# 480v shared phases



## Stl elec (Dec 18, 2012)

Do many people use three pole breakers for 480v lighting circuits, using a-b, b-c, and a-c for the loads? I understand this saves wire, but I still can't wrap my head around how the phases aren't "doubled up" with ampacities. If a-b is say .....17a, and b-c is 16a...... How is this not loading the b phase?over loading


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## Southeast Power (Jan 18, 2009)

I see it all of the time. Not really uncommon.


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## ohiosparky99 (Nov 12, 2009)

In a A-B, C-A, B-C setup, how many A, B, and C phases do you have, looks like 2 of each which would be pretty balanced,


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## Stl elec (Dec 18, 2012)

Not sure I understand the question. There are basically 3-2-pole 480v circuits. By feeding them from a three pole breaker instead of three 2-pole breakers, it would seem that there would be two circuits worth of amperes flowing through the shared legs. I've seen it, just hesitant without fully understanding how that's safe.


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## ohiosparky99 (Nov 12, 2009)

Stl elec said:


> Do many people use three pole breakers for 480v lighting circuits, using a-b, b-c, and a-c for the loads? I understand this saves wire, but I still can't wrap my head around how the phases aren't "doubled up" with ampacities. If a-b is say .....17a, and b-c is 16a...... How is this not loading the b phase?over loading


I think your looking at the breakers wrong, normally they go a-b, c-a, b-c, not a-b, b-c, a-c, that may be where the confusion is


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## Electric_Light (Apr 6, 2010)

Stl elec said:


> Do many people use three pole breakers for 480v lighting circuits, using a-b, b-c, and a-c for the loads? I understand this saves wire, but I still can't wrap my head around how the phases aren't "doubled up" with ampacities. If a-b is say .....17a, and b-c is 16a...... How is this not loading the b phase?over loading



Well let's say you've got 90 550VA MH loads (400W MH lamps) 

.55 * 90 = ~50kVA
16.5kVA per phase. 

If you've got 30fixtures each on A-B, B-C, A-C, then the current in each conductor is calculated by 50,000/480/sqrt(3) = ~60A 

However... each unshared pair (i.e. A-B feeding row 1, B-C row 2, so on) still carries 35A.


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## ohiosparky99 (Nov 12, 2009)

Stl elec said:


> Not sure I understand the question. There are basically 3-2-pole 480v circuits. By feeding them from a three pole breaker instead of three 2-pole breakers, it would seem that there would be two circuits worth of amperes flowing through the shared legs. I've seen it, just hesitant without fully understanding how that's safe.


Somebody is wasting a ton of money feeding 2 pole circuits with 3 pole breakers, that's crazy unless your in a bind


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## ohiosparky99 (Nov 12, 2009)

Stl elec said:


> Not sure I understand the question. There are basically 3-2-pole 480v circuits. By feeding them from a three pole breaker instead of three 2-pole breakers, it would seem that there would be two circuits worth of amperes flowing through the shared legs. I've seen it, just hesitant without fully understanding how that's safe.


Sorry, didn't read the question fully at first about the 3 pole breakers feeding 2 pole loads, still crazy though


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## Electric_Light (Apr 6, 2010)

ohiosparky99 said:


> Somebody is wasting a ton of money feeding 2 pole circuits with 3 pole breakers, that's crazy unless your in a bind


What kind of lighting do you mean anyways? A sporting venue maybe receiving 4160v, then feed 3ph at 480v to each court and then, spread out to different phases at the sub panel. 

Sometimes those multi-head poles for sporting illumination receives all three phases and heads are spread across phases to reduce strobing effect and prevent cameras from getting caught in the dim phase. Think of it like a single piston engine vs three piston engine when raising the RPM is not a practical means to reduce noticeable torque pulsation. The heads are arranged for specific "firing order" to ensure flicker free elimination.


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## ohiosparky99 (Nov 12, 2009)

Electric_Light said:


> What kind of lighting do you mean anyways? A sporting venue maybe receiving 4160v, then feed 3ph at 480v to each court and then, spread out to different phases at the sub panel.
> 
> Sometimes those multi-head poles for sporting illumination receives all three phases and heads are spread across phases to reduce strobing effect and prevent cameras from getting caught in the dim phase. Think of it like a single piston engine vs three piston engine when raising the RPM is not a practical means to reduce noticeable torque pulsation. The heads are arranged for specific "firing order" to ensure flicker free elimination.


Why not use a 2 pole breaker if only using 2 phases, I guess I'm having trouble seeing why you would leave 1 phase on a breaker empty, maybe I need to get into sports venue lighting


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## k_buz (Mar 12, 2012)

Is the OP saying that the 3P breaker is feeding two 480V (single phase) circuits and the B phase is double tapped?

There would be no reason to do this. You might as well install a 2P breaker and run one 480V circuit.


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## Stl elec (Dec 18, 2012)

All three poles are being used at the breaker. Three ungrounded 480v wires go to the first light pole. The first circuit of 480v single phase parking lot lights get tapped off of a-b phase, second set is b-c, third is a-c. Therefore giving you the equivalent of six wires worth of two pole circuits in just a three wire run from the panel. If had engineers design parking lot lighting this way, they were 30a circuits, 24a loads. Seemed like it would overload the shared phase wires.


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## RePhase277 (Feb 5, 2008)

If you have A-C, A-B, and B-C, then some of the amps on A-C are also flowing through the loads on A-B and B-C. And they all do this. With a full set, some of each phase's current is traveling through the loads on the other phases.


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## BBQ (Nov 16, 2010)

ohiosparky99 said:


> Sorry, didn't read the question fully at first about the 3 pole breakers feeding 2 pole loads, still crazy though


Not crazy at all, we do it all the time it saves money.


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## BBQ (Nov 16, 2010)

ohiosparky99 said:


> Why not use a 2 pole breaker if only using 2 phases, I guess I'm having trouble seeing why you would leave 1 phase on a breaker empty, maybe I need to get into sports venue lighting


There is no emtpy phase.


You leave the breaker with A, B, C, at the first light you use A&B, at the next B&C at the next A&C. Keep doing that as far as you need to.


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## Stl elec (Dec 18, 2012)

So it's safe, somehow the two circuits sharing these phases won't double the ampacity on the shared wires?


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## Bbsound (Dec 16, 2011)

Stl elec said:


> So it's safe, somehow the two circuits sharing these phases won't double the ampacity on the shared wires?


All the wires are shared. 

Draw it out as a diagram, you may understand it then.


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## Stl elec (Dec 18, 2012)

Never thought of it like that. Makes sense and the math makes sense. Thanks everyone for the help


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## Electric_Light (Apr 6, 2010)

If red wires each carry 100A load, black wires carry 173A (which is 100 x sqrt 3) 

A wye source has an advantage here even if the conductors going to fixtures are ungrounded, because it limits the voltage with respect to ground to 277v while providing 480v across lines.


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## Electric_Light (Apr 6, 2010)

ohiosparky99 said:


> Why not use a 2 pole breaker if only using 2 phases, I guess I'm having trouble seeing why you would leave 1 phase on a breaker empty, maybe I need to get into sports venue lighting


Sporting facilities often have multiple clustered loads, but clusters are fairly far apart.

So, it makes sense to minimize the wire needed to go between the clusters.


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