3 Phase 4 Wire System with single phase loads

[eestudent]

Hello everyone! Can you please explain to me how to calculate voltage drop calculations for Street Lighting systems, please see example below.

We have a 3-phase/4-core cable / feeder coming from a distribution panel (DP) inside a electrical room. The feeder supplies total 11 street lights. The distance from the DP to the first street lighting pole is approx. 100m and the distance between street lighting poles is 50m. Each lighting fixture is fed from line-neutral (e.g. L1-N, L2-N, L3-N, L1-N....and so on).
:?

 

[JDBrown]

First off, is this an NEC installation? Between your use of meters as a unit of measurement, and the fact that this is street lighting, I wonder if there is some other code that applies.

That being said, the method I use is the one outlined in the City of Los Angeles Electrical Code. The basic equation is: VD = Z * I * L, where Z is the impedance per unit length of your wire (Ohms), I is your current (Amps), and L is the length of your circuit. It is worth noting that the length of a single phase circuit is twice the distance between the source and the load, because you have to account for the length of both the supply and return conductors. Also, your current is not just the current required by your light; it's the current required by the light you're looking at, plus the current of all the other lights downstream from it on that same circuit.

Once you've calculated your voltage drop, you can convert it to percent voltage drop by dividing the voltage drop by the supply voltage and then multiplying by 100%.

Per City of LA requirements, I get my value of Z from NEC Chapter 9, Table 8 for conductors #2 AWG or smaller, and from NEC Chapter 9, Table 9 for conductors larger than #2 AWG. When using Table 8, I use the values for DC resistance of uncoated copper; if you're using copper conductors with coated strands or aluminum conductors, you would use the appropriate column from the table. When using Table 9, I use the values for Effective Z at 0.85 PF for uncoated copper wires. Again, if you're using aluminum, you need to use that column instead.

I've seen people use only the values from Table 8, others use only the values for Table 9, and still others use some other method altogether. The NEC doesn't say how you have to calculate voltage drop, it only suggests that you do so.

Looking back at my explanation, I'm realizing that voltage drop calcs can be more complicated than I normally think about. I guess that's what happens when you do them all the time. If you post some more information (supply voltage, wire size, number of lights, amps per light, etc.), we can probably give you a little more specific help.

 

[junkhound]

student problem

student problem

Hello everyone! Can you please explain to me how to calculate voltage drop calculations for Street Lighting systems, please see example below.

We have a 3-phase/4-core cable / feeder coming from a distribution panel (DP) inside a electrical room. The feeder supplies total 11 street lights. The distance from the DP to the first street lighting pole is approx. 100m and the distance between street lighting poles is 50m. Each lighting fixture is fed from line-neutral (e.g. L1-N, L2-N, L3-N, L1-N....and so on).
:?

Assume this is a student assignment? Easiest way is to model the line lengths and impedances )R, L, C, mutual inductances and couplings, etc) on PSpice or other FEA analysis program (a student version should be available as part of your student school curriculum) and hit the 'run' button.

 

[Besoeker]

Hello everyone! Can you please explain to me how to calculate voltage drop calculations for Street Lighting systems, please see example below.

We have a 3-phase/4-core cable / feeder coming from a distribution panel (DP) inside a electrical room. The feeder supplies total 11 street lights. The distance from the DP to the first street lighting pole is approx. 100m and the distance between street lighting poles is 50m. Each lighting fixture is fed from line-neutral (e.g. L1-N, L2-N, L3-N, L1-N....and so on).
:?

I'm not sure that forum rules should permit you to be posting this.....but, oh well

Does the same three phase feeder get looped to all the fixtures?
How are the lights distributed between the three phases? Possibly 3,3, 2?
How much current does each fixture require?
That would determine conductor size required to at least the first fixture.
It may be determined by current rating or permissible voltage drop.

As for modelling it....
You might consider one phase with three loads and a restive conductor between each.

 

[eestudent]

Street Lighting Voltage Drop

Street Lighting Voltage Drop

Thank you very much JDBrown, Besoeker & JunkHound for your reply.

I'm actually confused on how to calculate the voltage drop,let say for fixture no. 4(from attached pdf), considering that the cable from the source / Distribution panel is 4core (3phase) while the cable from JB to fixture is 2core (single phase)?

 

[JDBrown]

With the added information, I agree with junkhound: this looks like a homework assignment. That being the case, my answer changes somewhat. If this were a real installation, I would look at the 6 sq.mm. conductor size and conclude that was a minimum cross-sectional area. Then I would go to NEC Chapter 9, Table 9 to find that 6 sq.mm. falls between the sizes of #10 AWG and #8 AWG, so I would round up to #8's and continue on with my calculation using NEC data for #8 copper conductors. I would use the same method to determine the AWG size for the 10 sq.mm. and 4 sq.mm. conductors as well.

However, since this looks like a homework assignment, I'm a little more hesitant -- not because I don't want to help you, but because I don't know how detailed your instructor wants you to get. This question could be as simple as treating the installation as three separate single-phase circuits. You could then use the length, cross-sectional area and conductivity (or resistivity) of your conductor material (copper? aluminum?) to calculate the resistance of each section of conductor. From there, it would be a simple matter of determining the current in each section of conductors and using Ohm's Law to calculate the voltage drop.

However, depending on what the course is intended to cover (and on how particular/sadistic the instructor is), this question could be as complicated as using the AC impedance of the conductor (instead of the DC resistance) and taking into account things like skin effect. Then you might be expected to account for the reduced neutral current where you have loads on more than one phase. This would increase the complexity of the problem immensely, and I have no way of knowing what's expected of you.

In a real-life situation, I would treat this setup as three separate single-phase branch circuits (even though it's technically one multiwire branch circuit). Then I would take it one circuit at a time, figure out how much current is flowing in each wire segment and calculate the voltage drop for each segment based on that using the method I mentioned in my first post. Then I would total up the voltage drops for each circuit and see if they are acceptable. If not, increase wire size and repeat.

If we have misunderstood, and this is a real installation, not a question posed by an instructor at school, let me know and I'll try to walk you through it. But if it's a question for a class you're taking, there are too many unknowns for me to be sure I'm helping you with the method your instructor wants you to use.