Grounds, Neutrals & Transformers

[JSanders]

Not really an electronics question here more of a general wiring question, hope that’s ok.

Deal with frequent power outages and wanting to run parts of my house from a generator, mostly A/C & fridge. Have a generator with both 30 & 50 amp outlets but can only run one to the service panel. The idea would be to run the 50 amp to the service panel and then power the outside A/C compressor with the other 30-amp outlet.

The question is how would this effect the shared 24-volt AC transformer used by the system to control both the air handler & compressor? The air handler should be okay as it and the 24-volt transformer would be on the same circuit but the compressor would be isolated powered from the second outlet of the generator.

Don’t have a schematic of the generator but would think the grounds and neutrals between the 50 & 30 amp circuits are interconnected, even if true, still not sure if there would be an issue with the 24-volt transformer when trying to control the compressor.

Was able to get a schematic of the A/C compressor and highlighted the 24-volt control lines. The BLK wire is the 24V AC common line.

Tried asking this on both a home generator & HVAC forum but wasn’t able to get a definitive answer, most suggested buying a much larger generator and avoid the problem all together but that solution is far too expensive.

Really wanted to be sure I wasn’t going to damage anything before actually trying to hook things up. Thanks.

[CaptDon]

Why not just run the entire house from the 50 amp supply? You can't get 50 + 30 so just go with the 50. Is that 50A at 240VAC? I have a similar setup. I have a 50A 240VAC receptacle in the garage. I turn off the 100A master breaker in the main panel and simply backfeed my whole house from the generator plugged into the garage receptacle. The garage has a 50A breaker in the main panel in the house and also a 50A breaker in the subpanel in the garage.

[bdunham7]

I don't see an issue.  Everything will still share a common ground.  Is the smaller generator plug truly isolated from the other, as in there are separate generator sections?  That seems unlikely, but it wouldn't matter anyway.

[CaptDon]

Usually the 50 amp receptacle is provided for R.V. compatibility with the same cord used at R.V. parks and campgrounds. The 30 amp outlet is often used with a special store bought cable that takes the 30 amp 240VAC into a pair of 120VAC duplex 20 amp rated outlets. One duplex is on phase A 120VAC to neutral and the other duplex is on phase B 120VAC to neutral. Even though the generator has a 50 amp outlet it may only be capable of generating 30 amps. The 50 amp outlet is often for compatability to the owners already existing 'shore cord'. Depending on the generator's KW capacity I have seen 30A, 40A, 42.5A, 45A and 50A capacity being commonplace. I have a small portable generator with a 20A 240VAC receptacle but it only produces 10A or 2.4KW. Based on the receptacle you would expect a 4.8KW capacity. One other weird thing, many of these 240VAC generators feature 3-prong connectors, L1, L2 and neutral. The neutral is bonded to the generator frame and there isn't exactly an earth or ground connection.

[JSanders]

I’d be pushing things trying to run things on just the 50-amp circuit. I have a heat pump system with the air handler on a 60-amp breaker and the compressor on a 30-amp. I would never expect to run the emergency heat so the 60-amp rating for the air handler is way off for summer time use, probably more like 15-20 amp at best to run the fan, but even with that I’m already at 50-amps. By splitting things between the 50 & 30 amp outlets gives me room to keep the fridge going along with some lights, TV & computer.

Don’t know the run current for the compressor but guessing the startup is pretty high, thinking about installing a soft start to limit the inrush current. The generator is rated at 14.5k running watts so using both the 30 & 50 amp circuits is the only way to tap all the available power.

[indeterminate]

You should change the plug on the generator to a suitable size 100A etc  so that there is only one supply from the generator and one alternate supply entering the house.
what you are proposing is dangerous and against the wiring regs in many countries.

unless the generator is a large multi cylinder diesel with lots of rotating mass it will struggle with those loads.

[J-R]

Can you post the more info about the generator?  14.5kW at what voltage and phase configuration?

With regard to grounding a generator for a building configuration, the generally accepted method if it has GFCI is to use the neutral/ground bond that already exists in your building.  So when connecting the generator to the building, you will only connect the hot(s) and neutral, but not ground.  You don't want to remove the neutral/ground bond inside the generator because that is a safety issue when using the generator separately.

[BillyO]

Sorry, no help here, but just a comment in passing.  I really hate how the appliance industry draws schematics.  I had to fix a heating unit for a customer the other day and the schematic was equally cryptic.  In reality it was butt simple, but they take great pains to make it look like it's worth the $300/hr. labor to install/fix.   

[JSanders]

Briefly looked into things and yes trying to run 2 parallel lines from a generator to a single service panel is against local codes. It has the possibility of creating issues such as things being out of phase, I’m not suggesting that and both the service panel & the compressor would have there own transfer switches. The compressor would be totally isolated from the house & service panel whenever the generator would be operational. Might have to go back and read up more on the local codes.

As for generators handling that kind of amperage, many ‘whole’ house generators are capable of supplying that kind of load without being diesel. Generac whole house generators (22kw) typically use a 990cc OHV engines, not terribly large in size and not diesel.

In my situation we’re talking a max of 66 amps and my thermostat already stages the startup between the air handler & compressor so they wouldn’t create a simultaneous inrush of current. And as mentioned above I’d like to use a soft start kit for the compressor further reducing the startup amperage.

[J-R]

There are still other aspects to consider here, but without details it's hard to come in for a landing.

Most generators are rated for the largest output, but not the largest plus all the smaller continuously.  They also typically have separate maximum continuous and surge/startup ratings, and limits when drawing from a single phase only.

The schematic you posted shows a 208/230V requirement.  Some generators have a 30A output that is only 120V.

Maybe you already have this worked out.  Hard to tell from this end.

[JSanders]

Oh sorry, yes both the 50 & 30 amp outlets / circuits are 240v and the generator has a peak output of 18kw.