Genset Wiring Questions

[Jagmandave]

I have a Generac that's rated at 7000 watts @ 120V, 12,250 surge......so that's just about 60 amps, 30 amps @240 V normal running, right?

Should be more than enough to run my whole house I figured......

Here's my question....

It has several different places to plug into the main panel on the genset - a 30 amp -120V 3 prong twistlock, a 30 amp 120/240 4 prong twistlock, and one double 120V receptacle.

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We had a terrible storm two days ago and haven't had power since, I wired up a three prong 30 amp 240V plug on a pigtail and put it in the receptacle for my garage A/C unit to feed my main panel (Yes, I disconnected from the grid by opening the main breaker)

It easily runs everything I need in the house, including a small window A/C in the bedroom so we could sleep at night.

But, even with everything else turned off, it would not run the whole house A/C, which is also on a 30 240V amp breaker, instead it tripped one leg's circuit breaker on the genset . I assume the inrush current needed to start the compressor was just more than it could handle, even tho the loads seemed matched OK (30 amp 240V supply, 30 amp 240 draw) and it has surge capability of over 50 amps on 240V.

I reset the circuit breaker on the genset and went back to only running the 120V stuff (including 2 refrigerators, the window A/C, all the lights, computers, TV etc. and it's been going fine for 2 days now. The only things that make it draw down momentarily are when the compressor cycles on in the window A/C and when she fires up the coffee maker - and that's only a momentary thing.

Does this sound like what happened to you guys? Why wouldn't it handle that load? The main A/C is on a 30 amp 240 breaker, so it doesn't draw that much or it would trip that breaker too, right? What do you electrically smart guys think? The pigtail was made with 3 conductor 10 ga SO cord, the wires from the receptacle to the main panel are also 10 ga.

 

[MBfreak]

The big AC may not have an unloading valve. This gives a relatively high start (peak) current which can amount to 4x the FLA.
Installing an unloader valve is something any good AC technician can do.

Best regards

Ola

 

[Jagmandave]

Well, the unit hadn't been run for over a day, wouldn't that have let the head pressure equalize?

Also this unit has a time delay so it won't try to restart for a certain amount of time after a shut off, again I assume to let the pressure equalize?

Lastly, if it didn't have this feature, wouldn't that cause the regular breaker to trip too?

No comments about me using a 4 wire connector with only three wires in it?

 

[MBfreak]

Hi.
I am not a real pro on AC´s but lived in a country where you did most everything yourself to get it done. AC 24/7/365 and installed split units in my house there. They had expansion valves and when 3 hp compressor started took something like 4*FLA for 2 seconds. I installed a shunt from Hi to Lo with a solenoid valve that closed after 5 seconds and my troubles were over.

Ola

 

[LXCam]

Since you had your whole house picked up are you certain things as the refrigerator wasn't running at the same time let alone smaller loads such as lighting?. Also thermal bimetal resets found on equipment like that are much more prone to tripping on overcurrent then you're normal circuit breaker.

 

[Radix2]

All 30 breakers are not the same as far as tolerance to short time overload conditions, so it doesn't sound any more complicated than the genset breaker is just too sensitive for that load (as you said).

 

[Mustang51js]

So you have a 3 wire plug going to a 4 wire plug on gen, how did you wire it up, possible you might have neutral mixed up

 

[Jagmandave]

So you have a 3 wire plug going to a 4 wire plug on gen, how did you wire it up, possible you might have neutral mixed up

I wondered about that......

When I opened up the 4 wire plug to wire it, the green terminal was clearly identified, I assumed that the genny would be wired the same way so I metered it out while running to find the neutral ( again, I assumed) and wired it without anything on the green terminal. All the 120 stuff worked perfectly...

As I was feeding into both sides of a 240 breaker on the main panel, I ran things off of both legs and they all worked fine.

I also metered the house end plug with the other end of the pigtail plugged into the genset and it running and had 120V on both legs measured to the center pin. So, I think I had it wired right.......

I agree with those who posted that the circuit breaker might have just been too sensitive - for that matter I don't know what the CB's are rated for - 15 amps each per leg to get 30 or what exactly....if so how do they think you'll be able to utilize everything the genset is capable of delivering? If it has the ability to go to 12,250 watts under surge, how does it do that if the protection opens well before that?

Maybe if you had loads plugged into all three places?

 

[Jagmandave]

Since you had your whole house picked up are you certain things as the refrigerator wasn't running at the same time let alone smaller loads such as lighting?

Yes, I had opened all the other breakers to make sure there wasn't anything else drawing a load, however, thinking back now I did have the furnace breaker on so the 120V furnace fan would run and so the thermostat could start the condenser/compressor, maybe that was enough to unbalance the load - as only one breaker on the genny opened.

 

[MnemonicMonkey]

The problem with your wiring is that you're using the ground line from your ac as a neutral. If you have good ground and wiring, it's not a functional problem (as you've found), but if not, you've now put up to 120V on that line. Worst case scenario, you kill someone or burn your house down... so thankfully that didn't happen. The better way to do it would have been to connect the pigtail at the panel and land the neutral on the neutral bar, backfeeding the ac breaker.

As others have said above, your problem is more likely the combination of high inrush current and a cheap breaker.

Sent by my Trunk Monkey™

 

[theoldwizard1]

Install a generator interlock kit and feed directly into the main panel.

