voltage drop and parking lot lights

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squidbilly

Member
Location
Lucedale, MS
Basic Information:
volunteer work (adviser) for a local church
parking lot lighting w/ 4 GE M400 400W fixtures (3.9A each w/ HPS bulbs) on 2 poles (2 fixtures per pole)
fixture operating voltages: 108V - 131V per GE tech. support
120V AC (1 PH) 3-wire, copper conductor with PVC conduit
One 20A circuit feeds the fixtures
All 12 AWG wire (already is place when I arrived)

Relevant Distances:
segment 1: panel to center of parking lot (this point is geometrically between the poles): 175 feet
segment 2: center point to base of pole 1 or 2: 60 feet
segment 3: pole base to top of pole: 31 feet
segment 4: pole out to fixture at arms end: 14 feet

Wiring:
3 wires run from panel to center point of parking lot (175 feet) and feed:
3 wires running north to pole 1 (60 feet) and 3 wires running south to pole 1
These wires run to the base and up to the top of the pole (31 feet) where they feed:
3 wires running to fixture 1 (14 feet) and 3 wires running to fixture 2


My question: In calculating the voltage drop for each fixture via a spreadsheet, I did it in segments as follows:
using Vd=2kIL/Cm (with 12.9 for k and 6530 for Cm) for each segment

* segment 1 *
load amperage: 4 fixtures*3.9A = 15.6A
length: 175 feet

* segments 2 and 3 *
load amperage: 2 fixtures*3.9A = 7.8A
length: 60 + 31 = 91 feet

* segment 4 *
load amperage: 1 fixtures*3.9A = 3.9A
length:14 feet


I estimated the total voltage drop (sum of voltage drop from each segment) at each fixture to be 13.8V or 11.5% of the supplied 120V.
However, I have several "knowledgeable" people within this church arguing that #12 is adequate for the parking lot lighting.
Based on the calculations (and I do realize they are mere estimates if they are indeed correct), the lights will burn dim if the even come on at all.

Since I am probably rusty, I just wanted some advice on my calculations before proceeding.

Thanks in advance
 
S

Strathead

Senior Member
Location
Ocala, Florida, USA
Occupation
Electrician/Estimator/Project Manager/Superintendent
squidbilly said:
Basic Information:
volunteer work (adviser) for a local church
parking lot lighting w/ 4 GE M400 400W fixtures (3.9A each w/ HPS bulbs) on 2 poles (2 fixtures per pole)
fixture operating voltages: 108V - 131V per GE tech. support
120V AC (1 PH) 3-wire, copper conductor with PVC conduit
One 20A circuit feeds the fixtures
All 12 AWG wire (already is place when I arrived)

Relevant Distances:
segment 1: panel to center of parking lot (this point is geometrically between the poles): 175 feet
segment 2: center point to base of pole 1 or 2: 60 feet
segment 3: pole base to top of pole: 31 feet
segment 4: pole out to fixture at arms end: 14 feet

Wiring:
3 wires run from panel to center point of parking lot (175 feet) and feed:
3 wires running north to pole 1 (60 feet) and 3 wires running south to pole 1
These wires run to the base and up to the top of the pole (31 feet) where they feed:
3 wires running to fixture 1 (14 feet) and 3 wires running to fixture 2


My question: In calculating the voltage drop for each fixture via a spreadsheet, I did it in segments as follows:
using Vd=2kIL/Cm (with 12.9 for k and 6530 for Cm) for each segment

* segment 1 *
load amperage: 4 fixtures*3.9A = 15.6A
length: 175 feet

* segments 2 and 3 *
load amperage: 2 fixtures*3.9A = 7.8A
length: 60 + 31 = 91 feet

* segment 4 *
load amperage: 1 fixtures*3.9A = 3.9A
length:14 feet


I estimated the total voltage drop (sum of voltage drop from each segment) at each fixture to be 13.8V or 11.5% of the supplied 120V.
However, I have several "knowledgeable" people within this church arguing that #12 is adequate for the parking lot lighting.
Based on the calculations (and I do realize they are mere estimates if they are indeed correct), the lights will burn dim if the even come on at all.

Since I am probably rusty, I just wanted some advice on my calculations before proceeding.

Thanks in advance
Click to expand...

