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Author Topic: 120v/208V vs 120/220V  (Read 3636 times)

Sebastian Vidales

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120v/208V vs 120/220V
« on: October 05, 2015, 08:35:56 pm »

Greetings to you all,

Im currently working on a show for a big pop artist, and just did a show for a big rock artist. Our generator vendor supplies 120v phase to ground, and 220v phase to phase. We work like this always, and our country voltage is 120v and our conector is Edison.
In riders they spec they want 120/208v.
 Last weekend we had 2 synchronic generators running at 220v between phases, in their distro  (for consoles and monitors)  they received 127v: They insisted it was to Hot, they couldn't work this way, so we hooked them up to a 208v  phase to phase converted generator. They had their distro with 120 an everything worked out ok. That generator had no backup.

For our next show, a production manager is insisting on changing the whole system to 120/208v.

I intuit this would not be a good idea.....

Any insight?


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Tim McCulloch

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Re: 120v/208V vs 120/220V
« Reply #1 on: October 05, 2015, 09:21:06 pm »

Greetings to you all,

Im currently working on a show for a big pop artist, and just did a show for a big rock artist. Our generator vendor supplies 120v phase to ground, and 220v phase to phase. We work like this always, and our country voltage is 120v and our conector is Edison.
In riders they spec they want 120/208v.
 Last weekend we had 2 synchronic generators running at 220v between phases, in their distro  (for consoles and monitors)  they received 127v: They insisted it was to Hot, they couldn't work this way, so we hooked them up to a 208v  phase to phase converted generator. They had their distro with 120 an everything worked out ok. That generator had no backup.

For our next show, a production manager is insisting on changing the whole system to 120/208v.

I intuit this would not be a good idea.....

Any insight?

Greetings Sebastian-

I think there may be a misunderstanding of 120/220v.  In electrical utility power service the "120v" is derived from a center-tapped secondary winding of a transformer; the center tap becomes the grounded current-carrying conductor (we call it "neutral").  If you measure the voltage from center tap to "hot" and find 117v, you should measure 234v from the two points either side of the center tap (the "hot legs").  In other words it should double, and if you have 220v across the 2 hots you should measure 110v from neutral to hot.  This presumes reasonably equal loading of each hot leg, or measured without load.

A no-load voltage of 127v on each leg is within the -5%/+10% voltage tolerance that most nominally 120v. devices are designed to accommodate.  With the PA running I'd expect to see a voltage drop of 3 to 5v, and you're still within tolerance.

To take this another step, consider that if your equipment is designed to run with a nominal 120v. supply voltage, the rest - 208 vs. 240 (or 220) - doesn't factor into the voltage supplied.  In a 3 phase 120/208 wye service the voltage from neutral to any hot leg is 120v.  The voltage between any 2 hot legs is 208v.

My question, since the numbers don't add up:  Was the voltage at the generator(s) and the production company distro(s) measured with the same voltmeter?  I ask because unless they have a certified calibration I've almost never seen 2 voltmeters agree, and I've seen plenty of errors from meters built into distros.

Edit ps.  We have forum specifically for electrical and grounding issues.  Your thread will probably get moved to that forum, and it's a great place.
« Last Edit: October 05, 2015, 09:23:33 pm by Tim McCulloch »
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Brian Jojade

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Re: 120v/208V vs 120/220V
« Reply #2 on: October 05, 2015, 09:46:46 pm »

Tim is spot on.

If the artist is using 120v devices, they shouldn't care whether you use single phase (120/240) or 3 Phase (120/208) to supply power.  When you supply power, especially from a generator, it is important that you load each leg of the generator about equally, or you can get more voltage fluctuation, but it doesn't need to be exact.  127v on an unloaded supply with an accurate meter is within spec.

Now, if they are operating 3 phase devices, such as motors, then you can't get away with providing them a single phase supply.  Doesn't sound like that's the case here.
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TJ (Tom) Cornish

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Re: 120v/208V vs 120/220V
« Reply #3 on: October 05, 2015, 10:59:05 pm »

Greetings to you all,

Im currently working on a show for a big pop artist, and just did a show for a big rock artist. Our generator vendor supplies 120v phase to ground, and 220v phase to phase. We work like this always, and our country voltage is 120v and our conector is Edison.
In riders they spec they want 120/208v.
 Last weekend we had 2 synchronic generators running at 220v between phases, in their distro  (for consoles and monitors)  they received 127v: They insisted it was to Hot, they couldn't work this way, so we hooked them up to a 208v  phase to phase converted generator. They had their distro with 120 an everything worked out ok. That generator had no backup.

For our next show, a production manager is insisting on changing the whole system to 120/208v.

I intuit this would not be a good idea.....

Any insight?
It seems to me that you have a 3-phase generator running too hot - 220V phase to phase instead of the usual 208V phase to phase.  Regulating that back down closer to 208V should lower the phase to neutral voltage, but if you're only reading 127V at the generator, that's probably not too far off considering distribution wiring.

Saying this another way - 220V or 240V is a different kind of power system - a center-tapped single phase service, where ideally the line to line voltage is closer to 240V rather than 220V.  This is not what you should be aiming for with a 3-phase generator, and I agree with your production manager that 208V/120V is the right call (or maybe 212-215V/123-125V to allow for a little bit of loss).

There are lots of places to read up on the difference between 3-phase power and single/split-phase power that explain this in much more detail.
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Cailen Waddell

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Re: 120v/208V vs 120/220V
« Reply #4 on: October 05, 2015, 11:24:20 pm »

Sebastian where are you located?

120/208 3 phase is common production power here in states... But if you are somewhere else, it might or might not be...




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jason misterka

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Re: 120v/208V vs 120/220V
« Reply #5 on: October 05, 2015, 11:48:20 pm »

It seems to me that you have a 3-phase generator running too hot - 220V phase to phase instead of the usual 208V phase to phase.  Regulating that back down closer to 208V should lower the phase to neutral voltage, but if you're only reading 127V at the generator, that's probably not too far off considering distribution wiring.

Saying this another way - 220V or 240V is a different kind of power system - a center-tapped single phase service, where ideally the line to line voltage is closer to 240V rather than 220V.  This is not what you should be aiming for with a 3-phase generator, and I agree with your production manager that 208V/120V is the right call (or maybe 212-215V/123-125V to allow for a little bit of loss).

There are lots of places to read up on the difference between 3-phase power and single/split-phase power that explain this in much more detail.

That's what it sounds like to me. Generator set to three phase mode running 220 - 240volts instead of 208.  In single phase mode on the generator 220 - 240 volts would be fine.  There should be a voltage knob on the generator with enough room to adjust down from 240 to 208.

Jason
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Re: 120v/208V vs 120/220V
« Reply #5 on: October 05, 2015, 11:48:20 pm »


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