question on home generator/electrical

Discussion in 'Technical Corner' started by alanbreck, Apr 16, 2020.

1. alanbreckPre-Flight

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Someone gave me this generator, with no manual. And I have a question about it.
I'm to lazy to explore the internet for an answer, other than looking for an owner's manual, which what I could find wasn't much help.

See attached photo.

See the L14-20 plug? It's labeled 120/240V, and 20 amps.

My question ... What determines whether the electricity coming out of that plug is 120 or 240V? It can't be both. Is it the user configuration of the plug that goes in which makes the determining factor?

Thanks.

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2. TarheelpilotEn-Route

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Mine has a switch

3. kylebFinal Approach

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I'm not an EE and don't know that much about generators, but it looks like the 120 V side has receptacles for 3 prong outlets and the 240V side has receptacles for 4 prong outlets. So the type of plug you insert, and where you insert it is the deciding factor (I think). You can probably pull load off of both sides at once as long as the total load doesn't exceed the generator's capacity.

4. Arrow76RPre-takeoff checklist

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The 240 socket is actually 2 X 120 depending on whether the wiring connects across the two 120 phases = 240 VAC (180 degrees out of phase with each other) or one phase to neutral (= 120 VAC). Typically the 4 prong twist lock socket is only used for 240 VAC devices but you could use that socket for 120 VAC by appropriately wiring the plug. BUT you want to make sure you keep the current draw from both phases approximately equal to minimize neutral line current (should be zero if phases are matched exactly).

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We’ve got a Coleman Powermate 7500 that we just used during 48hrs of power outage after an EF3 ripped through the area. I’m quite familiar with this setup.

The receptacle is marked "120/240V" because that's what it is. It is able to supply both 120 and 240 volts. It’s really the same as your dryer is, 240 across the outside legs and 120 between the inside and outside leg.

Take a camper for example, with a 240V service, the breaker box in the camper would accept two hots, the neutral and the ground. From this breaker box, a single pole breaker would supply a 120V lighting or receptacle circuit. A two pole breaker would supply 240V to a large air conditioner unit or something similar.

For us, we just take a 40ft, 30amp plug from the generator and plug into a 30amp power inlet box on the side of the house. Within this inlet box is the main 200amp breaker to the house, so we’ll just shut off the main breaker and flip on the 30amp breaker that the generator will supply to. It’s enough to run several things in the house. 20 amp is enough to keep your fridge cold and keep a few lights on.

Last edited: Apr 17, 2020
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Correct - if you have 120/240 20amp than you’ll have to have that specific plug, likewise for the 30amp side. It’s two different plugs.

7. PaulSTouchdown! Greaser!

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Get the model number, google the number and the words "owner's manual". All your questions will be answered.

My only other suggestion is to run it using Stabil in the gas, shut the fuel off when done, run until the carb is empty (engine stalls). Then drain the remaining fuel, burn it in your car. Leave it empty until the next time you need it, then repeat. It will always start for you, or at least you won't have fuel problems.

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8. jsstevensFinal ApproachPoA Supporter

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What was said above about being both is correct. Technically it's both because it's a 4 wire outlet so it has 2 hot legs, a neutral and a ground. Each hot leg is 120V potential to the neutral and 240V to each other. The alternating current measured between hot legs is 180 degrees out of phase so they cancel each other out at the neutral. They can be used either way.

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9. Capt. Geoffrey ThorpeTouchdown! Greaser!PoA Supporter

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10. Jim_CAKLine Up and Wait

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I do something similar. Just a caution to the OP - you have to be very careful and connect the plug before you start the generator. It is worth repeating the obvious - once the generator is running both ends of that plug are hot - and lethal. And make sure you shut off the incoming breaker. You don’t want the power coming back on while you are feeding the panel with the generator. My next house is going to have the proper generator plug hard wired near the meter so anyone can do it.

This is a good suggestion. In addition - I have gone to using non-ethanol fuel in on my small engines. The carbs get ruined with the ethanol fuel. You want the generator to start in an emergency.

11. chartbundlePattern Altitude

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Hopefully OP uses a proper inlet, which means a normal extension cord is used. Also some breaker panels have the ability to use an interlock slider that will only let either the main breaker or one feed breaker turned on at the same time. For mine once the interlock is installed you turn off the all the breakers, turn off the main, slide the interlock over, start the generator, turn on the feed breaker, and then turn on the circuits you want.

Last edited: Apr 17, 2020
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Absolutely!
It is well worth the investment. Once the plug is connected to the 30amp breaker outside, you can simply pick and choose what things you wish to power from the breaker panel in the house.