A year or 2 ago there was someone else here trying to do the same. They wound up buying a bigger generator.

 

[Charles (in GA)]

You should be using the L14-30 connector, the 4 prong one, and providing a neutral, ground and two hots to the panel. You also should be using an interlock kit on the main breaker and the backfeed breaker.

A whole house A/C implies a central unit. Is this two separate units? an air handler in the house somewhere and a compressor/condenser outside? or is this one of the self contained packages that gets return air ducted to it though the foundation and in return supplies air back to the house thru a duct?

If there is an air handler, it will be fed by a separate breaker and you cannot expect to feed a 30 amp draw with a 30 amp supply and also power the air handler, etc. Its a pretty small unit if all it draws is 30 amps.

Charles

 

[Jagmandave]

The problem with your wiring is that you're using the ground line from your ac as a neutral. If you have good ground and wiring, it's not a functional problem (as you've found), but if not, you've now put up to 120V on that line. Worst case scenario, you kill someone or burn your house down... so thankfully that didn't happen. The better way to do it would have been to connect the pigtail at the panel and land the neutral on the neutral bar, backfeeding the ac breaker.

As others have said above, your problem is more likely the combination of high inrush current and a cheap breaker.

Sent by my Trunk Monkey™

OK, let me see if I understand what you wrote, first of all, my house was built in 1964 - the neutral and grounds all land on the same bar. The socket I plug my garage A/C into uses three leads, two hots and a neutral. When I plugged my genset in I only used three wires from the genset plug, 2 hots and what I presumed was a neutral as the green ground was clearly identified in the plug.

You should be using the L14-30 connector, the 4 prong one, and providing a neutral, ground and two hots to the panel. You also should be using an interlock kit on the main breaker and the backfeed breaker.

A whole house A/C implies a central unit. Is this two separate units? an air handler in the house somewhere and a compressor/condenser outside? or is this one of the self contained packages that gets return air ducted to it though the foundation and in return supplies air back to the house thru a duct?

If there is an air handler, it will be fed by a separate breaker and you cannot expect to feed a 30 amp draw with a 30 amp supply and also power the air handler, etc. Its a pretty small unit if all it draws is 30 amps.

Charles

Charles, the house A/C does use an external condensor/compressor unit, and the A-coil mounted in the furnace plenum, then it uses the furnace fan to cool the house. So yes, even tho it's a small load to run the furnace fan, it could have been just enough to overwhelm the breaker in the genset when trying to start that and the compressor and condenser fan. It's possible that had I run the furnace fan off a separate plug on the genset - say with an extension cord - it might have worked. It is a 3 ton unit and cools our 1700 sq ft house efficiently and well. It's about a 5 year old Lennox.

I agree that it should be wired properly to the main panel, and I'm going to buy and wire in an either/or switch - when I do I can use all four leads off the genset, including the ground.

But this was an unexpected emergency, so I took the shortcut just so we could survive the heat with the window A/C in the bedroom. When I originally bought the genny I figured the only time I'd need it was winter!

I appreciate all the input, guys!

 

[TractorJeff]

Those cheap little Push Button circuit breakers work fine for normal usage. But trying to spin an A/C compressor with possibly the air handler Fan coming on at the same time, they won't do.
Starting inrush value for the first half second could be 4 times FLA or higher. Then as the motor starts, the Inrush value will decay as the motor comes up to speed. I seen 7.5hp.480 volt Fan motors go as high as 80 amps on a Fluke Clamp-On.
Also, somewhere I read of a person having a similar issue with a portable/standby generator. It was determined by fellows way smarter than us that there just isn't enough copper in the windings to hold the voltage up high enough to stay below the manufacturers "Peak" advertised value! The field collapses and the voltage goes way too low for too long to even consider starting the motor.

 

[brewchief]

I've seen amp draws of 70-80 amps for a couple of seconds on start up on a 3 ton A/C.

You might get it to run by adding a hard start kit to the A/C, It will add a dedicated start capacitor and a potential relay to the unit.

What model# A/C? I can see if a hard start kit is available.

 

[Jagmandave]

5 year old Lennox is all I remember - tomorrow in the daylight I'll take a pic of the data plate.

It's not critical that it be able to run the whole house A/C, as long as it will carry the window unit in the bedroom like it does at least we can sleep - I just thought it had enough capacity to carry it, maybe it doesn't. We'll still be OK.

The one thing I wondered about was how they arrive at the ratings, but I assume that's what it's capable of carrying if you utilize all the plugs.

The next question would be is it possible to wire up just one plug that would carry all the power it can make, then hardwire that to an either/or switch to the main panel?

 

[TractorJeff]

Yes, you should be able to get tot the leads out of the gen barrel and wire a dedicated outlet. That panel is a "convenience distribution panel".

 

[LS6 Tommy]

The big AC may not have an unloading valve. This gives a relatively high start (peak) current which can amount to 4x the FLA.
Installing an unloader valve is something any good AC technician can do.

Best regards

Ola

Unloaders don't decrease starting load, they are for capacity control. They artificially ADD load to the compressor under low demand conditions. Adding an unloader to a residential hermetic compressor is no small task, either.


Pressures will only equalize on shutdown if you have a fixed orifice metering device or a TXV with and equalizing line.

7000 watts probably won't run your A/C properly either if it's bigger than 18000 Btu.

Tommy