Commercial Electrical Help Requested

[Woodtroll]

Hello, folks,

Not gun related, but I'm hoping someone here can help me.

I'm trying to help a friend put a compressor in a garage that has three-phase electrical power. I'm pretty good at residential wiring, and understand the basic concept of three-phase power supply, but I am no three-phase expert and this one has me stumped.

This box used to power an approximately 10HP motor, which is what he wants to put back in (a 10HP compressor). However, there are only two wires running into this box, I assume one hot and one neutral (?), with 280V across them according to my meter. I've never seen three-phase without three hot leads (but know they have no dedicated neutral), so I can't figure out what this supply is so I can request the right motor for his compressor. Is this a single-phase 277/280V circuit, (which I would assume means this is a 480V three-phase building) or how would I describe it?



Thanks for your help!
Regan

[draperjojo]

I doubt anyone would fuse a neutral. Is there a ground pulled? What's the voltage from ground to each incoming wire? How many spaces does the breaker that feeds this take up in its panel, two? Do you trust your meter?

[Tanqueray]

That fuse disconnect has three terminals - the continuous one on the right would typically be for either neutral or ground. This is for either a single phase or two phase load, but I’d guess it’s single phase with a fused neutral.
As previously mentioned, the easiest way to establish this would be with a meter.

[Catshooter]

A 480/277 volt service can indeed have a dedicated neutral. It can also have no neutral as in ungrounded. Then one would almost never use a single pole breaker in such a panel. And one should almost never fuse a neutral. So what you're looking at may have been installed by someone who doesn't know what he is doing.

I'd go to the panel that feeds that disconnect and put my meter to work.

If your meter is telling the truth then the panel that feeds this disconnect should have one of those conductors under a single pole breaker. Then yes, it's a 480/277 panel with a neutral.


Cat

[4t5]

Do not attempt another hot leg into that box,the unused connection is for a neutral and is bonded the metal box (dead short).This appears to be wired for single phase 208 volts,or 240 volts depending on the incoming voltage,the previous load(motor) seems to have needed two hot legs and a mechanical ground, the ground coming from the emt(metal tubing) connected to the fused switch box and no neutral.

[marlinman93]

If you read each leg to the unfused leg and each of them read 280v. then across the two fused legs should read 480v. approximately. That means you don't have a 3 phase system there. but rather a single phase 480v. system, and it is using two legs of the 480 and a ground.
Be careful messing around with these systems, as just being good at wiring around the house could mean you get yourself hurt if you aren't careful, or sure of what you have there! I did this for over 40 years, and have plenty of horror stories I could tell!

[Griff]

I don't find anything like that in my Time-Life books on home repair... which is far beyond the extent of my electrical expertise! I just spent two days trying to fix a low voltage issue in my barn & shop, (my 120V in becomes 107V by the time it reaches my new LED lights in the shop)... I found all the common & ground wired together... tried to separate them and found NOTHING then worked! Ended up replacing some wire, disconnecting some runs to stuff that I never used (water heaters in the horse stalls), and ended up putting jumpers between the common bar and newly installed ground bars... place is 43 years old... my most pleasant (NOT!) surprise was a run of from the barn's subpanel to a sub-subpanel (workshop) had been 4 gauge wire, but sometime before I bought it had been cut to feed the water heaters and jumpered with 10 gauge! Evidence of high heat in it... now I'm wondering just how much of the house and the 250' run from the main to the sub-panel will need to be redone...

[Woodtroll]

Thanks, everyone, for your help. I didn't make it clear in my original post that this building DOES have three-phase power, and I'm just trying to figure out what is in the box so I can get his motor ordered. I'm not about to start messing with this circuit myself and because of the complex building layout and the inability to trace it back to its source panel FOR CERTAIN, I am not going to be the one messing with any rewiring. There is no ground coming into the box (the only two wires coming into the box are the switched/fused ones you see), so I think the ground must be back through the EMT conduit like was acceptable back in the old days.

This building is over 50 years old, has been added to and modified several times, and I have no idea when this circuit was added. I am pretty sure this is a 277/480 three-phase system, but I did not think at the time to check each leg by grounding it to the box itself. I was just so puzzled to see only two wires in the box, with no dedicated ground and 280V between the two wires, that I did not think to do any other checking.

Thanks for everyone's concern, too - I know these things are nothing to play with. I have a healthy respect for household current and am comfortable with residential wiring, but the big numbers scare me. I am no coward - I will stand and fight anything I can see if forced to, but you can't see electricity!

Thanks again, everyone, take care!
Regan

[marlinman93]

Since the conduit it the ground, and you have 280v. between the two wires, then you likely have only one hot in that box, so it's a 277v. single phase feed. If you read each wire to ground you'll likely find only one of them reads the same 280 volts you get between wires, as the other should be a neutral. The neutral wire should have been marked with white or natural gray tape to indicate it is a neutral.
I can see that someone installed the identified ground screw in the unfused buss on the right, so it is indeed a ground. It's the colored screw that is seen in the upper part of that buss bar.
If you went back to the feed or source for this disconnect and opened the cover, you'll likely see it's fed by a single fuse, or single pole breaker. The neutral should not be fused, although it's not a code violation to do so. It's just a waste of money to fuse it, and without the neutral marking it creates confusion and a dangerous situation in this setup.
If your friend wanted a 480v. single phase feeder, the neutral could be moved at the source to a hot feed, and then this disconnect could be used as a 480v. single phase feed. The disconnect should also have a tag on the cover showing it is rated for "600 volt" use. If it has a tag stating it's "250v." rated, then it's illegal even for 277v. use.