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13. jsstevensFinal ApproachPoA Supporter

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Also, make sure to turn off the main disconnect or breaker so you're not feeding power back into the grid. You can zap some poor repair person who's trying to fix the problem...

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14. Greg BockelmanTouchdown! Greaser!

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Any LEGAL installation will include a device that will not allow grid power to power the house while the generator is powering it.

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15. jsstevensFinal ApproachPoA Supporter

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Absolutely true. How many installations do you suspect have never been inspected?

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Correct. Ours will trip the breaker if that happens.

17. flyingronTouchdown! Greaser!PoA Supporter

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How so? Unless you exceed the capacity of either the generators breaker or the main, it won't trip. The generator will happily run 240V back through the meter to and cause potential injuries to the lineman.

You need an electric or mechanical interlock.

I've got an automated transfer switch on mine. Still there's warning stickers all over the place warning you that the system is fed from multiple points and to be careful (duh).

Having a generator is only half the battle. You better make sure it runs from time to time. My goes through an exercise cycle every Thursday. I've also built a little Raspberry PI unit that talks to the maintenance port on the unit and lets me know what's going on (the control panel has an alarm but it is so feeble and on the far side of the hangar from our living space, the only time we know it is going off otherwise is when one of our neighbors walking their dog tells me my generator is beeping). I also have a "TankMonitor" on the propane tank that feeds the thing.

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18. redtailEn-Route

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and each line or phase leg is a half cycle (time) apart on the sine wave. Reaching peak voltage at a different time in each cycle, at 60hz (in the US). I've seen this confuse people into thinking that makes it a two-phase circuit. However, It's still single phase as we all know.

Last edited: Apr 18, 2020
19. flyingronTouchdown! Greaser!PoA Supporter

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Yes, it's really only one phase with a center tap. I get into arguments about this all the time, but I give up and let it slide most of the time.

20. flyingronTouchdown! Greaser!PoA Supporter

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For if single phase power is "two phase" then three phase power should be "five phase."

21. forsonsincPre-takeoff checklist

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The automatic transfer switch will power the system with loss of grid power...will it also reset the system when it is restored?

22. flyingronTouchdown! Greaser!PoA Supporter

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Yes. In my case the ATS is more than just a transfer switch. It also controls the generator. As soon as it senses loss of power it tells the generator to start. As soon as it sense the generator is providing power it switches the load to the generator. When the utility comes back (and there's a timer, it has to be back for some number of seconds), then it throws the switch back to the utility and tells the generator it is no longer needed. The generator has it's own timer to determine when it needs to actually stop the engine.

It works well enough when it works. The generator is on the far side of the hangar from the house. If we're inside and the windows aren't open, we really can't tell we're on the generator. The problem comes in that the generator as installed has this sonalert which is loud enough at the generator, but we won't hear that inside the house either. I usually knew my generator had faulted because my neighbor would hear it when walking her dogs and let me know.

Fortunately, some enterprising guy had figured out the control port for the thing and wrote software (and the electrical diagrams) for a Raspberry Pi generator monitor. I now get alerts anytime anything happens on the generator (the thing runs every week just to keep things smooth). It has sensors all over the place (temperature, coolant levels, etc...). It's also designed to be serviced. There are two hoses that you just flip out of the cabinet to drain either the oil or radiator. Only thing that's a bitch to get to is the cooling fan belt. (Most of the side panels are designed to be opened easily, but that one you have to remove a dozen screws). It's driven by the propane tank in the ground just behind it (we also use propane for domestic hot water, two fireplaces, fire starters in a few others, and my grill). I also put a remote monitor ("Tank Utility") on the propane tank gauge as I don't oft go out in the back yard and look down the hole where the tank is to see what the level is.

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23. Skip MillerFinal Approach

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Don’t forget that the 240 volts will energize the low voltage side of the transformer on your local pole and emerge as potentially thousands of volts to the line your friendly local lineman is working on. Potentially lethal? Yes!

Although I do believe it is standard practice for linemen to ground the line(s) they are working on.

-Skip

24. PaulSTouchdown! Greaser!

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Soooo, how does that work exactly? You hook up your generator to a line that requires a bazillion amps to power up, and your generator that has a 30 amp circuit breaker in it. Won't the circuit breaker just kick out?

I'm just wondering, would never try it, I have a generator switch wired to my panel, but just wondering.

25. Skip MillerFinal Approach

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Yes it should... are you willing to bet someone else's life on it?

-Skip

26. Capt. Geoffrey ThorpeTouchdown! Greaser!PoA Supporter

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Depends.
Where is the break?
How many people are on your side of the break?
How much **** did they leave turned on?
What breaker?
The system may be able to supply a bazillion amps, but that doesn't mean that it is going to suck up that many amps if you try to backfeed a section of it.

