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Will 277 V. read on meter? #129579
05/04/05 04:20 PM
05/04/05 04:20 PM
L
Last Leg  Offline OP
Member
Joined: May 2005
Posts: 41
Houston, Texas
We just turned power on at a project. It was supposed to be 480/277 V. However, the Power company hooked up straight 480. We burned some ballasts. The Journeyman says he checked voltage before flipping breakers. Question: If the Power company does not bring a neutral from their transformers, will a meter read 277V, from phase to bldg. ground, essentially?

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Re: Will 277 V. read on meter? #129580
05/04/05 04:35 PM
05/04/05 04:35 PM
P
pauluk  Offline
Member
Joined: Aug 2001
Posts: 7,520
Norfolk, England
Welcome to ECN.

If the xfmr secondary is a wye with the center point grounded, then yes, you would read 277V to ground on each phase.

How would 277V loads in the building be connected if no neutral was provided though? [Linked Image]

If the service is straight 480V delta, then the voltage to ground will depend upon how (and if) the secondary on the transformer is grounded, e.g. if corner grounded (i.e. one leg of the three grounded) you would see a full 480V to ground on the other two phases, or if the secondary is ungrounded the voltages to ground could vary quite widely.

Re: Will 277 V. read on meter? #129581
05/04/05 04:54 PM
05/04/05 04:54 PM
L
Last Leg  Offline OP
Member
Joined: May 2005
Posts: 41
Houston, Texas
The transformer was not grounded, or at least no ground from the transformer was connected to our service. The power company was out within an hour switching things around, so I am not sure if they were connected wye or delta; do not know for sure if the man on the job knows how to tell by looking. The premise wiring was all connected to our grounding electrode system. The question being, in the above situation, would a meter be able to read the 277 Volts, phase to grounding electrode without the transformer reference to ground? In other words is it a reliable method to check voltage with a tester, or was someone doing a CYA? Power company admitted fault, but the question remain.

Re: Will 277 V. read on meter? #129582
05/04/05 10:02 PM
05/04/05 10:02 PM
L
Larry Fine  Offline
Member
Joined: Feb 2005
Posts: 693
Richmond, VA
According to 250.20(B) ('02 NEC), there should have been a grounded neutral brought in, if you were expecting, and spec'ed to the POCO, that you wanted 480/277 and a neutral. It sounds like you had a ground, but no neutral.

To answer the question directly, with no load, and the transformers' Y-connected secondaries' common point grounded, a meter should have read 277, but the impedance of earth is not low enough to assure 0 volts on the neutral with loads.

However, "straight 480" suggests a delta-connected secondary, which would limit you to either corner-grounding or a center-grounded secondary (both of which would show up on a voltage test), but the latter isn't done on 480.

I can't help but wonder how any competent electrician could not notice a service with no neutral conductor. Where did the panelboard neutrals go? Was this a simple main-breaker panel, or did the electrician assume the ground was "neutral'ed"?


Larry Fine
Fine Electric Co.
fineelectricco.com
Re: Will 277 V. read on meter? #129583
05/05/05 07:24 AM
05/05/05 07:24 AM
L
Last Leg  Offline OP
Member
Joined: May 2005
Posts: 41
Houston, Texas
This morning, I finally got the scoop. It was hooked up 480 delta, no neutral. Our Superintendent said they had the neutral made up straight to their pole ground, which was not tied to the transformer at all. He said the lineman said, get this; we get our neutral for 277 from our transformer inside.
Okay, daily problems aside, would this situation allow a meter to read 277 volts? I listed this under the theory area because I want it to gel in my mind. Thanks for your response.

Re: Will 277 V. read on meter? #129584
05/05/05 08:18 AM
05/05/05 08:18 AM
R
resqcapt19  Offline
Member
Joined: Nov 2000
Posts: 2,148
IL
The ungrounded delta will read ~277 to ground in many cases, especially if the loads are not connected or are all 3 phase.
Don


Don(resqcapt19)
Re: Will 277 V. read on meter? #129585
05/05/05 10:33 AM
05/05/05 10:33 AM
B
BobH  Offline
Member
Joined: Oct 2004
Posts: 135
Newfane, N.Y USA
Being that many digital meters are high impedance I would suspect you would read 277 volts to ground. Mistakes happen to even the best of em.

Re: Will 277 V. read on meter? #129586
05/05/05 02:20 PM
05/05/05 02:20 PM
P
pauluk  Offline
Member
Joined: Aug 2001
Posts: 7,520
Norfolk, England
Quote
He said the lineman said, get this; we get our neutral for 277 from our transformer inside.

That sounds almost as if the PoCo was expecting you to install a delta-wye xfmr on the premises.

Re: Will 277 V. read on meter? #129587
05/06/05 11:50 AM
05/06/05 11:50 AM
Scott35  Offline

Broom Pusher and
Member
Joined: Oct 2000
Posts: 2,708
Anaheim, CA. USA
LastLeg;

Welcome to ECN!

So, the PoCo connected their Xformers in a Closed 3 Wire Delta configuration, instead of a (commonly used) 480Y/277 VAC 3Ø 4Wire config.?!?!?!
Very odd (to me), especially since (in my areas) the 480 VAC 3Ø 3 Wire Delta is a special order / situation only setup!

As to the Meter's reading of ± 277 VAC to Ground, this must have been measured with a High Input Impedance Volt Meter - like a DVM (Digital Voltage Meter) or a DMM (Digital Multi Meter).

Was the measurement done with a DVM/DMM, or a typical "Wiggy" Solenoid + Neon type Voltage Meter?

If DVM/DMM, these will read the "Capacitive Coupling Effects" between an Ungrounded AC System's Line Conductor(s), and a Conductor / Metallic Enclosure which has been Bonded to an Earth Reference.
"Shunting" the Meter's leads with a "Wiggy" will remove nearly all the Cap. effect, and give a much closer to accurate response.
(Shunting the leads = connect the "Red" leads of both meters to one test conductor, and the "Black" leads of both meters to the Grounded reference).

If the tester was a "Wiggy", and the readings were a "Solid" + 240 VAC (between 240 VAC and 480 VAC), and each Line was equal to Ground, then this is a new one on me!
The Wiggy will nearly always act like it is discharging a Capacitor (quickly "flash" a reading in a high range), then either drop off completely, or slightly hum a few seconds until responding with a dead indication.

If the Wiggy showed a "Solid" reading, which was "Somewhat Equal" on 2 Lines to Ground, but "Noticeably Higher" on the 3rd Line to Ground test, then this would indicate the Identified Conductor (the intended to be Grounded "Neutral" Conductor) was connected to the Center Tap of one Transformer, and the configuration is a Delta.

Let me know what tester(s) used, and any other questions.

Scott35


Scott " 35 " Thompson
Just Say NO To Green Eggs And Ham!
Re: Will 277 V. read on meter? #129588
05/09/05 09:21 PM
05/09/05 09:21 PM
Scott35  Offline

Broom Pusher and
Member
Joined: Oct 2000
Posts: 2,708
Anaheim, CA. USA
***BUMP***

Last Leg, did you see my latest reply?

I may have some data to contribute regarding this scenario.

Scott35


Scott " 35 " Thompson
Just Say NO To Green Eggs And Ham!
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