Rotary phase converter: local ground or all the way to the panel?

Ok. Thanks for everybody's help in the earlier thread. The 20HP rotary phase converter is working, but I now need to place it in the garage, about 70+ feet wiring run distance from the main breaker panel.

Given the cost of copper wire these days, my inclination is to run just the two hots to the panel and to put a grounding rod close up to the garage and run a local ground to it. I see no reason to run a neutral line from the panel (the machines are all 3 phase).

I know that I could run a ground line to the panel AND run a local grounding rod, but is it a bad idea to just do a local ground?

What gage wire for a 20HP RPC, but the main/biggest load/machine will be a 10HP spindle and a couple of 2HP machines, never all at the same time? ($ signs get much bigger with the wire gage/diameter :-)

Anybody near San Jose, CA have a spool of #4 or larger gage for cheap? :-)

Thanks in advance!

Reply to
rpseguin
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bad idea it can create a ground loop and hazardous vlotages between grounds, install a ground rod, but definetely bond it to the main building ground.

Reply to
hallerb

I used 6/3 SO cable for mine. For that 70+' distance however, you might be better off with 4/3. The SO cable might cost a little more, but it makes for a clean installation. Unless you plan to run conduit.

Also keep in mind, if you run wire instead of SO cable, your ground doesn't need to be as big as the hot wires. A #10 ground wire will work.

Matt

Reply to
Matt Stawicki

That is illegal and unsafe. I assume the "primary" side is a 240V motor? You need an equipment ground conductor all the way back to the

240V service. If one of the hots shorts out to the motor case, for example, you need this bonding path to carry enough current to trip the breaker. You can not rely on the earth for the bonding path, its resistance is too high.

As for the secondary side, you need to look up in the NEC how to wire a "separately derived system". You can also look up how to wire motor loads to determine the size of the feeder conductors for the primary side.

Cheers, Wayne

Reply to
Wayne Whitney

SO cable is not for permanent installation, you need to use SER or UF cable.

Wayne

Reply to
Wayne Whitney

=46orgot to mention...... Go all the way to the panel with the ground.

Matt

Reply to
Matt Stawicki

You can get by with #10 Green..but...

Go all the way to the panel.

While life is cheap, replacing you can be expensive

Gunner

Reply to
Gunner

copper is costly but why install something you know wouldnt pass code, or a future home inspection.

do it right do it once then relax and forget about it

Reply to
hallerb

I'll do it the right way and run it to the panel.

I'm just renting the house, which, by the way, doesn't have any grounded outlets other than a couple of GFIs in the bathrooms. The landlord did put 3 prong grounded receptacles in, even though there's no ground wires in any of them.

Reply to
rpseguin

Put the converter right near the entrance panel so the highest current only has to run the short distance. Then run smaller wire to the remote 3 phase loads. Requires an additional lead but the lower current req. should work out a lot cheaper. Now as to "remotely developed grounds" This was legal when we did a bunch of "cabins in the woods" supplied from a main lodge back quite a few years ago (I havent checked the code since). We ran the power with two direct burrial wires and installed a standard grounding rod at each cabin. This was all code compliant (at that time). ...lew...

Reply to
Lew Hartswick

Lew sez:

"Put the converter right near the entrance panel so the highest current only has to run the short distance. Then run smaller wire to the remote 3 phase loads. Requires an additional lead but the lower current req. should work out a lot cheaper."

Not a good idea and not even remotely close to any "code". A RPC consists of one or more idler and load motors. The combination comprises a sort of electrical network. Complex circulating currents flow between the idler and load. It is not like there is a "generator" side and a "load" side. The same current circulates throughout the network; wiring should be sized accordingly.

Bob Swinney

Now as to "remotely developed grounds" This was legal when we did a bunch of "cabins in the woods" supplied from a main lodge back quite a few years ago (I havent checked the code since). We ran the power with two direct burrial wires and installed a standard grounding rod at each cabin. This was all code compliant (at that time). ...lew...

