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Michael Katz

How much line loss is ok?

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We are shooting an outdoor location which has 2 50amp spider boxes available. There aren't enough 50 amp twist lock extensions to get the boxes where we need so we will have to do it with stingers. The run is about 300 feet, and they will mostly be powering monitors, computer chargers, etc. I'm worried about experiencing line loss and having the monitors turn off and on with the lower voltage. I know you typically get about 5v of loss per 100ft of stinger, so does anyone know how this will affect our video village situation? Thanks in advance.

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http://acegenerators.com

 

ACE carries the 50amp Twist Lock cables and lunchboxes.

 

I'm sure other shops have them too.

 

There was an issue with being able to hide the cable, but we're renting it now. Do you know how well the twist lock will hold up voltage wise? All in all its about a 500ft run with the 6/4 cable.

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It is a bit more complicated. According to the National Electrical Code (NEC), the amount of voltage drop that still allows acceptable performance from equipment operating on it, and does not cause harm to the equipment, is the definition of “Allowable Voltage Drop.” As to what that figure is, lets look to the specific section of the NEC that covers line loss.

 

The NEC does not regulate allowable voltage drop specifically, but in a fine print note (FPN) in Section 215.2(A)(3), the code reads:

 

FPN No.2: Conductors for feeder as defined in Article 100, sized to prevent a voltage drop exceeding 3 percent at the farthest outlet of power, heating and lighting loads, or combinations of such loads, and where the maximum total voltage drop on both feeders and branch circuits to the farthest outlet does not exceed 5 percent, will provide reasonable efficiency of operation.

At 120V line level, this translates to a voltage drop of no more than 6 Volts, or the voltage should not drop below 114V from line loss. How likely is it that you will encounter unacceptable line loss over 300ft of AC Extensions?

 

There is an equation to calculate voltage drop but the math is pretty complicated. Fortunately there are Line Loss Calculators for this kind of thing available online at http://www.stealth316.com/2-wire-resistance.htm. But to calculate allowable voltage drop, according to the NEC, you must take into account “feeders and branch circuits to the farthest outlet” which means that you not only have to take into account the 300’ stinger run to the spider box, but also the 50A cable run from the spider box to the receptacle it plugs into, and also the cable from the receptacle to the breaker on the panel and also the feeder cable from the breaker to the service head. Since you probably don’t have all that information, lets just run the numbers on the 300’ stinger extension alone and see where that ends up.

 

If we enter 300’ of 14 Awg cable at nearly full load into the calculator we get the results in the table below, or a line loss of nearly 15 Volts (14.781). Where the allowable voltage drop according to the NEC is 6V at 120V line level, our drop without taking into account the length of the feeder cable to the panel and the length of cable to the spider box, is more than double the allowable amount. If we look at the effect of this voltage on the switch mode power supplies (SMPSs) powering the monitors, computer chargers, etc, we see why it is not allowable by the NEC.

 

VoltageDrop_19A300ft.jpg

 

SMPSs will draw more current to compensate for the drop in voltage to maintain the power supplies Apparent Power which in turn leads to the generation of heat, which increases the resistance of the cable, which in turn increases the voltage drop, and sets up a self perpetuating cycle until you have a brown out and your equipment shuts down.

 

One solution is to use, instead of a spider box, a 240V-to-120V step-down Transformer/Distro, like the ones we sell for the Honda portable generators that have variable taps that allow you to boost the voltage on the secondary side. Our standard Transformer/Distro is designed to boost the voltage on the load side (secondary) of the transformer by 5 percent. For instance, if you were to plug the Transformer/Distro directly into a generator running with no load and feed the supply side (primary) of the transformer with the generator's 240V output, you will get 126 Volts out on the secondary side where you would plug in lights. We have designed this slight boost into our standard Transformer/Distro to compensate for the line loss that is unavoidable over a long cable run, and the voltage drop on the generator under load. We also offer a "Select" model that enables you to adjust the amount of voltage boost in two 5% steps. This enables you to maintain full line level (120Vs) regardless if the supply voltage has dropped to 228V, or even 216V, from line loss and load running on the generator. To find the optimum switch setting, our "Select" model also includes a built-in voltmeter that tells you if the line level is too low or too high.

