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#129658 - 07/01/05 09:22 PM Voltage Drop  
poorboy  Offline
Joined: Jun 2005
Posts: 124
Central Maine
It's 200 feet from a ltg contactor to a lite pole, 400 feet to the next pole, and 300 feet to the last pole. You run #1 AL to feed a total of 6- 1000 watt HID pole heads (277 V). All is as engineered, should work fine. Only a 30 Amp ckt.

What if the 200 feet from the contactor to the first pole is only able to be #6AL due to an undersize conduit. How much of the ability of the circuit to accomodate the distance is lost.

I'm really looking for the general concept here, maybe even better asked by saying what if I ran #12/2 Romex 100 feet, then spliced it to 200 feet of #8/2 Romex and hooked up a 120 V table saw. How bad did the 100 feet of #12 hurt the circuits ability to keep the voltage up?

Does the oversized wire need to originate at the source?

Tools for Electricians:

#129659 - 07/01/05 10:18 PM Re: Voltage Drop  
winnie  Offline
Joined: Sep 2003
Posts: 649
boston, ma
The general principals that you need are your basic 'ohms law' which says that the voltage drop across any circuit element is (including the wires) is simply the current times the resistance of that element.

Each of the conductors in this circuit add resistance and cause voltage drop.

The only problem with putting the skinny wire first is that it has to carry _all_ of the current for _all_ of the lights, which means that you will have your highest resistance section where you have your highest current, meaning high voltage drop...but this is also the shortest run.

I am guessing at 24A of current (6000W, 277V, something for power factor).

#6Al has a resistance of 0.808 ohms per 1000 feet. You have 400feet (200 out, 200 back) and so a resistance of 0.3232 ohms. V=IR, so the voltage drop is 7.75V for this first section.

The second section carries 16A. #1Al has a resistance of 0.253 ohms/1000 feet. You have 800 feet and a resistance of 0.2024 ohms with a voltage drop of 3.24V.

The final section carries 8A, 600 feet of wire, 0.1518 ohms, voltage drop of 1.21V.

Total voltage drop of 12.2V or about 4.5%. Kind of on the high side but possibly okay, depending on the way the ballast regulates versus reduced voltage.

I've made the simplifying assumption that the voltage drops are small, and that the lamps draw the same current. This is actually not true, since the current consumption will change with voltage.

The bulk of your voltage drop is in the first section. You might consider doing that stretch in copper; using #6Cu the voltage drop is reduced to 4.8V


#129660 - 07/02/05 05:40 AM Re: Voltage Drop  
poorboy  Offline
Joined: Jun 2005
Posts: 124
Central Maine
Unfortunately, Jon, the #6 Al I gave in the example is going to be #8 copper (the largest I can go to get 4 ckts in the 1 inch PVC left out thru the slab of the large mall store by another company) or I would use #6 CU. Maybe I'll look into "superconductors" (LOL).

This is one of the drawbacks of the multi-contract, multi-electrical-sub nature of mall developments. Coordination between entities is essential, yet lacking. All the elec sub who is doing the big box store did was look at his panel schedule and see the four 30A ckts, size his stub-out for nine #10 copper wires, and put in a one inch stub out. He has no site plan, that is our work, bid on and started months before he ever came on site with his contract.

#129661 - 07/02/05 05:00 PM Re: Voltage Drop  
DiverDan  Offline
Joined: Mar 2005
Posts: 38
Sacramento, CA
Hi poorboy,

Series voltage drop computations are best done using the segment method. Without getting very complicated, very quickly with IEEE voltage drop formulae and its application to segmental voltage drops, it is far better and eaiser to download Volts (links located on the left @ ECN's Webstore) and use its Series Voltage Drop module. It will solve your problem and afford you unlimited "what if" analysis abilities.


Dolphins Software

#129662 - 07/07/05 07:47 AM Re: Voltage Drop  
winnie  Offline
Joined: Sep 2003
Posts: 649
boston, ma

Remember that you only get voltage drop with current flow [Linked Image]

If you have a multiwire circuit with no current flow on the neutral, then there is no voltage drop in the neutral.