Reply to
Robert Swinney

| I'm just renting the house, which, by the way, doesn't have any | grounded outlets other than a couple of GFIs in the bathrooms. | The landlord did put 3 prong grounded receptacles in, even though | there's no ground wires in any of them.

They don't make very many 2-prong ungrounded GFCI receptacles, even though such things would work and can even do a self-test without a ground wire.

Reply to
phil-news-nospam

According to rpseguin :

Yes, for the reasons expressed by others.

Remember that there are losses in the RPC. A 10HP spindle actually loaded to 10HP (as opposed to an unloaded 10HP motor) is going to present something more like 12-13HP on the RPC. So, you're going to have to factor in 20-30% losses.

Secondly, yeah, wire is expensive. You may wish to consider using aluminum instead. Done correctly (to-code connectors and conductive grease for the application), it's just as safe, to-code, and a lot cheaper.

When I installed the 100A subpanel to my detached garage (100' of burial), I saved something near $300 by using aluminum instead of copper, and the inspector was perfectly happy with it - despite catching the electrician forgetting the grease on the subpanel end... ;-).

Reply to
Chris Lewis

True. But to clarify: three prong GFCI receptacles that aren't grounded are legal. Ungrounded three prong outlets _downstream_ of a GFCI are also legal (as a retrofit), as long as you use the stickers saying "ungrounded outlet" on them.

Reply to
Chris Lewis

Folks:

I gotta say, when you start tossing 20HP rotary convertors and 10 hp motors into the pot, economy starts to shed its meaning. Best to do the job right. If a 10 hp motor can't pay for its own infrastructure it might as well go to pasture. I'd say this even goes if the motor is for recreational use

-- fun does have value, but fun can turn bad if you cut corners. Don't skimp on your hot sticks when turning on your Tesla coil.

But, man, ten horses...some people have big fun, I guess.

A P

Reply to
autobus_prime

In alt.engineering.electrical Chris Lewis wrote: | According to : |> In alt.engineering.electrical rpseguin wrote: |> |> | I'm just renting the house, which, by the way, doesn't have any |> | grounded outlets other than a couple of GFIs in the bathrooms. |> | The landlord did put 3 prong grounded receptacles in, even though |> | there's no ground wires in any of them. |> |> They don't make very many 2-prong ungrounded GFCI receptacles, even |> though such things would work and can even do a self-test without a |> ground wire. | | True. But to clarify: three prong GFCI receptacles that aren't grounded | are legal. Ungrounded three prong outlets _downstream_ of a GFCI are | also legal (as a retrofit), as long as you use the stickers saying | "ungrounded outlet" on them.

How about one of those grounded plug adapters ... the kind where you have

3 holes on one end to plug in a grounded plug, and only 2 prongs on the other end to plug into a legacy 2-hole no-ground outlet ... that integrates GFCI protection as part of the adapter? I've seen GFCI cord sets, but only with a grounded plug. How about with an ungrounded plug?
Reply to
phil-news-nospam

Don't know your actual installation plan, but you may be able to pick up a coil of #4 AL triplex from the local power company - some of it gets scrapped, either from teardown, or a leftover chunk off of a reel. Ken.

Reply to
Ken Sterling

Shhh but in a pinch I've used a "strand"--twisted together from 3 individual pieces of #12 or 14 copper.

Reply to
Jeffrey Lebowski

:-) I get the message. I won't cheap out. My credit card is feeling the strain though.

Still looking around for some #2 or bigger. I've got a lead on some #4 wire.

My plan, after all the advice is to put a single phase 100A subpanel in the garage. That subpanel will have a breaker for the RPC.

Main Panel | | #2 wire, 70+ feet run | 1ph Sub-panel | | 60A 2 pole breaker, #2 wire, 10 foot max run | RPC | | #2 wire, 10 foot max run | 3ph Panel (3ph/pole breakers for machines)

Reply to
rpseguin

As has been mentioned, you don't need a full size ground. You should check the code, but my recollection is that a #10 is a suitable grounding conductor for a 60A circuit.

Reply to
Ned Simmons

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