Electrical Question

Looks like you've identified the 2 hots (120 volts each to ground, 240 across them). Is this circuit connected to the 30 amp double breaker?

The other 2 lines should be grounded (check). Your saw and compressor will only need one. You can replace the 4-wire receptacle with a 3-wire and just tape the extra ground wire.

 

The terminal with the t*t on it would be the ground and the nickel screw on the back would be neutral so you could also wire a plug to give 120volts. Are you measuring between the 2 hot legs to get 250 volts?
If that's a generator input to your house it's the wrong sex and could fry someone working on a pole if you used it to power your house if you lost power. You'd need a transfer switch.

 

The 4-prong outlets are basically 2 phases (two hots), neutral and ground. The 3-prong outlets are 2 hots and netural.

The extra ground is a good thing. Code now for 220V is the 4-prong outlet. The 3-prong is the older style. I would say not to change the 4-prong outlet to a 3-prong unless you are sure you will only ever use it with the saw and don't want to change the cable on the saw.

To get it to work, you can do this one of two ways... change the socket on the wall to a 3-prong, which if you do, you would just not use the ground wire and use the 2 hots and the neutral. OR, you could convert the saw to a 4-prong plug (would be my choice).

Or, you can buy an adapter that just plugs on the end of the 3-prong saw connector and lets it plug into the 4-prong outlet. Or you can make an adapter cable from a 4-prong plug, 3-prong socket, and small bit of 220V wiring that you can get at Home Depot.

If you change the cord on the saw to a 4-prong cord, open the electrical panel where the cable goes in and there will be a little jumper to remove (that, on a 3-prong system, just connects the neutral to the ground). On a 4-prong wire, you have seperate neutral and ground, so connect them accordingly.

 

On a 3-wire 240 volt circuit there is no "neutral" only a ground, as the current flow is between the "hots". The 4-wire circuit uses a second grounded conductor as a dedicated "neutral" only for any 120 volt accessories (such as a clock) that may be powered from one leg of the 240 volt source.

 

It's not "brain surgery" but as Dirty Harry reminded us, "a man's got to know his limitations".
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Make sure your electrician does something about the 30 amp breaker protecting the 20 amp plug. If the wire feeding the plug is #12, you need to replace the 30 amp breaker with a 20. If the wire is #10, you could replace the plug with a 30 amp receptical.

 

I know what you're saying but it's technically incorrect

In a 220V 3-prong, it's two hots and neutral. On a 4-prong, it's 2 hots, neutral and ground. In that 3-prong cable, he's going to have red and black which are "hot" and white which is "neutral" - connected back to the center tap on the utility pole transformer. This is the problem with the 3-prong setup and why it was changed in the electrical code in the 90's. The neutral wire serves the function of ground due to a jumper on the appliance, but technically it's still the neutral, not ground, even though it's used as ground. You are right, there is normally no current flow between the hot wire and neutral, but if you look at the box wiring for a 3-prong 220V outlet, the connections back to the panel will be hot, hot, neutral instead of hot, hot, ground. Using neutral as ground was allowed back in the day to save on copper, but it doesn't allow for 110V circuits (like you said for clocks and such) and it also isn't as safe as having a dedicated ground.

In some ways it's a semantics thing, but IMO there is an important and often misunderstood (not by you, by the average consumer) difference between what neutral is and what ground is. IMO it's sort of crazy that it was ever allowed by code to be using the neutral wire as ground!

 

The color of the wire insulation doesn't determine what it is called.

The third wire in a 3-wire 240 volt circuit is for equipment grounding (the motor will still run without it). It was misused as a "neutral" in the past.

This subject is confusing and you and I mostly agree, it's largely a matter of semantics.

The NEC refers to "neutral" as "the grounded conductor" and the "ground" as "the grounding conductor". They are both bonded together at the Service Panel and connected to water pipes, ground rods, etc. as well as the center tap of the transformer.

The 4-wire plug added the second grounded wire which really became a dedicated "neutral" for those 120 volt accessories that had been sharing the ground wire for this purpose.

 

Aren't you glad you asked?

 

Believe it or not it actually is easier to follow than some things on Italian cars.

 

Bottom line - don't be a chicken... grab a pair of pliers, bend the prongs on that 3-prong plug until it fits into the 4-prong socket, just pick any 3 slots and get bending.

You've got kids... get one of them to plug it in for it's first test

But seriously, you can get 4-prong plugs on a wire at Home Depot for this very purpose. Just pop the panel on the saw where the wire goes in - you'll see a strap or brace connecting ground to neutral (combining them at the saw) and you'll also see two hot connections. Take the strap off, now you have 4 connections - 2 hots, neutral and ground. 4 conductors on your cable... hook 'er up and you're good to go!

The 4 conductor plug is "safer" and your saw has the necessary connections - just need to pop that strap off and wire up the new cable. If I were in your area, I'd do it for you on barter for a free colonoscopy. Maybe 2NA is closer and will barter that colonoscopy in exchange for hooking you up?