicide cords because the second male end (the one being used to plug into an outlet instead of an inlet) is live when the generator is running and could super easily zap someone handling it.

Once you are already installing an inlet, it is no big deal to get it with the proper 4 wires (hot, hot, neutral, ground). Also put it outside so that you are not tempted to bring the generator inside, which far too often kills people with carbon monoxide poisoning.

Interlock

In order to legally (i.e., according to the electrical code) connect a generator, you must have an interlock. This is a fancy name for a piece of metal that fits on your panel and is designed so that it is impossible to have the interlock in place and have both the utility feed (main breaker) and generator (breaker to the inlet) on at the same time. This prevents you from plugging in and turning on the generator during an outage without turning off the main breaker first. Why does that matter? Because electricity from your generator at 240V will go out through the main breaker to the lines that are being worked on. If it doesn't zap someone working on the wires on your street then it can get transformed into high voltage and zap someone a long distance away. And it doesn't take much zapping to kill someone.

A number of commenters have pointed out that there are some really good practical reasons why you wouldn't want to have your generator running with the main breaker on, including that you could overload your generator or have big problems when the utility power comes back on. But without an interlock, most of the time running your generator with the main breaker on won't cause your generator to self-destruct, particularly if you only run it for a few seconds before realizing you need to flip the other breaker. The problem is that those few seconds can be deadly to someone working on the lines. So an interlock is a must - procedures of "step 1 turn off main breaker, step 2 turn on generator, step 3 plug in generator, step 4 turn on generator breaker in panel" are too easy to mess up, particularly for something that you might do only once every few years. An interlock guarantees the most crucial part of the sequence of operations.

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    Everyone focuses on the possibility of killing line workers, but I bet a lot of people who do this are more worried by the possibility of breaking their generator by trying to power the whole neighbourhood with it
    – user253751
    Sep 11, 2023 at 20:27
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    @user253751 I doubt most people even realized that generator power can even feed back though the main breaker/meter. Power off, the generator powers what they want. They don't care about the neighbourhood or the lineman since they are only powering their stuff. Which is true if they use an extension cord to plug stuff into.
    – crip659
    Sep 11, 2023 at 20:58
  • @crip659 that is true (I didn't, once), but I also doubt when you tell them it can backfeed the grid and kill line workers, they'll think that's a problem, but when you tell them it will overload their generator and break it, they'll suddenly care.
    – user253751
    Sep 12, 2023 at 5:10
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    @user253751 Not to mention the fact that, even if powering the neighborhood somehow worked out, eventually the utility power will come back on -- chance are utility and the generator will be out of sync and the generator will be violently forced to align itself (mechanically) to the electrical speed and phase angle of the grid power.
    – Greg Hill
    Sep 12, 2023 at 17:02
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I want to also suggest that you use a TRANSFER SWITCH for this configuration. They come in several flavors, both electromechanical (relays) and solid state (thyristors/triacs). They serve the same purpose as previously described: If electric power resumes from Edison (aka power company), it will disconnect your generator. It also separates your house from the power company when your generator is running so you don't ZAP a power company worker trying to fix the outage.

Without getting too complicated, I will tell you that transfer switches can be very smart and check for the STABILITY of Edison's "return to service" power before switching you back to it. However, the smarter you want, the more expensive they get.

Okay, so let's talk about configuration. If you only want 1 device to function during power failure, you put the transfer switch near it and wire a MALE plug to go into the generator:

1.) Power fails 2.) Transfer switch cuts out Edison and connects to plug. 3.) You start generator and continue.

If you want to get more intricate (i.e. keep refrigerator, some lights, electric stove, furnace (But NOT AIR CONDITIONER)),running, you will need to run special wires in the house. If these items already have their own circuit breaker, a simple reconfiguration in the breaker panel (where you will then locate the transfer switch) is all you need.

Hope this helps!

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    Interlock on a decent panel is a lot more flexible than a manual transfer switch and cheaper too. An automatic transfer switch is great, but costs a lot. Sep 11, 2023 at 21:47

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