If there is some way that you could re-arrange your lighting circuits so that you have _three_ balanced circuits, then you could eliminate voltage drop in your neutral, and probably get to an acceptable over-all voltage drop.

You will need to consider sequencing; will there ever be some circuits on and others off? Because you will have current flow in the neutral whenever some of the circuits are off.

You might also consider using 480V line to line loads, which will eliminate the neutral entirely and again reduce the apparent voltage drop.


#129663 - 07/07/05 08:56 PM Re: Voltage Drop  
poorboy  Offline
Joined: Jun 2005
Posts: 124
Central Maine
I am planning for each of the 4 ckts to have their own neutral. I did consider sharing neutrals, but don't want to. Some circuits are off all nite, so yes, some are off while others are on. I like the 480 volt idea too, but not sure if the ballasts will be multi-tap. Developer supplies these pole lites and I haven't checked the cuts on them to see. The other electrical sub won't want to supply 4 two-pole 480V breakers, and the switchgear package which was prob supplied by the large dept store chain won't necessarily have room in the contactor panel. Too many things controlled by other people in the whole scheme of this deal. I will do the math before I pull the wires in the 1" PVC, possibly I can downsize 2 of the ckts which supply light poles close to the bldg to #10 and get the others up to #6 copper. That would be (4) #6 and (4) #10 in a 1" PVC. Codebook is in the truck and I'm too tired to walk downstairs to get it, will check later.

#129664 - 07/08/05 06:25 PM Re: Voltage Drop  
winnie  Offline
Joined: Sep 2003
Posts: 649
boston, ma
Hmmm. As a final note, remember that voltage drop is a 'design issue'. The issue would be things such as efficiency (power wasted in the wires) and power quality ( sudden change in voltage caused by other loads being applied to the line).

Efficiency costs money over time, but is not a safety issue.

Power quality is generally an issue with things like how easily motors start, and if lights flicker when other loads are operated.

In your case, you have circuits with well defined, fixed loads. If you have a 277V source, and a _10%_ voltage drop caused by your load, then everything will be fine as long as your loads are happy operating at 250V. You will need to look at the ballasts, and see what sort of taps or supply voltage range they have. It may be that this is an entire non-issue in this particular case.


#129665 - 07/09/05 06:44 PM Re: Voltage Drop  
Scott35  Offline

Broom Pusher and
Joined: Oct 2000
Posts: 2,707
Anaheim, CA. USA

I feel your pain here!

My maiden voyage into the fun and exciting Circus World, better known as:

"Large Mall Developement Projects",

has left me wondering, just who the heck came up with this newer, improved version of Counter-Production and Anti-Coordination! [venting big time!]

Must find this individual and remove (Him, Her, It, Them, whatever) from this Planet.
Stamp a big-old "Return To Planet Of Origin - Defective Unit(s)" label on their head(s), and request operative replacements!

Ok, now that I have vented slightly, on to the Luminaire situation.

I would love to say "Of course the Ballasts will have a 480 VAC tap - after all, these are 1000 watt HIDs, obviously used for Parking Lot or Street/Highway applications, which tend to have large distances between poles and source", but for some reason, I totally see these fixtures being shipped with the most assinine Ballast possible!

You might even wind up receiving something with only a 120 VAC lead!

If so, then things are going to suck!
(see, I told you there is a "wee-bit" of frustration boiling away, from my funderful mall experience!)
[Linked Image]

Chances are the Ballast kit will be like the Advance Transformer's "Quadri-Volt" CWA and HX Family.
It would have taps for 120 VAC, 208 VAC, 240 VAC and 277 VAC.

If you are lucky, the Ballast kit might be one in the "F" code for input Voltages (per Advance Transformer).
These come with taps for 277/480 VAC, 277/347 VAC, 277/347/480 VAC and 347/480 VAC input ratings at the "Primary" side of the CWA (or the "actual" Primary winding of a Regulated Lag Ballast).

With Lamps of this size, you will most likely see a CWA Ballast driving the Lamp - typical for >750 Watt Metal Halides and HPS.
There is an optional Isolation version of the CW Ballast, and it has 480 VAC provisions at the Primary.

If the Ballast is CWA "Type T Input", then it should have a 480 VAC tap, along with 277 VAC and even 120 VAC "T" (just if you want job security, as to the endless resetting of the Breakers!... joking).

