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[Stephen Swaffer]

Mike,

I could be wrong-but it seems like from measurements that part of the reason for the name "wild leg" is that the voltage to neutral is variable-depending on load balance and type (inductive-reactive-resistive).  My understanding is that the only way the wild leg can be used is for a phase to phase 208 load-either 3 phases, or between the wild leg and either of the other 2 phases?  Trying to run a 208 load from the wild leg to neutral is not a good idea.

[Mike Sokol]

Mike,

I could be wrong-but it seems like from measurements that part of the reason for the name "wild leg" is that the voltage to neutral is variable-depending on load balance and type (inductive-reactive-resistive).  My understanding is that the only way the wild leg can be used is for a phase to phase 208 load-either 3 phases, or between the wild leg and either of the other 2 phases?  Trying to run a 208 load from the wild leg to neutral is not a good idea.

That's a really interesting question. Of course in a delta high-leg system you only connect 3-phase motors that don't require a neutral at all, or 120-volt loads between the center tap and either leg of the 240-volt transformer. There's no good reason to hook anything up to 208 volts between the neutral and the "high" or "wild" leg. But while there's not as much current available on this leg, I think that's due to the reduced size or the second "high" leg transformer.

But I've got an electrical buddy who was a substation engineer, and knows all sorts of weird stuff about 25 Hz 2-phase power at Niagra Falls, and other early electrical history. I'll ask him if he knows the history of the word "wild" as in wild-leg delta power. Maybe there's something to it.

If you want a real brain teaser, take a look at the Scott-T transformer, which was invented in the 1890's to convert earlier 2-phase 90-degree power to 3-phase 120-degree power without any moving parts. http://en.wikipedia.org/wiki/Scott-T_transformer

[Tim McCulloch]

Mike,

I could be wrong-but it seems like from measurements that part of the reason for the name "wild leg" is that the voltage to neutral is variable-depending on load balance and type (inductive-reactive-resistive).  My understanding is that the only way the wild leg can be used is for a phase to phase 208 load-either 3 phases, or between the wild leg and either of the other 2 phases?  Trying to run a 208 load from the wild leg to neutral is not a good idea.

One of the nightclubs I used to work in (a century ago) had a 208v wild leg that powered the AC compressors.  The split phase 120/240v service powered everything else. 

[Frank Koenig]

Here's a thought. You guys check me on this. I don't do three-phase every day. And I have issues with polarity 

If you flip the polarity of one of the the transformers then the neutral-to-wild-leg is 317 V instead of 208 V.

(120 @ 0 deg) + (240 @ 120 deg) = (208 @ 90 deg)
(120 @ 0 deg) - (240 @ 120 deg) = (317 @ -41 deg)

Maybe this is where the "wild" reputation comes from.

Properly connected single-phase loads won't know the difference. Properly connected three-phase loads will just see the phase rotate in the opposite direction, which will cause plant technicians to flip two of the motor motor leads to get the motor to spin the way they want.

If this is correct then I think there's a joke in here about the polarity of the third leg.

--Frank

[Jonathan Johnson]

Main panels are 2 200 amp 3 phase panels, one of which has no high leg delta phase, rather phase B is fed to both bus B and bus C.  ...  Is feeding 2 buses with one phase a common practice?

My concern would be that someone could install two 120V circuits on adjacent breakers with a single, shared neutral (a fairly common practice). If the breakers are one on bus B and one on bus C (rather than one on bus A) they will both be on the same phase. This means that the shared neutral has the potential to be overloaded.

[Jeff Bankston]

a friend had a auto body shop in a 80 something year old commercial building surrounded by houses. i rewired the building for him. he had 2 seperate services coming in. a 120/240 3 wire single phase for standard 120v recepticals and lights and a 240v 3 wire 3 phase for equipment and machinery and motors. his old compressors had 3 phase 240v motors. theres also the 120/240 4 wire 3 phase high leg system. the high leg is on C phase at the service and here in cali the dept of building and safety require the high leg to be on B phase at the circut breaker panels. C gets orange phase tape. black , orange , red , white , green in the panels.

[DavidTurner]

It's interesting to me how small the kVA rating can be in relation to the notional size of the service.

In the eighties I was production manager for a singer who had attended school in a small town in Alabama. There was a homecoming concert planned for her at the gymnasium at that school and a section of highway was to be named for her with the Governor of Alabama to present the award. I live a short distance from that school and so was dispatched to do a site survey to be sure we could do a proper show in the gym. Checking for power availability, I found two 400a 3 phase disconnects across the hall in the industrial arts shop...GROOVY.

Showtime, Governor on stage ready to speak, lights are run up and POW! power to both sound and lights goes down! As luck would have it, there was someone from the local power company attending. A quick trip to his truck and he returned with a long pole to reset the breaker on the pot hanging from the pole outside the gym. It seems that the xformer on that pole could not supply even 200 amps!

We did the governor's presentation with only follow spots and audio while the power company hung a larger transformer and tied it in LIVE!

I suppose it is not uncommon to have higher breaker ampacity at the service than the transformer can supply.

[Frank Koenig]

Properly connected three-phase loads will just see the phase rotate in the opposite direction

Well, I guess you were all waiting for the other shoe to drop as old Uncle Frank figures it out.

The trap I fell into was believing that flipping one of the transformer windings (source) is equivalent to flipping two of the motor leads (load). It is not -- I drew the picture.

The transformer polarity must be correct or three-phase loads will blow up. As a result the neutral-to-wild-leg must be 208 V.

I'll be quiet now an leave this to the electricians. Thank you all.

--Frank

Edit: spelling

[Mike Sokol]

The transformer polarity must be correct or three-phase loads will blow up. As a result the neutral-to-wild-leg must be 208 V.

Isn't 3-phase power a lot of fun? 

[Jeff Bankston]

wasnt their someone named "pole pot" ? dont drop your pole in the pot. i think i'm out of phase right now. Mr. Douglas was on top of a pole many times

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