Moving Light Load Calculation

[Greg MacPherson]

I need to calculate the load for a number of 208v moving lights so I can determine the amount of power I need to order from a venue. There used to be a post on this but I can't locate it. Any direction will be appreciated. Thx.

 

[Greg MacPherson]

Okay, finally found it. Back in 2008 derekleffew posted this simple formula: # of Fixtures X current draw of each/1.732 for the load on each leg. He had several caveats (load must be exactly balanced, startup current would be higher, voltage drop, etc) but the formula gives me the basics and I will add 20% and then make sure the total of all that is 20% less than the service is rated for.

 

[MikeJ]

That's correct. With basic planning and smart power distribution balance of +/-10% is pretty doable, but 20% is a safer buffer. Having some headroom and very close load balance is especially important with a generator.

Consider though, just adding up everything in watts then divide by your power legs. This is easier if you have a combination of 208 and 120v, like moving lights and a bunch of lekos and Moles.

 

[Greg MacPherson]

Thanks Mike. I always allow 20% headroom on total load. And I have stored the 208v formula as it allows a quick computation. And I concur on the generator situation, though of course in entertainment the actual working load is almost never balanced. But no leg should be loaded to more than 80% of capacity.

 

[TJCornish]

Part of the reason the buffer is needed is the non-linearity of the load of the fixtures. This creates heating and stress on the distribution system in excess of the nameplate rating of the fixture, unless the fixture is power factor corrected. Most of the bigger movers are now and therefore their real load is pretty close to the nameplate rating, but most dimmers aren't, and older or lower-end movers may not be, either. Might be worth a look at the gear you're going to be using to see how much has PFC functionality.

 

[RickR]

There is one more critical factor: 3 hour rule.

Most circuit breakers are not rated for continuous use, more than 3 hours. NEC declares a maximum 80% loading for longer durations. That 25% additional can be part of your in-rush factor but needs to be included. (For the math deprived folks, if your load is to be 80% then add 25% of the load, to get 100%. 80A + 20A (25% of 80, not 20%) = 100A)