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sk1939
Premium Member
join:2010-10-23
Frederick, MD

sk1939

Premium Member

[Electrical] 3 Phase Power

Is there any easy way to balance out a 3 phase distribution system that has entirely single-phase loads relatively easily? Having small issues like registering 12VAC on ground.
bbear2
Premium Member
join:2003-10-06
dot.earth

bbear2

Premium Member

What do you mean by easily? Dynamically? Automatically? Or just with a spreadsheet and watt-o-meter?

mackey
Premium Member
join:2007-08-20

1 edit

mackey to sk1939

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to sk1939
Assuming you have a bunch of MWBC's in groups of 3, you can always shuffle circuits around as long as you stay within the group. I.E. if 1/3/5 was a group, you could swap 1 and 3, 1 and 5, or 3 and 5. However, you CANNOT swap 1 and 4, 5 and 7, etc.

However, getting 12v on the neutral means you have a dirty open/high resistance splice somewhere, someone broke up a group and you now have a seriously overloaded neutral, or too small a gauge was used for the length of the run.

/M

nunya
LXI 483
MVM
join:2000-12-23
O Fallon, MO

nunya to sk1939

MVM

to sk1939
How are you measuring 12V on the ground? What points are you measuring?

sk1939
Premium Member
join:2010-10-23
Frederick, MD

sk1939 to mackey

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to mackey
No MWBC's present.
sk1939

sk1939 to nunya

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to nunya
Click for full size
Neutral to Ground at the panel. There's no way I'm working on this myself, just trying to get a feel for the issue at hand.

John Galt6
Forward, March
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join:2004-09-30
Happy Camp

John Galt6 to sk1939

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to sk1939
said by nunya:

How are you measuring 12V on the ground? What points are you measuring?

Using a DMM?

nunya
LXI 483
MVM
join:2000-12-23
O Fallon, MO
·Charter

nunya to sk1939

MVM

to sk1939
You may not even have an issue. A lot of factors come into play:
How far is it to the main disconnect (where N and G should be bonded). A lot of influence can be picked up between hither and yon.
Common mode voltage can be misleading. In a perfect world, N-G V would be 0. That doesn't happen often.

You start getting into harmonics and field side grounding / bonding issues.

TBQFH, unless you are running some uber-sensitive equipment, I wouldn't be concerned at all with 12V.

I wouldn't even bat and eye at anything less than 20V on a typical 4w system.

guppy_fish
Premium Member
join:2003-12-09
Palm Harbor, FL

guppy_fish

Premium Member

Further on nunya's post, your measuring of the neutral is not at the bond point, all your doing is measuring the volt drop in the line from the distribution panel to the ground point, which isn't the end of the completed circuit.

On 3 Phase, the closer the loads are on all three phases, they will lower the neutral current, and hence, where your measuring the voltage at, but its meaningless as the neutral is spec'ed to handled a full imbalanced line ( all three phases and neutral are the same gauge wire ), meaning one phase fully loaded and then return current at the same current down the neutral.

tschmidt
MVM
join:2000-11-12
Milford, NH
·Consolidated Com..
·Republic Wireless
·Hollis Hosting

tschmidt to nunya

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to nunya
said by nunya:

I wouldn't even bat and eye at anything less than 20V on a typical 4w system.

Could you go into more detail. I realize the OP is measuring difference far from the bonding point but 20 volts seems like an awful lot. What is producing such a large difference?

I would have though the difference in potential between Neutral and Safety Ground should not be more the the voltage drop of the neutral conductor - just a few percent at most.

/tom
iknow_t
join:2012-05-03

iknow_t to sk1939

Member

to sk1939
it seems even 12V on 200F of wire would make it very hot.. and that's 1200A wire.. unless it's high harmonics..

sk1939
Premium Member
join:2010-10-23
Frederick, MD

1 recommendation

sk1939 to John Galt6

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to John Galt6
Yes, calibrated Fluke 179.
sk1939

sk1939 to nunya

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to nunya
Well we had some issues with some audio equipment, but nothing a dedicated and isolated ground didn't fix.
iknow_t
join:2012-05-03

iknow_t

Member

said by sk1939:

Well we had some issues with some audio equipment, but nothing a dedicated and isolated ground didn't fix.

1200A 3 phase and audio equipment sounds like a major recording studio..

ArthurS
Watch Those Blinking Lights
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join:2000-10-28
Hamilton, ON

ArthurS to sk1939

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Modern audio equipment shouldn't powered by three phase power. All my designs require single phase 3 wire service with isolated ground.

sk1939
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join:2010-10-23
Frederick, MD
ARRIS SB8200
Ubiquiti UDM-Pro
Juniper SRX320

sk1939 to iknow_t

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to iknow_t
said by iknow_t:

said by sk1939:

Well we had some issues with some audio equipment, but nothing a dedicated and isolated ground didn't fix.

1200A 3 phase and audio equipment sounds like a major recording studio..

Not quite, live sound.
sk1939

sk1939 to ArthurS

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to ArthurS
said by ArthurS:

Modern audio equipment shouldn't powered by three phase power. All my designs require single phase 3 wire service with isolated ground.

Single phase power wasn't an option due to the amperage required.

guppy_fish
Premium Member
join:2003-12-09
Palm Harbor, FL

guppy_fish to sk1939

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to sk1939
That's why isolation transformers ( signal level ) are standard operating procedure for sound re-enforcement, you will always have imbalances and ground loops.

