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#10352 06/07/02 02:06 PM
Joined: Jun 2002
Posts: 2
Mustang Offline OP
Junior Member
When measuring voltage from ground to each leg on a 240 volt 3 phase circuit what ahould be your volt readings on each leg?

To answer some of your questions, it is a 3phase3wire setup, we have a central bus running across the plant that supplies the presses. But I dont know which leg to come off to get my 120 volts?

[This message has been edited by Mustang (edited 06-14-2002).]

#10353 06/07/02 02:10 PM
Joined: Dec 2001
Posts: 2,498
If it is 240 V phase to phase it should read 140 V phase to neutral.

#10354 06/07/02 03:06 PM
Joined: May 2002
Posts: 30
The voltage phase to ground depends on if the service is a corner grounded 240v delta or center tapped 240v delta or if the service is true three phase three wire delta (ungrounded). If the service is the last, the voltage phase to ground is probably power less, meaning the voltage drops to zero if a load is applied. (The load can be as small as a wiggie.) This phanton voltage is usually detected with a DMM or DVM, which have high impedence.

What type of service do you have? & what are the voltage readings you are getting?

#10355 06/07/02 05:34 PM
Joined: Jan 2002
Posts: 324
Well it does depend on your Xfmr configuration, but your most common set up is a delta 240/120 which gives you a 208V to ground on your high leg which should be your B phase in your panel (and tagged orange). The C xfmr is center tapped and carries all of your 120V loads. In an industrial application where no 120V leg is needed the set up will be different.

[This message has been edited by arseegee (edited 06-07-2002).]

#10356 06/07/02 06:22 PM
Joined: Apr 2002
Posts: 2,527
Not being there it’s a bit hard to speculate, but in the US it is characteristic for utilities to ground the midpoint of one transformer on a delta or open-delta service, even of the grounded conductor is not carried in to the building. If it is corner grounded, then one phase {typically Bø} must be marked white as a grounded circuit conductor. Grounding the midpoint of one transformer allows serving additional 1ø3w loads, even if only three conductors are brought to the 3ø servce.

Ungrounded 240V ∆3ø3w is more commonly found in larger induistrial buildings served from 480V-primary dry-type transformers for servce to localized machinery, and with the exception on drive-isolation transformers, a wye 240V source is not to prevalent, for it requires 139V coils on the transformer. In most cases locally served 240V 3ø ungrounded servce is immaterial if ∆ or Y.

To some extent it depends on the age of the installation, with 208Y and 480Y outpacing ∆ service for standardization and flexibility. It also depends on who owns the serving transformer.

[This message has been edited by Bjarney (edited 06-07-2002).]

#10357 06/07/02 06:33 PM
Joined: May 2001
Posts: 717
Simple answer, and I see you are an industrial type guy.

240 volt, 3 phase, 3 wire, 120 volts to ground.
That said, that's IF it is a grounded case.

240 volt, 3 phase, 4 wire, A & C phase 120 volts to ground, 208 to ground B phase. 384-16 (e)(f)

Ungrounded, wierd readings to ground based upon load at the time, if you have single phase type loads on it, voltage can vary widely.

#10358 06/07/02 09:29 PM
Joined: Mar 2001
Posts: 2,056

Wouldn't the 120 to ground only appear on the 2 ungrounded legs? The 3rd (grounded) phase being 0 volts to ground?

#10359 06/07/02 11:06 PM
Joined: May 2001
Posts: 717
There's at least 4 ways to connect a Delta. That's why it's REAL difficult to answer his question.

Reading his profile, he's an industrial engineer, so he could have nearly anything under the sun.

1 - 3 wire ungrounded. Very sensitive industrial equipment, hospital type situations. Voltage fluctuates without ground reference. Rare

2 - 3 wire grounded. Don't use the neutral, same equipment as above but with stable ground reference. You would render a high leg with this, but with no neutral, it would not matter, but you would read 208 to ground from B phase ( or 198 if it is a 230 volt system)

3 - 4 wire center tap ground. Use the neutral, this is the one, of course, that creates the high leg.

4 - 4 wire corner ground (there can also be 3 wire corner grounds) the wierdest of the bunch and danged rare, usually industrial, never seen one around Wash. DC (no industry) but worked 'em a lot in Baltimore and Richmond.

Then you've got open or closed delta.

Only on one of those would your meter register 0 to ground on one phase. The center tap would be A to G= 120, C to G=120, B to G= 208 (or phase to ground voltage from A or C phase times 1.73)

I have not hit my books since reading his question, with the voltage reference he gave, it is also possible he's dealing with an old Scott type connection, or 2 phase. He would have 4 hot wires on this system normally. Or to put that another way, typically he would have 4, there are 2 wire Scotts as well.

[This message has been edited by George Corron (edited 06-08-2002).]

#10360 06/07/02 11:26 PM
Joined: Dec 2001
Posts: 440
Likes: 3
How's the leg George? I'm with George on this one guys. The most common set up I've seen is 3 ungrounded conductors at each corner of the delta, and one coil center tapped. This will give you the "B" phase high leg marked orange. I've come across some delta systems that were grounded on one corner, but they were few, and far between.
George are you an industrial "Guru"? Sound like it.
Mustang, I wouldn't worry about "high leg" as much I would "dirty leg", or those two "legged deer", or in George's case, a broken leg, depending on your lifestyle. Unless of course your job depends on the answers given on this forum, which may leave you on your "last leg", and never forget "three legged Willie Williams" (only his wife knew for sure).

One leg up,

The Watt Doctor
Altura Cogen
Channelview, TX
#10361 06/08/02 12:14 AM
Joined: Nov 2000
Posts: 2,236
Likes: 1
"Unless of course your job depends on the answers given on this forum, which may leave you on your 'last leg'..."

Oh, man... You're talking about me again...

Don't know where I'd be without y'all taking me by the hand...

(As always, many, many thanks...)

Residential/Commercial Inspector
5 Star Inspections
Member IAEI
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