Using a 120 generator with a UL interlock to back feed a panel

[penright]

I have seen lots of threads on backing feeding a panel while using the proper UL interlock. Just asking for a friend. No really, just asking for a friend, and true it might be more academic than anything else. He was looking to buy a portable generator and was looking at a 120 30A. My advice was if you are going through that much effort then go a little bigger with true 220. Then talking about just handling it with extension cords and there is a lot that does that, but that makes me nervous.

But to my question, when you back feed the panel with a breaker and interlock. Is the breaker just a switch? Could you just feed one side of the breaker if the generator is just 120? I know depending on which leg you would get every other outlet, but if the refrigerator happened to be on the other leg, maybe there would be an outlet close by that would be on the powered leg. Somehow that feels safer than stringing a bunch of extension cords through the house. How is this addressed by code?

 

[pattenp]

The interlocked breaker still serves as a breaker and can feed only one side providing 120V. Technically the hot can be pigtailed to supply both legs of the panel using a 2-pole breaker but you will only have 120V on both legs and no 240V.

 

[nadogail]

You may inform your friend that the purpose of the interlock is to guarantee that while the generator is feeding the house no electricity can escape to the grid. They are to protect the lineman working on a circuit that was presumed to be dead.

 

[theoldwizard1]

The interlocked breaker still serves as a breaker and can feed only one side providing 120V. Technically the hot can be pigtailed to supply both legs of the panel using a 2-pole breaker but you will only have 120V on both legs and no 240V.

Correct ! This will NOT damage any 240V appliances because the voltage at the connection will be ZERO volts.

 

[Stuart in MN]

You typically want standby power for the essential circuits - heat, a few lights, a few outlets - and assuming he doesn't have an electric furnace, 120v is probably all he needs. Central a/c will be 240v but they can maybe get by with a 120v window unit. If he has an electric stove that's going to be 240vac, but for a power outage they may be able to get by with a microwave and possibly a toaster oven. On the other hand, if he has his own well, it's an essential circuit and the pump is probably also 240v, so that needs to be taken into consideration.

 

[penright]

My purpose for stressing the interlock was not to get hung up on it. That we understand code requires it.

The interlocked breaker still serves as a breaker and can feed only one side providing 120V. Technically the hot can be pigtailed to supply both legs of the panel using a 2-pole breaker but you will only have 120V on both legs and no 240V.

Where would the 2nd leg be tied into? I am assuming it could not be at the breaker, my understanding is only one wire can be on a lug. Made up in the receptacle that is the feed from the generator?
I still don't feel comfortable that the generator will be enough. I still think a minimum would be in the 6000 range. But that is why I don't have one yet. I start off with a 6-7K and then start looking at whole home, then decide that I don't want to spend that much money just yet. Hoping that prices will come down once the backlog from COVID finishes up.

 

[penright]

You typically want standby power for the essential circuits - heat, a few lights, a few outlets - and assuming he doesn't have an electric furnace, 120v is probably all he needs. Central a/c will be 240v but they can maybe get by with a 120v window unit. If he has an electric stove that's going to be 240vac, but for a power outage they may be able to get by with a microwave and possibly a toaster oven. On the other hand, if he has his own well, it's an essential circuit and the pump is probably also 240v, so that needs to be taken into consideration.

All essential circuits are 120. I just did not know you were allowed to spit the 120 into the two legs like that. Even with 240 there would not be enough to run AC. His stove is gas. I need to double-check the gas heater fan is 120. I bet it is. I think hot water is tankless gas, so that vent motor should be 120. It was the back feeding with 120 to both legs that was the part I was questioning.

 

[pattenp]

Where would the 2nd leg be tied into? I am assuming it could not be at the breaker, my understanding is only one wire can be on a lug. Made up in the receptacle that is the feed from the generator?
I still don't feel comfortable that the generator will be enough. I still think a minimum would be in the 6000 range. But that is why I don't have one yet. I start off with a 6-7K and then start looking at whole home, then decide that I don't want to spend that much money just yet. Hoping that prices will come down once the backlog from COVID finishes up.

The one hot from the gen is split into 2 legs by using a wirenut and 2 pigtails inside the panel. The 2 pigtails then connect to the lugs of the 2-pole breaker giving you 120V on each leg of the panel.

 

[tyme2par4]

The one hot from the gen is split into 2 legs by using a wirenut and 2 pigtails inside the panel. The 2 pigtails then connect to the lugs of the 2-pole breaker giving you 120V on each leg of the panel.

You could also use a pigtail intended for RVs. I have a 30-50A dogbone that ties the 2 hot legs so you power both side of the panel.

What kinds of loads are you looking to power? I had a 7.5kW generator, and unless I wanted to run the AC, I rarely ever went above like 20% load on the generator. But I had a 240V well pump, so I needed a 240V generator.

 

[penright]

You could also use a pigtail intended for RVs. I have a 30-50A dogbone that ties the 2 hot legs so you power both side of the panel.

What kinds of loads are you looking to power? I had a 7.5kW generator, and unless I wanted to run the AC, I rarely ever went above like 20% load on the generator. But I had a 240V well pump, so I needed a 240V generator.

We were looking at the pigtails. Most of the genders were wrong. I bet if we looked a little harder we could have found one. Talking this morning he could run most of what they want with the 3.3kW. He found an 8+kW that he liked, but one of the "justifications" is the generator pulling double duty for tailgating. The 8+kW would get awkward in that use. Like all engineering compromises, you can't have it all. Just trying to get the facts to help make the decision.

 

[quickfarms]

My backup generator is actually a construction light tower. They run off of a 6kw diesel generator and it has worked just fine.

 

[slow]

https://www.amazon.com/dp/B07GMVJV7X/?tag=atomicindus08-20 Something like this L5-30 to L14-30 female with X/Y jumpered together.

 

[u2slow]

You'd be doing all the same work as wiring in a 240v gen... and only have half the panel being able to use it.

I run extension cords with a 30A/120V gen - and I'm an electrician.

 

[theoldwizard1]

You'd be doing all the same work as wiring in a 240v gen... and only have half the panel being able to use it.

Not if you do it this way (completely "legal")

The outlet from the generator is likely a either a 6-30 or L6-30 (120V). The inlet to the house is likely a 14-30 or L14-30 (240V). Connect L1 to L2 inside either end of the cord. You will be feeding a circuits in the house from the 30A generator outlet. Use the power wisely and you won't have any problems.

With a 120V generator of that size you can easily run a window A/C as long as you are not running to many other big loads (microwave).

 

[u2slow]

^ that works... I read too fast and assumed OP didn't have an inlet installed yet.

I'm not bothering with an inlet and interlock until I at least have a 240v gen.

 

[dcg9381]

Correct ! This will NOT damage any 240V appliances because the voltage at the connection will be ZERO volts.

Sorta. Voltage at the connection is still there if you were to stick your finger on it. Voltage between either hot (assuming you span at the panel) will be zero, but you'll still see 120v to the neutral. You're right on no appliance damage. I don't know the details on if some 240V appliances take advantage of the neutral and power up some or part with 120V.

 

[theoldwizard1]

I don't know the details on if some 240V appliances take advantage of the neutral and power up some or part with 120V.

Some very old appliances may power up a light bulb or timer. They used ground as neutral.