Three phase or Single Phase VD?

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Mike01

Mike01

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So I have been searching the fourm and have seen this in a couple of thread's and trying to make sense of this. When installing single phase loads [L-L] across three phase circuits [L-L-L] how do you calcualte the voltage drop at each load? For example [this seems to be the most common applicatoin] when installing exterior lighitng that is either 480V L-L or 208V L-L, it seems to be common practice to install a three pole breaker and balance the loads [fixtures] across all three phases [AB, BC, AC, repeat] by doing this the voltage drop is calcualted as a three phase VD calcualtion due to having two multiple return paths to the source. My questoin is how would you calcuate the voltage at each location where your loads are connected L-L and not L-L-L? Would this create an imbalacend load for this portion of the cirucit untill the next load or two away where it would become balanced again?
 
Here's how I would do it;

If all three phases are balanced, just add up the total amp-feet for the phase with the longest run. If unbalanced, then do all 3 and see which one is the worst. If your longest run doesn't have the most amps, it might not come out as the worst case.

To be strictly rigorous, it's a bunch of vector algebra.
 
Mike01 said:
So I have been searching the fourm and have seen this in a couple of thread's and trying to make sense of this. When installing single phase loads [L-L] across three phase circuits [L-L-L] how do you calcualte the voltage drop at each load? For example [this seems to be the most common applicatoin] when installing exterior lighitng that is either 480V L-L or 208V L-L, it seems to be common practice to install a three pole breaker and balance the loads [fixtures] across all three phases [AB, BC, AC, repeat] by doing this the voltage drop is calcualted as a three phase VD calcualtion due to having two multiple return paths to the source. My questoin is how would you calcuate the voltage at each location where your loads are connected L-L and not L-L-L? Would this create an imbalacend load for this portion of the cirucit untill the next load or two away where it would become balanced again?
Click to expand...

Suppose there is a 3 phase panelboard and three single phase loads in each phase are connected,

To know about voltage at the end of each load, calculate voltage drop calculation same as you do for branch,

425px-Voltage_Drop_Calc_-_Single_Phase.png

load balance calculation will be done on main panel, if the loads are typical there would not be imbalance otherwise there could be, but per standard the imbalance load may not increase 5% or so.
 
Mike...

Are you looking for a way to optimize (by EXCEL perhaps?) the cable-sizing procedure for a 3-phase+ground, radial system supplying a number of nearly-identical sized fixtures, having nearly-equidistant separation?

Regard, Phil Corso
 
Phil Corso said:
Mike...

Are you looking for a way to optimize (by EXCEL perhaps?) the cable-sizing procedure for a 3-phase+ground, radial system supplying a number of nearly-identical sized fixtures, having nearly-equidistant separation?

Regard, Phil Corso
Click to expand...

Phil, not so much a spreadheet just trying to wrap my head around and understand voltage drop on a three phase branch cirucit that connects loads at single phase L-L. I have see this type of configuration mostly in outdoor lighting and just trying to understand the VD calc. If you have a three phase cirucit with ground, you woud use the three phase approach but for example at the first load you are connected at L-L not L-L-L, so how would you figure out the VD at that point or any other downstream regardless of distances being equal?
 
Mike01 said:
Phil, not so much a spreadheet just trying to wrap my head around and understand voltage drop on a three phase branch cirucit that connects loads at single phase L-L. I have see this type of configuration mostly in outdoor lighting and just trying to understand the VD calc. If you have a three phase cirucit with ground, you woud use the three phase approach but for example at the first load you are connected at L-L not L-L-L, so how would you figure out the VD at that point or any other downstream regardless of distances being equal?
Click to expand...
Quite simply, voltage drop is current times wire impedance. So you take the current at the source of the circuit and multiply by the impedance of the first leg. Do this for both legs that the first load is connected to. The voltage drop is the sum of the two calculations, while the voltage at the load is the source voltage minus this sum. Cascade the preceding calculation through the entire circuit while reducing the current of each leg-section by the already served load.
 
Smart $ said:
Quite simply, voltage drop is current times wire impedance. So you take the current at the source of the circuit and multiply by the impedance of the first leg. Do this for both legs that the first load is connected to. The voltage drop is the sum of the two calculations, while the voltage at the load is the source voltage minus this sum. Cascade the preceding calculation through the entire circuit while reducing the current of each leg-section by the already served load.
Click to expand...
I should note that the preceding makes some assumptions and as a result will include some error. For example, youmay base your circuit current on X number of loads times the rated current of each load. Commonplace. However, if the load current diminishes due to drop in supplied voltage, the current at the circuit source will not add up to the calculated value using rated current. Nevertheless, if one keeps the voltage drop at the end of the circuit to 5% or less, there should be too much deviation to be concerned with.
 
Smart $ said:
I should note that the preceding makes some assumptions and as a result will include some error. For example, youmay base your circuit current on X number of loads times the rated current of each load. Commonplace. However, if the load current diminishes due to drop in supplied voltage, the current at the circuit source will not add up to the calculated value using rated current. Nevertheless, if one keeps the voltage drop at the end of the circuit to 5% or less, there should be too much deviation to be concerned with.
Click to expand...
...shouldn't...
 
Mike? back at you in the next few days.

I?m searching my files for the formula I derived during electrical design of the Verrazano-Narrows Double-Deck Bridge between Brooklyn and Staten Island, NYC. It involved some 560, 480 single-phase highway lamps and 560, 480/120V transformers (for maintenance receptacles) at each lamp post!

(Ps: The receptacles are another story)
 
Phil Corso said:
Can anyone explain how to attach an EXCEL sheet to this forum!
Click to expand...
You can't... at least not in native format. The only method I've found that works is to embed it in a MS Word .doc.

Alternatively, upload to a storage site and provide a link. Having a Google account specifically for this forum, I use Google Drive associated with that account. There are many other options...
 
Phil Corso said:
Anyone...

E-mail me for a free copy of the working ECEL solution. You only have to provide your name, location and e-mail address!
Click to expand...
I think the email address goes without saying. How else are they going to email you.

However, I fail to see why you are so adamant about name and location. :thumbsdown: You were initially going to post to a public website, where anyone could download without you knowing name and location. You rejected my request simply because I choose to remain anonymous. I could just as easily furnish you with a fake name and location... but I also choose to retain my integrity.
 
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