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Ask Mr. Tight-Watt: Barn lighting  Smart Choices Archive

Ask Mr. Tight-Watt: Barn lighting

Dwight Kramer, northwest Iowa electrical supervisor, Iowa Department of Public Safety, answers a reader's question about lighting and wiring.

Ask Mr. Tight-Watt

Q: I want to add five lighting units (chandeliers) to my barn. Each unit will take 16 bulbs, for a total of 80 20-watt bulbs. Will my standard wiring and light switch be OK? –Bary, a Smart Choices reader

To answer this question, Mr. Tight-Watt called on Dwight Kramer, northwest Iowa electrical inspector supervisor, Iowa Department of Public Safety.

A: Bary, since conductors are rated on the amount of amperage (load) that they can carry without overheating let’s start by calculating the load of these chandeliers. 

First, your total wattage is 80 bulbs times the 20 watts per bulb, or 1600 watts. Wattage is a function of the voltage and amperage (W=VA). The National Electrical Code requires us to use 120 as the nominal voltage. The Code also requires in 210.20(A) that the rating of the breaker be no less than 125 percent of the continuous load (three hours or more). I can only assume that this will be a continuous load, which will increase the wattage in this case to 2000 watts. Therefore, that equation now becomes A = W/V, or amperage equals 2000 watts divided by 120 volts, which results in an amperage load of 16.66 amps. If all five of the lighting units are installed on one circuit, this circuit will require at least #12 copper conductors protected by a 20A breaker. 

Here are a few things that you should consider:

1. This anticipated load will be pushing the limits of a 20-ampere circuit. If you consider splitting the load into two circuits, they could be installed using #14 copper and protected by 15A breakers.

2. If you use actual chandelier bulbs, these usually have no visual characteristics to distinguish them as 20W, 40W, or even 60W. If any burned out bulbs are inadvertently replaced with larger wattage bulbs, the load could possibly be doubled or even tripled, which will cause dangerous overheating to your building’s electrical system.

3. With the assortment of many types and sizes of energy-efficient bulbs available today, you should be able to find a style that will work for your application and conserve energy, thus decrease the cost of operation.

Thanks for the question. Please contact me at kramer@dps.state.ia.us if I can be of more assistance on this lighting project.

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