Without even doing all the math you did, I agree with you. In Florida, you couldn't install that way, because the State Building Code actually requires a maximum voltage drop of 3% on Branch circuits. But, you light would not burn dim. They either fire or the don't fire. That is the nature of the HPS type fixtures.
 
augie47

augie47

Moderator
Staff member
Location
Tennessee
Occupation
State Electrical Inspector (Retired)
I came up with roughly the same values. Not ideal, but in real life you would probably get by. Have you checked for actual voltages ? In many cases a "120v" circuit runs a bit higher.
Any chance the fixtures are dual voltage rated ? (many ballast are).
 
S

Strathead

Senior Member
Location
Ocala, Florida, USA
Occupation
Electrician/Estimator/Project Manager/Superintendent
GoldDigger said:
Only to the extent that the ballast is an electronic constant-current supply. If a purely magnetic ballast is used, the current will still be dependent on the applied voltage.
Click to expand...

That is not my understanding of the operation of HID fixtures. High voltage is required for them to fire, but once they are fired, the ballast regulates the voltage to allow the amount of current needed by the lamp for operation, so as long as the lamp fires, the ballast will the limit the current to that needed. Someone with an extremely deep understanding of the physics will probably chime in and clarify which understanding is the correct one.

Note I am not trying to be technical, just outline the basics that would be needed for troubleshooting and explaining to laymen.
 
G

GoldDigger

Moderator
Staff member
Location
Placerville, CA, USA
Occupation
Retired PV System Designer
Strathead said:
Someone with an extremely deep understanding of the physics will probably chime in and clarify which understanding is the correct one.
Note I am not trying to be technical, just outline the basics that would be needed for troubleshooting and explaining to laymen.
Click to expand...

If someone made the same statement (lamp output is independent of voltage) about a fluorescent lighting system, you would probably argue from experience that the statement was not true. The basic physics of a fluorescent lamp versus an HPS lamp is the same in terms of the need for a ballast to regulate current, but in the case of an HPS lamp the output from the discharge itself is useful visible light so no phosphors are needed as an additional step.

Another analogy which may help is that of a simple neon bulb (not a neon sign tube). You typically have a series resistor to limit the current, and the difference between the lamp arc voltage drop and the applied voltage has to be dropped across the resistor at the desired working current. If the neon drops 60 volts, the resistor will have to to drop 60 volts also for a 120 volt input. But if the input voltage is 130, the resistor will drop 70 volts, with a corresponding increase in current. If the input is 110 volts, the resistor will have to drop only 50 volts, with a corresponding decrease in current. (The description is 100% accurate for DC input. For AC, the effects of non-linearity in the neon's voltage drop make it more complicated.)
In the case of HPS, the situation is not quite as simple, since the series element will typically be an inductor and so complex impedances come into play. But the overall effect will still be similar.
 
Smart $

Smart $

Esteemed Member
Location
Ohio
squidbilly said:
...
120V AC (1 PH) 3-wire, copper conductor with PVC conduit
One 20A circuit feeds the fixtures
All 12 AWG wire (already is place when I arrived)

...

Wiring:
3 wires run from panel to center point of parking lot (175 feet) and feed:
3 wires running north to pole 1 (60 feet) and 3 wires running south to pole 1
These wires run to the base and up to the top of the pole (31 feet) where they feed:
3 wires running to fixture 1 (14 feet) and 3 wires running to fixture 2

...
Click to expand...
Would appreciate some clarification...

When you say three wires are you counting the EGC or not? If not, when you say 20A circuit, is this a 3-wire circuit on a 2-pole 20A breaker?
 
S

squidbilly

Member
Location
Lucedale, MS
Smart $ said:
Would appreciate some clarification...

When you say three wires are you counting the EGC or not? If not, when you say 20A circuit, is this a 3-wire circuit on a 2-pole 20A breaker?
Click to expand...

Yes, I am counting it as one of the three.
Green
White
Black ---> runs to a 20A single pole breaker.

I also did verify that this is a multivolt fixture.
 
