Cable Run for Honda Generator

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#1 Ryan Emanuel

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Posted 08 February 2017 - 08:42 PM

Wanted to check if my math was correct on line loss based on the Set Lighting Technicians Handbook.

I used the voltage drop formula VCm/2KI=L to determine the longest length I could run bates 60 AMP #6 AWG, with a load of 40 amps and stay within the 3% recommendation.

(3.6)(26240)/(21.6)(40)=109.3 ft.

So if I wanted to stay within NEC recommendation I can run 2 50 ft. 60 Amp bates.

Is this the way you guys determine the cable you rent for runs or is there another methodology.

Also how do you check voltage drop at the end of the line when the current needs to be flowing for the voltage drop to take place?

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#2 Jack OGara

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Posted 08 February 2017 - 09:27 PM

IDK American gauge so I wont answer your first question and I am not a "electrician" per se, I just own a few generators and do it on low budget features a bit

But I run 6mm (that is given name to the total core size or something here) for 25A generator and have 100m of it that works fine.

Though we are 240v here, and I am okay with -10v drop on the line at 5Kw. Most of the HMIs and stuff all work off 210-250v so loads to play with, but got to watch on the generator side with how many Watts one is pulling because cable lose.

As for testing voltage, load it up, then test.

It is the only way other than doing it on a calculator, but actually testing it with load is best.

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#3 Guy Holt

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Posted 04 March 2017 - 08:00 PM

So if I wanted to stay within NEC recommendation ...

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 geometrically as the voltage decreases. For example a 1k lamp operating at 90% rated voltage (108V) produces about 68% of its normal light output – a 1kw lamp is now a 650w lamp. But, that is not all, 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.

... to stay within NEC recommendation I can run 2 50 ft. 60 Amp bates.

Which is not nearly enough to not pick up the sound of the generator on your audio tracks. A common problem with portable generators, even the super quiet Honda Inverter generators, is that by the time you move them far enough off set that you don't hear them you have significant line loss. To the problem of line loss, you have the added problem that as you add load, the voltage drops on portable generators (it is not uncommon for a generator to drop 5-10 volts under full load.) The combination of voltage drop on the generator and line loss on a long cable run can cause voltage to drop to the point where HMI and Kino ballasts cut out unexpectedly or won't strike at all. For these reasons, portable gas generators are typically operated too close to set where they are picked up on audio tracks. The trick to recording clean audio with a Honda EU6500is is to use a 240V-to-120V step down transformer, like our 60A Full Power Transformer/Distro, that has a slight voltage boost built into it, which enables you to operate the generator at a distance without suffering from voltage drop.

Our 60A Full Power Transformer/Distro provides 7500 Watts of power in a single 120v circuit from the Honda EU6500is Generator or the new Honda EU7000is Generator

To record sync sound without picking up any generator noise, you need to add 200'- 300' of extension cable between the generator and set. This is usually enough cable to place the generator around the corner of a building, or to run it out of a van or truck - which is usually all the additional blimping you need with these generators. A heavy-duty 250V twist-lock cable runs between the generator and the Transformer/Distro, which eliminates multiple long cable runs to the generator and eliminates the severe voltage drop you would have using standard electrical cords. And, to assure full line level (120V) on set, our Full Power Transformer/Distros are designed to compensate for the unavoidable line loss you will have over an extended cable run.

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. Our "Select" model of Transformer/Distros, 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.

.....how do you check voltage drop at the end of the line when the current needs to be flowing for the voltage drop to take place?

To find the optimum switch setting, our "Select" Transformer/Distros have a built-in volt/amp/kva meter that tells you if the line level is too low or too high.

A good example of how our Transformer/Distro makes it possible to record clean audio tracks even under the worst of conditions is the indie short "Toothbrush." In this story of mistaken identity produced by Guymanly Productions, a pivotal scene takes place in the middle of a near vacant parking lot of an all night convenience store.

With no building or other sound barrier within a reasonable distance to block the sound of the generator, Gaffer Aaron MacLaughlin had no recourse but to put it behind their grip truck as far from set as possible. As you can see from the photos below, he ran 300' of twist-lock extension cable from the generator to our Transformer/Distro hidden behind a newspaper box. From the Transformer/Distro he then ran 200' of 6/3 Bates Extension to set where he broke out to 20A Edison receptacles using a 60A snack box. While running the generator near full capacity with a lighting package that consisted of three 1200W HMI Pars and two 1k Baby Quartz Fresnels, he experienced no appreciable voltage drop on set even after a 500' cable run because our Select Transformer/Distro was able to compensate for both the line loss of the cable and voltage drop of the generator under near full load.

Left: Scene in parking lot. Center: Transformer/Distro hidden behind newspaper box (set 200' in distance.) Right: Generator baffled by truck (Transformer/Distro 300' in distance.)

This example shows how the variable boost of our Select Transformer/Distros, not only enables you to place the generator further from set where you won't hear it, but also assures that the supply voltage on the secondary side of the transformer does not drop too low. By comparison, had Aaron run 500' of standard 14 Awg electrical cord he would have experienced a line loss alone of 24.5V. With that severe a voltage drop, his HMI ballasts would probably have cut out from low voltage as he added additional loads on the generator. Without the line-loss compensation of our Transformer/Distros, he would have had to move the generator closer to set where it would be picked up on the audio tracks.

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

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