Think of the neutral wire as a low ohm resistor, when your measuring at a sub panel, your measuring the voltage drop midway to the ground bond, the neutral from ground bond to subpanel ( one low ohm resistor ) and then the subpanel to the load.

ArthurS
Watch Those Blinking Lights
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join:2000-10-28
Hamilton, ON

ArthurS to sk1939

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to sk1939
I've designed professional sound and video production systems for large performing arts spaces, houses of worship, even sports stadiums. Single phase powered every one of them. With all the switching power supplies in the equipment, it was a requirement to keep triplen harmonics to a minimum.

sk1939
Premium Member
join:2010-10-23
Frederick, MD

sk1939

Premium Member

Well short of installing an expensive three to single phase transformer, there was no way for the utility to provide 800A of single phase service, they wouldn't do it at the time from my understanding.

ArthurS
Watch Those Blinking Lights
Premium Member
join:2000-10-28
Hamilton, ON

ArthurS

Premium Member

800 A for a sound system??? What is it that you're amplifying?? Biggest service that I have spec'd for a typical amplifier room (four racks of amps) is a 200A panel!! With proper circuit sequencing, you don't need anything more.

sk1939
Premium Member
join:2010-10-23
Frederick, MD

sk1939

Premium Member

800A for the building, dedicated transformers. The gear dosen't draw enough power (or justify the expense) for a dedicated service drop.

chamberc
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join:2008-08-05
Addison, TX

chamberc to ArthurS

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to ArthurS
said by ArthurS:

I've designed professional sound and video production systems for large performing arts spaces, houses of worship, even sports stadiums. Single phase powered every one of them. With all the switching power supplies in the equipment, it was a requirement to keep triplen harmonics to a minimum.

Quite normally done in large settings such as NCAA division 1 stadiums, see it all the time.

ArthurS
Watch Those Blinking Lights
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join:2000-10-28
Hamilton, ON

ArthurS to sk1939

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to sk1939
With new construction or refurbishments, it's not that much more expensive to put a single phase isolation transformer to convert 3 phase to single "split phase" power, than it is to convert higher voltage three phase to lower voltage three phase. Very common with new stadiums, performing arts halls, churches with sophisticated sound systems. Older venues and stadiums still have 3 phase technical power panels as part of legacy systems back in the good old days when switching power supplies were not as common as they are now with audio equipment, or the specifying consultant didn't know any better.

Back to the OP, the voltage you're seeing between neutral and ground is commonly observed with large AV systems with plentiful switching power supplies in the equipment supplied by three phase power. Make sure the neutral conductor is sized appropriately.
Mr Matt
join:2008-01-29
Eustis, FL

Mr Matt to iknow_t

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to iknow_t
said by iknow_t:

1200A 3 phase and audio equipment sounds like a major recording studio

More likely sites like these:

»www.crownaudio.com/galle ··· iew/id/5

sk1939
Premium Member
join:2010-10-23
Frederick, MD
ARRIS SB8200
Ubiquiti UDM-Pro
Juniper SRX320

sk1939 to ArthurS

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to ArthurS
It's still on the old three-phase and has been. This isn't new construction, but the addition of new amps has exacerbated the noise floor problem. We also pick up radio stations through the ground at times (or used to at least).

All the neutrals are the same size as the hot conductors, none are oversized or undersized.

ArthurS
Watch Those Blinking Lights
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join:2000-10-28
Hamilton, ON

ArthurS to sk1939

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to sk1939
Helpful guide here:
»www.middleatlantic.com/~ ··· per.ashx
and here:
»www.middleatlantic.com/~ ··· dum.ashx

John Galt6
Forward, March
Premium Member
join:2004-09-30
Happy Camp

John Galt6 to sk1939

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to sk1939
Rent one of these for a week:

»www.fluke.com/Fluke/inen ··· ID=73939

I had some real serious problems (bad harmonics and conductor heating) and a PQ analyzer like this made it easy to -see- the problem.

Do the line side and all of the loads, one at a time. Main, distribution, panelboards and then branch circuits. Make notes and use the device logging and report functionality to keep track of what is going on.

Once I understood what the problem was, and what was causing it, it was an easy fix. The larger problem was that the building was so large with so much equipment that it was impossible to find and fix it with regular test equipment.

mackey
Premium Member
join:2007-08-20

1 recommendation

mackey to sk1939

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to sk1939
said by sk1939:

All the neutrals are the same size as the hot conductors, none are oversized or undersized.

How do you know they are not undersized? Due to the harmonics generated by switching power supplies, they may need to be much larger than what they would be with a resistive load. It's not uncommon for places with lots of computers to have neutrals which are 2 sizes larger than the hot conductors.

»ieeexplore.ieee.org/xpls ··· r=396580

/M
InvalidError
join:2008-02-03

InvalidError to sk1939

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to sk1939
If you find three-phase loads that have grossly uneven load across phases, that would need investigating before doing anything else - you might have some pieces of high-power gear with blown line input rectifiers, fuse, transformer winding, etc.

If three-phase circuits check out fine, your three-phase panel might have some high-power single-phase circuits, perhaps on a sub-panel or power distribution unit. If you are lucky, you might be able to solve your problem by simply switching stuff between outlets on a PDU.