Another electrical puzzle: 120 from 240?

[David Nebenzahl]

Was called to look at an electrical problem. I looked at the panel that seemed to be the culprit, determined it to be a 240 v. 3-phase (probably) panel with a few breakers that were causing problems. One side of one of the power lugs was completely burned off, and the breaker on the other lug was making sizzling sounds. Turned it off pronto.

The panel has 3 big fat wires coming into it. Measured between all of them: 240 volts between all 3, which makes me think it's 3-phase (3 legs, equal voltage).

But here's the weird thing: there are some 120-volt outlets apparently attached to this panel that function correctly. When I turned some of the breakers off, the outlets lost power, so I assume that they're directly powered from this panel.

How can this be? How do you get 120 volts from 240? (No big fat transformers visible anywhere.) What am I missing here?

In case anyone's wondering, no, I don't mess with 3-phase power or any other kind of heavy-duty commercial/industrial stuff. My customer's reaction was "I'll call a licensed electrician", which would have been my advice. It appears the entire panel needs replaced.

I'm just curious about this situation.

[metspitzer]

Just curious. What kind of work do you do? Why were you called to look at an electrical problem?

[Pete C.]

If you read 240V and not 208V it's most likely a three phase delta configuration. To get single phase 120V service out of it one of the three distribution transformers in the three phase configuration has it's secondary center tapped to provide a neutral that gives 120V relative to each of that transformers poles. Since this transformer represents one side of the delta configuration and is connected to two of the three phase legs, that third phase leg is *not* 120V relative to the neutral and is normally color coded orange to identify it. The third leg is ~208V relative to the neutral and is typically called the "wild leg" or some less PC terms. I'm sure a 'net search on "wild leg" and "delta" will turn up some diagrams.

[John Grabowski]

*On a three phase delta service you would get 120 volts between two of the legs and ground or a neutral. The third leg would have a much higher voltage to ground and should be identified with orange tape if there was a neutral in this panel.

If there was no neutral available in this panel then there should be a transformer somewhere to provide the 120 volts.

Are you sure that the voltage was 240? It could be a three phase wye service which would have four conductors and the voltage would be 120/208.

[David Nebenzahl]

On 2/9/2009 5:38 PM metspitzer spake thus:

None of your beeswax.

[RBM]

What you missed is the neutral. And it's probably lucky that you didn't start messing with what appears to be a Delta service.

[David Nebenzahl]

On 2/9/2009 5:38 PM Pete C. spake thus:

Yes, it was 240 volts. Thanks for the explanation, by the way.

Found a diagram that shows a delta configuration with a wild leg. Makes sense.

But that panel only has 3 big fat wires coming into it, so apparently there's no neutral (or no center-tap) connection. Is it possible that this wire was upstream of this panel and connected elsewhere?

Not going to lose any sleep over this; just curious.

[Speedy Jim]

The outlets receiving 120V may be fed via one of the 240V breakers which is connected to an "autotransformer" or to an isolation 240/120V xfmr. In that case, the "derived" Neutral would not appear at the panel and the service would be plain Jane 240V Delta.

Jim

[Pete C.]

Quite common for a building with delta service to have separate three phase only and 120/240 single phase panels to help avoid confusion and mistakes. Three hot legs and ground in the three phase panel and the neural, two correct hot legs and ground in the single phase panel.

If you were turning off breakers in the three phase panel and 120V loads were going off things could be a bit of a mess as is often the case with older industrial services. 240V delta is mostly used in industrial occupancies with big three phase loads as opposed to general commercial where 208/120V wye service is preferred due to the large number of 120V loads.

[Kevin Ricks]

Sounds like its standard 240V panel with an open neutral. When the neutral is open you get full voltage readings on the neutral from the other leg via any loads that are attached and turned on. The loads may be turned on but not functioning. ie a filament of the light will be dark. Since no, or very little, current is flowing then

100% of the voltage is passed through the filament when read with a volt meter.

Remember there is no voltage drop through a resistor where there is no current flowing... so with a meter you will measure the same voltage on both ends of that resistor.

Open neutrals do strange things Some lights will be dim, others very bright and other will appear to be off depending on what load are attached to each leg.

Kevin

[Pete C.]

That's a possibility. The presence of any three pole breakers in the panel would be a tip off that it's three phase. Either way, with sizzling breakers and burned off lugs it needs serious attention and likely replacement.

[Davej]

Don't you know software guys can fix anything?

[Stormin Mormon]

With a snippy answer like this, we can only leave to imagine that you're stealing power from some where. And that you're up to no good.

[Stormin Mormon]

I have a few questions, and a few ideas. However, you didn't provide enough information in your post. And, I doubt you'll answer any questions. Seeing as how you're being snippy with folks, today.

[David Nebenzahl]

On 2/9/2009 7:57 PM Stormin Mormon spake thus:

I think this one's been pretty well answered, Stormin. Check the other posts.

[David Nebenzahl]

On 2/9/2009 7:55 PM Stormin Mormon spake thus:

Yeah. I'm stealing power and selling it out-of-state. Big bucks.

[Stormin Mormon]

You better be using an old truck to haul all that electric. The Feds might confiscate the truck that you're using to haul the electricity. Interestate thef, and all.

[David Nebenzahl]

On 2/10/2009 5:37 AM Stormin Mormon spake thus:

Nah, I just use a bunch of 55 gallon drums.