Here's some data regarding your pending scenario:

Per Appendix C, the maximum wire sizes, number of wires, and types - vs. duct size and types are as follows:

1" PVC Schedule 80:
THHN "non-compact" Conductors;
* #6 = 5 Conductors maximum,
* #8 = 7 Conductors maximum.

It would work out using #8 cu, and running two - 3 wire circuits.
Conductors would equal 7 - including a #8 cu EGC.

Current carrying Conductors would be 6.

Derated capacity would be 44 Amps max.
( 55 amps × 0.8 )

Voltage drop per segments:
*Figured as:
  • 277 VAC input to Ballasts,
  • CWA type Ballasts,
  • 1000 Watt Lamps - HPF Ballasts,
  • 4.2 Amps per Ballast @ 277 VAC,
  • 3 Ballasts per 2 wire Circuit between contactor panel and first pole, and between second and third poles,
  • One 1Ø 3-Wire Multiwire Circuit

Volt loss for 1st segment (200 feet run to 1st pole) - each L-N 2 wire circuit: 3.65 Volts

This equates to around 1.3% Voltage drop on a 277 VAC circuit, carrying 12.6 Amps on 200 feet of #8 cu. in a non-magnetic raceway, with a 90% Power Factor.

For the 300 foot segment, a 2-wire circuit carrying the load current of 3 Ballasts (12.6 Amps) would be a loss of 5.47 Volts, so adding the previous loss of 3.65 Volts, the loss is now 9.12 Volts.

This is now a 3.29% drop in Voltage, and the nominal 277 VAC is something like 267.8 VAC.

Same 2-wire circuit carrying the load of only 2 Ballasts (8.4 Amps) would have a loss of 3.65 Volts - equal to the first segment's loss.

Collectively, the total Volt loss is now 7.3 Volts, and resulting as a 2.63% Voltage drop.

For the last segment (the 400 foot run), figuring 2 Ballasts (8.4 Amps) on a 2-wire circuit, results in a loss of 4.86 Volts.

Worst-Case scenario would result in 13.98 Volts lossed, and now the circuit is down to 263 Volts.

This value is 95% of the original 277 VAC, so the resultant loss is 5%.

Looks like you just made it! #8 cu just may work!

The CWA Ballast draws a pretty much uniform level of current throughout the operation of the Lamp.

It draws the same Amperage during Lamp Starting as it does during normal operation.

Also, the CWA can tolerate Voltage sags - to some degree. Not as well as the Isolated type counterparts, but overall the light output should not suffer drammatically from a 5% Voltage drop - I think the output will be between 90 and 95% of the possible wattage related output (Lumens), with a drop of 10% in Line Voltage.

As the Lamp ages, it will begin to act wierd, since it will require an ever increasing Voltage at the Arc Tube.
This will exibit the infamous "Cycling On-Off" effect, as seen with HPS Lamps.

The figures change drammatically as the above data is fine tweeked.

Also, if the raceway is PVC Schedule 40, you could use "Compact Conductors" with THHN insulation, and get the 7 #6 cu's in the first segment!
Refer to Appendix C of the NEC for further information.

Good luck!


Scott " 35 " Thompson
Just Say NO To Green Eggs And Ham!

#129666 - 07/09/05 07:04 PM Re: Voltage Drop  
iwire  Offline
Joined: Jan 2003
Posts: 4,391
North Attleboro, MA USA
I will be a little shorter than Scott [Linked Image] but I do want to say by all means use a mutiwire branch circuit for these lights.

There are few reasons not to and plenty of reasons to do it multiwire.

About the only argument against multiwire circuits is a 'lost' neutral.

This can be easily overcome by careful splicing with the best connectors for the job.

The benefit is of course fewer conductors and less voltage drop.

Bob Badger
Construction & Maintenance Electrician

#129667 - 07/12/05 06:19 PM Re: Voltage Drop  
Larry Fine  Offline
Joined: Feb 2005
Posts: 693
Richmond, VA
Poorboy, once you're outside the building, can't you use a junction box and splice onto larger conductors for the runs?

Larry Fine
Fine Electric Co.

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