An electrician question

[MotorDoctor]

While I was upgrading the shop heating system and bugging our resident HVAC guru to death I noticed in the power panel (fuse box?) (breaker box?) that there are four large gauge wires coming in. A red, white, black and blue. The blue is taped off and not being used. The white is neutral, the red and black are hot. Would I be correct in assuming that it (blue) is the third leg of Three phase? Happy happy, joy joy if it is I really want to get a bigger air compressor and I'm constantly seeing the 3 phase variety on the local craigslist. If it turns out to be 3 phase I guess my next step will be to figure out how to make that work for an air compressor. Must take a different power panel or sub panel? Thanks for your help.

[rickairmedic]

Dan if you dont have an answer by morning I will ask my neighbor electrician buddy .



Rick

[LKeithR]

Not saying it's not there but you'll only get 3-phase power if it's actually connected at the pole or meter. Sounds to me like someone may have just used a 4-conductor wire when only 3 were needed. You may find that the blue wire is not hooked to anything at the other end as well...

[terry lingle]

That blue wire is indeed the conductor for the third phase.
you need to look at the incomming source to see if it has the third phase available.
More likely the installer just had a piece of 4 conductor available and the blue is jusr an extra wire in the cable.

[Paychk]

X2 on use what you got, the blue is most likely safed off at the other end.

[Dr Dean]

My brother is an electrican. On long underground runs he will use 4 conductor wire instead of 3. The 4th wire will not be connected to anything untill one leg fails underground, at that time the 4th wire will be used and the failed wire is then disconnected. It's cheaper to run 4 conductor than trench in a whole new run. Wires do get skinned or damaged from time to time, crap happens.

[MotorDoctor]

Sounds like the thing to do is check it for voltage to see if it's hooked up. Looked on the local craigslist last night, no 3 phase compressors cheap

[Bender]

Originally Posted by Dr Dean

My brother is an electrican. On long underground runs he will use 4 conductor wire instead of 3. The 4th wire will not be connected to anything untill one leg fails underground, at that time the 4th wire will be used and the failed wire is then disconnected. It's cheaper to run 4 conductor than trench in a whole new run. Wires do get skinned or damaged from time to time, crap happens.

Just happened to my neighbors irrigation pump. Sparky just deleted the bad wire and hooked up the 4th one. Saved a lot of $$.

[LKeithR]

Originally Posted by Dr Dean

My brother is an electrican. On long underground runs he will use 4 conductor wire instead of 3. The 4th wire will not be connected to anything untill one leg fails underground, at that time the 4th wire will be used and the failed wire is then disconnected. It's cheaper to run 4 conductor than trench in a whole new run. Wires do get skinned or damaged from time to time, crap happens.

I did the same thing with buried water pipes. Our water has a lot of organic material in it so it only takes a few years and the line gets plugged to the point where flow is greatly restricted. I got tired of having to keep digging up the main line between the pump and our house so the last time it happened I laid 4 pipes in the ground. Plastic is dirt cheap and now when one line plugs up all it takes is a few minutes at both ends to connect a clean one and we're good to go. That was 8-9 years ago and we're still on pipe #2 so what's left will probably last as long as we're going to be on the place. That's a good thing 'cause at my age the idea of digging up more water lines has no sex appeal whatsoever...

[MotorDoctor]

I checked the blue wire for voltage when I got to work today. Just a spare leg
What's funny is it does show some voltage on it, about 32vac between it and the white neutral. Must have some stray voltage on it I guess. The old thermostat on the old heater was the same way. When it was switched on, it showed line voltage (120vac) but with it off it had about 75vac. So I replaced it. Now when it's off, there's no voltage.