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27. flyingronTouchdown! Greaser!PoA Supporter

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This is exactly my point. No the circuit breaker won't trip. If the generator can put only out 30A and it has a 30A breaker, it will happily backfeed the system. In order for the breaker to trip, the generator needs to be able to put out more than the breaker rating. The current has to come from somewhere. The line does not take a "bazillion amps" to power up. And it only takes a few amps to kill the lineman.

It's illegal to not have some sort of interlock.

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28. redtailEn-Route

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Before we did maintenance or cleaning on transformers and other switchgear, we used to make our high-tension switcher (our supervisor) touch the equipment that he grounded, just to be sure.

I don't have any experience with home emergency generators (basic principal is the same), but the 3-phase 480/277 emergency generators here on my job facility, must be manually disconnected (via circuit breaker) and shut down for that very reason.

Certain buildings are supplied with two 13.8kV feeders (one NY & one NJ supply). If we lose either feeder, the tie breaker will close and continue to feed all loads normally from the other one. There's a lot of redundancy in our systems (6 high tension feeders).

However, if we lose both (as during hurricane Sandy), with both main breakers open, the generator will power up, breaker closes and ATS to feed various essential loads.
In this case, the ATS is a one-way switch. The power authorities don't want high tension 3-phase switchgear flopping back and forth. Too many bad things can happen!

29. PaulSTouchdown! Greaser!

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Yeah, yeah , yeah, I already said I wouldn't do it, nobody should do it, blah, blah, blah.....

But if I hook my 30 amp generator up to a 50 amp load, say a big motor, the generator is going to labor and that breaker is going to trip. Why wouldn't back feeding a system that has megawatts of load on it do the same?

Don't just come back and tell me I'm stupid, if you understand it you can explain it, educate me.

30. GheryTouchdown! Greaser!

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Someone did explain it earlier. Where is the break in the supply line? How many loads are hot on your side of the break? Yes, there may be sufficient loads to trip your 30 amp breaker, or there may not. If there aren't, the portion of the HV line that you are on will be energized, and if a lineman makes contact with it, he's dead. You must keep your side of the load isolated from the mains feeding your house via the incoming feed when your generator is running. The law requires it as does the safety of anyone also connected to the feed to your house.

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31. Greg BockelmanTouchdown! Greaser!

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It would.

32. Greg BockelmanTouchdown! Greaser!

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Any lineman worth his salt would make darned sure the line was dead before working on that line.

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33. PaulSTouchdown! Greaser!

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Thank you, that's what I thought.

34. redtailEn-Route

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Treat it like it's hot!

Don't know why that reminded me of a song I heard... "drop it like it's hot" LOL

7200V

35. PaulSTouchdown! Greaser!

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Those guys have some skills, I watched a pole climbing class one time, one guy wasn't getting it, was kind of comical in the controlled setting, but looked tougher than it appears. A lot to know and a little mistake can be lethal.

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36. forsonsincPre-takeoff checklist

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Would you share the make and model of your generator , I am assuming the switch is same as generator

37. redtailEn-Route

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When I started my career at EWR, over thirty years ago, the old-timers told me a horror story of an electrician who was working in a manhole, repairing an airfield lighting circuit and was electrocuted!

If I remember correctly, they said it happened in the 60's or 70's.

There was a mix up, miscommunication and improper safety procedure/complacency, etc. He wound up hack-sawing into a live 4160V cable!!!
Fried the poor guy down there in that manhole.

As you can imagine, I never forgot that story!

38. Capt. Geoffrey ThorpeTouchdown! Greaser!PoA Supporter

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If the system you are back feeding had megawatts of load on it, there is a good chance that your power wouldn't be out.
If your power is out, as often as not, something locally is broken. There may not be even close to megawatts of load on it. There may be next to no load at all. Depends on what is broken, how far away it is, how many other people are hooked up between you and the broken stuff, how much **** they have still plugged in. etc.
Your generator may or may not trip a breaker depending on the size of the breaker (if it exists) and how much current the generator is capable of producing.

39. flyingronTouchdown! Greaser!PoA Supporter

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Yes, both Generac. HTS transfer switch and an 80 KVA 4.6L (I think it's a Ford engine, really) generator.

Last edited: Apr 22, 2020
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40. PaulSTouchdown! Greaser!

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My generator will never back feed the grid. Most of the power outages we have here are of the errant squirrel blowing up on a line tripping an circuit breaker at the transfer station a couple miles from my house. Everyone is still connected to the grid as is their appliances, so yes, there is still quite a bit of potential load on that grid. But that's neither here nor there, it's stupid to not properly connect a generator to your house. Doesn't stop thousands of people from doing though.