 

If the transformer is outfitted with standard Bates receptacles you can use heavier gauge #4 or #6 Awg extensions in place of stingers to eliminate further voltage drop over the cable run from the Transformer to set. I once made the mistake of not taking into account the cable run from the service head to the panel where I tied in #2 banded feeder. Voltage was fine until I switched on a 300W Fresnel and it browned out. For more details on how to calculate voltage drop use this link.

 

Guy Holt, Gaffer, ScreenLight & Grip, Lighting Rental & Sales in Boston

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Do what JD said, figure it out.

 

The three things that affect line loss or a voltage drop are length, load (in amps) and diameter size of your cable. If you have a long run but are only powering up small load of electronics, you can get away with small diameter cable.

 

Some things will work regardless of the voltage drop. If you have several 10k’s or maxi brutes at the end of a 1000 foot run, the lights will turn on if you have too small cable. But they will be warmer operation at a lower than optimal voltage.

 

If you need 3-18k hmi’s to work at the end of that 1000 foot cable run you had better do your homework or your lights won’t turn on. You may get one or possible 2 to work, then when you turn on the 3rd, the amperage draw will knock off the first 2 lights.

 

The Harry Box book does cover calculating line loss for detailed info.

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Some things will work regardless of the voltage drop. If you have several 10k’s or maxi brutes at the end of a 1000 foot run, the lights will turn on if you have too small cable. But they will be warmer operation at a lower than optimal voltage.

 

If you need 3-18k hmi’s to work at the end of that 1000 foot cable run you had better do your homework or your lights won’t turn on.

 

You definitely want to stay within the NEC recommendation. Low voltage because of "line loss" can cause problems such as reduced efficiency and excessive heat in equipment, unnecessary additional load on the circuit, and a dramatic shift in the color temperature and in the output of lights. For example, the effect of line loss on tungsten lights can be dramatic because their output falls off exponentially as the voltage decreases. For example that 10k lamp operating at 90% rated voltage (108V) produces about 68% of its normal light output. And as Tim points out, as the light intensity decreases, so does the Kelvin color temperature of the emitted light. In the case of fluorescents, HMIs, and LEDs, because their power supplies are typically of a “constant power” type, they will draw more current as the line voltage decreases in order to maintain constant power to the lamp which may lead to breakers tripping. In the case of generator output, voltage loss translates into an exponential loss in power. That is because, if you double the ampere load on the cable, the voltage drop also doubles, but the power loss increases fourfold. What this means is that when a distribution system has a large voltage drop, the performance of the generator (its maximum effective load) is reduced.

 

Rather than run out multiple runs of 4/O cable, big budget shows also use step-down transformers to compensate for line loss. On The Judge the generators supplied 277/480V power that was then stepped down to 120V/208V by transformers a 1000’ away (see photos below.) The higher voltage allowed the use of fewer runs of smaller cable and the variable taps on the transformers enabled the electricians to compensate for what voltage drop there was over the long cable run. Taking a similar approach with the Honda EU6500 and EU7000 generators, our step-down transformer/distros enable you to get the generator well off set where they won’t be picked up on your audio tracks and compensate for line-loss over the long cable run to set.

 

For more detailed information on line loss, I would suggest you read an article I wrote on the use of portable generators in motion picture production. The article is available at www.screenlightandgrip.com/html/emailnewsletter_generators.html.

 

Guy Holt, Gaffer, ScreenLight & Grip, Lighting Rental & Sales in Boston

 

Forum_Judge_1generators_480V_supply.jpg

300KVA generators generate 277/480 power.

Forum_Judge_120V_cable_runs.jpg

4/O cable carries the 277/480 power to step-down transformers a 1000’ away

Forum_Judge_Transformer_Primaries.jpg

277/480V power supplied to the primaries of step-down transformers

Forum_Judge_1transformers_120V_secondari

120/208V stage distro connected to the secondaries of the step-down transformers

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