Sandman1110

Sandman1110

Member
Location
Coastal Oregon
squidbilly said:
Basic Information:
volunteer work (adviser) for a local church
parking lot lighting w/ 4 GE M400 400W fixtures (3.9A each w/ HPS bulbs) on 2 poles (2 fixtures per pole)
fixture operating voltages: 108V - 131V per GE tech. support
120V AC (1 PH) 3-wire, copper conductor with PVC conduit
One 20A circuit feeds the fixtures
All 12 AWG wire (already is place when I arrived)

Relevant Distances:
segment 1: panel to center of parking lot (this point is geometrically between the poles): 175 feet
segment 2: center point to base of pole 1 or 2: 60 feet
segment 3: pole base to top of pole: 31 feet
segment 4: pole out to fixture at arms end: 14 feet

Wiring:
3 wires run from panel to center point of parking lot (175 feet) and feed:
3 wires running north to pole 1 (60 feet) and 3 wires running south to pole 1
These wires run to the base and up to the top of the pole (31 feet) where they feed:
3 wires running to fixture 1 (14 feet) and 3 wires running to fixture 2


My question: In calculating the voltage drop for each fixture via a spreadsheet, I did it in segments as follows:
using Vd=2kIL/Cm (with 12.9 for k and 6530 for Cm) for each segment

* segment 1 *
load amperage: 4 fixtures*3.9A = 15.6A
length: 175 feet

* segments 2 and 3 *
load amperage: 2 fixtures*3.9A = 7.8A
length: 60 + 31 = 91 feet

* segment 4 *
load amperage: 1 fixtures*3.9A = 3.9A
length:14 feet


I estimated the total voltage drop (sum of voltage drop from each segment) at each fixture to be 13.8V or 11.5% of the supplied 120V.
However, I have several "knowledgeable" people within this church arguing that #12 is adequate for the parking lot lighting.
Based on the calculations (and I do realize they are mere estimates if they are indeed correct), the lights will burn dim if the even come on at all.

Since I am probably rusty, I just wanted some advice on my calculations before proceeding.

Thanks in advance
Click to expand...

12.64% here (Of course I'm using a spread sheet calculator, so hopefully it's right). It looks like you'd have to go with #4 copper to get it under 3% drop.
 
Last edited:
M

mauk

Member
Location
NY
Occupation
Electrical Engineer
squidbilly said:
Yes, I am counting it as one of the three.
Green
White
Black ---> runs to a 20A single pole breaker.

I also did verify that this is a multivolt fixture.
Click to expand...
a 2-pole breaker and reconnecting at 208/240V would make the most sense to me

that's a pretty long run for a heavily-loaded 120V circuit with #12's
 
H

hillbilly1

Senior Member
Location
North Georgia mountains
Occupation
Owner/electrical contractor
Smart $ said:
But multivolt in what respect?

The OP says "fixture operating voltages: 108V - 131V per GE tech. support"
Click to expand...

Good question, though in laymens terms it usually means multiple voltage taps. The OP may have only asked GE the operating range of the 120 volt tap. Maybe the OP will clarify.
 
S

squidbilly

Member
Location
Lucedale, MS
hillbilly1 said:
Good question, though in laymens terms it usually means multiple voltage taps. The OP may have only asked GE the operating range of the 120 volt tap. Maybe the OP will clarify.
Click to expand...

Yes, I was initially told they were 120V. In doing my calculations, I called GE and asked the operating range based on the information I had been given (which is when I was given the range of 108V-131V). Upon actually visiting the site and inspecting the lights, I can say for sure that the lights can be configured to operate at other volatges, like 240V.
 
Smart $

Smart $

Esteemed Member
Location
Ohio
squidbilly said:
Yes, I was initially told they were 120V. In doing my calculations, I called GE and asked the operating range based on the information I had been given (which is when I was given the range of 108V-131V). Upon actually visiting the site and inspecting the lights, I can say for sure that the lights can be configured to operate at other volatges, like 240V.
Click to expand...
Wiring at 240V will definitely reduce voltage drop for same size size wire compared to 120V. But the existing white conductor presents a problem.
 
S

squidbilly

Member
Location
Lucedale, MS
Thanks again for all the responses.

Just an update .....

Against my strongest recommendations, they went ahead with the wiring as is (#12 all the way) and had the local power company stand up the poles. Upon testing them tonight, none of the four lights will burn. Now, they are back to me asking why.
 
iwire

iwire

Moderator
Staff member
Location
Massachusetts
squidbilly said:
Is the NEC 215.2(A)(4) informational note No. 2 the only part of the NEC that makes voltage drop recommendations (< 5%)?

Thanks.
Click to expand...

There are more informational notes like that in after 210.19, 310.15, 455.6, 547.10, 551.73 maybe some others

Beyond that there are some code sections that actually have voltage drop limitations, 630.31 (welders), 647.4(D) (sensitive Electronic Equipment), 695.6 / 7 (Fire Pumps)
 
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