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Running a long line of power for lighting

lighting power resistance

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#1 Owen Cant

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Posted 24 April 2013 - 06:50 PM

Hello all,

 

Im wondering if any of you, particularly lighting gaffers & qualified sparks are able to offer me advice on the following.

 

I am on a location shoot for three days next week on a very low budget.

The only available power source at the location is 150 metres from the set, in the form of 4x 16amp sockets on separate rings.

I want to run 4 separate lines of power to the set as I will need up to 10K. I am trying to work out what the power loss due to cable resistance would be if i used 16A cables for each line (cheaper), or 32A cables (less resistance i assume?)

 

I am weighing this up with hiring a portable 10K generator instead.

Has anyone rented one of these before? And can reccomend a model & hire company in the north west?

 

Also, I had planned to use HMI's 575/1.8/2.5, does anyone foresee any issues using these units plugged up to a genny? Or even to a very long cable run? Apologies if this is a novice question for some,

 

Kind Regards

 

Owen


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#2 Guy Holt

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Posted 26 April 2013 - 05:54 PM



... I am trying to work out what the power loss due to cable resistance would be if i used 16A cables for each line (cheaper), or 32A cables (less resistance i assume?)... I am weighing this up with hiring a portable 10K generator instead. Has anyone rented one of these before?

 

 

The problem with portable generators under such circumstances, 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” (often referred to as “Voltage Drop”) from the long cable run (if you use regular cable) back to set. Running standard extension cords 450M from a building will result in significant line loss as well. It sounds like your f**ked either way.Low voltage on set can cause problems such as reduced efficiency and excessive heat in equipment, unnecessary additional load on the generator, 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. 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 severely – use this link for details.)  

 

 

Guy Holt, Gaffer, SceenLight & Grip, Lighting and Grip Rental & Sales in Boston


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