Another What size wiring question w/ a twist

[aar_man]

Here are the parameters:

I have an outbuilding that I want to run a 100A feed to from my home.
The home has a 200A (AL) entrance.
The run would be approximately 150 feet. Of that, 45 would be in the basement and then approx. 92 feet of conduit under the attached garage and continuing underground to the detached building..leaving 13 feet of make in wiring for inside the outbuilding.
The conduit is 1 1/2 inch PVC.

The twist is that years ago I purchased a Quincy QR series Duplexing Air Compressor with a 200 Gal tank & (2) 7.5 3-phase motors 230/460V...22.2A/11.1A each.

Since I don't have 3-phase power and after looking at Rotary phase Converters, I came across a smokin' deal on a 33KW Onan Diesel GenSet powered by a 4 cyl. Cummins complete with a 120 Gal attached tank...Wife takes husband to hospital to have head checked.

I figure set the GenSet to 120/240 3-phase...power the compressor when I need it and then have it ready for stand-by when the power goes down for the house.

Talked to an electrician...
1. Genset to a 3-phase transfer switch. Switch up to feed Quincy, switch down to send the power to the second transfer switch.
2. Second transfer switch. Would have the house feed in the middle. Switch up powers the panel in the outbuilding(during normal conditions from the house),switch down would be the feed from the GenSet. I would have to take the house off line at the main before energizing the GenSet feed.

I would also need (2) grounding rods and separated neutral/ground bars on the outbuilding panel...right?

I told the electrician that I was leaning toward CU for my feed instead of AL.
He suggested 2-2-2-8 CU if that was what I was thinking.

COntacted several jobbers and was really pushed toward 2-2-4-6 (AL) twisted direct burial. Is that the trailer feed wire? Price around 200.00

The 2-2-2-8 (CU) was around 520-600.00 WOW!

Is the 2-2-4-6 adequate or am I just living in the old days when most wiring was copper.

Thanks for the space.

 

[Charles (in GA)]

What is the gen set capable of???? You don't normally find gen sets capable of just switching from three phase to single phase, though, depending on the three phase it produces, you can get certain single phase voltages from it.

I'm sure the pros will chime in on why this really is not a good thing in several respects.

Charles

 

[930dreamer]

I can't help here, but I have a question on the Gen/Air compressors. How large of an amp load can the gen-set handle? I started two 5 hp, 3 phase Champion pumps/motors and the in rush was 142 amps. Does the Quincy start with a lead/lag alternator setup? Post some pics when you can.

 

[brewchief]

Talked to an electrician...
1. Genset to a 3-phase transfer switch. Switch up to feed Quincy, switch down to send the power to the second transfer switch.
2. Second transfer switch. Would have the house feed in the middle. Switch up powers the panel in the outbuilding(during normal conditions from the house),switch down would be the feed from the GenSet. I would have to take the house off line at the main before energizing the GenSet feed.

This part scares me, how will you insure the house panel is disconnected from the utility feed before backfeeding the panel from the generator? Simply saying you will throw the main breaker does not fly.

 

[pattenp]

You can't use 2-2-4-6 Al for 100A. 90A is the max. And the cable is commonly called mobile home feeder.

 

[Norcal]

120/240V 3Ø is not something to mess with, connect to the wrong leg & you let out the magic smoke.

Backfeeding a panel & relying on turning off a main to run under gen power is dangerous & illegal.

 

[Aceman]

This sounds like a disaster in the making. I think you're going to spend more money trying to make this work than buying the proper components the first time.

Like Norcal said, you screw up with 240 three phase and hit some appliances with the high leg and that'll be the last time those appliances work. So say you try and use A and C phase only, and just not use B phase, the high leg. Well, I haven't heard of any 3 phase generators made to pull only single phase from.

A few years ago for a job I needed a single phase 480v backup to feed a transformer, no one made one that I know of, so I asked Caterpillar and they said no way to using a 480v three phase unit to pull only single phase from it.

 

[pattenp]

I can't speak to any of the 3 phase stuff, don't know a thing about it. But as far as the main panel goes with back feeding it it is illegal with out some type of safety to prevent power from the gen going back out on the utilities lines. The manual solution is to put a safety interlock on the main breaker that prevents the gen breaker from being switched on without switching of the main breaker. Some jurisdictions allow this, some may not.

 

[KenC]

Not a pro here but: I think this will workif you send the feed to the house to a transfer switch before your main breaker. That probably will require another set of conductors sized to the house load when running on the genset.

 

[nehog]

If I understand you correctly, you will have two feeds between the building and your house. That won't work: fails to meet code. There is no safe and reasonable way to do what you want using a single feed.

 

[jbberns]

Sounds like a phase converter is a better solution, But what describing would work. Key is two transfer switches. Wouldn't recommend for a diyer. Make sure your electrician is familiar with 3 phase. Lot of residential guys aren't.