Code Calculations

Jan 1, 2001 12:00 PM, By James Stallcup

Industrial Calculations: Lighting Loads

This is the first in a multi-part series that will cover industrial calculations for service and feeder conductors in an industrial facility. You must size each service and feeder so it can carry a load current that is not less than the sum of all branch-circuits it supplies in the electrical system. To do this successfully, you must work methodically. Your first step is to lay out the lighting loads.

Art. 220 is the basis for this task. Unfortunately, Table 220-3(a) doesn't list industrial occupancies. Thus, we must calculate industrial lighting loads without the use of Table 220-3(a). Fortunately, Sec. 220-3(b) lists specific types of "other loads."

What if you have an unlisted VA rating? Let's take a look at Sec. 215-2(a) and 230-42(a)(1). What these basically say is that you must size the feeder-circuit conductor to have an allowable ampacity equal to or greater than the noncontinuous load plus 125% of the continuous load.

Example: Suppose you have 160 lighting ballasts operating at 120V and rated at 1.5A each. You want to know the load in VA if the ballasts run 12 hr a day.

Step 1: Compute load per Sec. 220-4(b).

160 x 1.5A = 240A

Step 2: Compute continuous load per Sec. 215-2(a) and 230-42(a)(1).

240A x 125% = 300A

Step 3: Compute VA.

300A x 120V = 36,000VA.

Solution: The lighting load for the unlisted occupancy is 36,000VA.

Show window lighting load. When you think of lighting in an industrial setting, you probably think of 277V fluorescent or metal-halide lamps. Other types of loads exist in that environment, too. Most manufacturing plants, for example, have show windows. In the lobby area, you may see displays of products or company history. In the plant itself, you'll find show windows that contain ISO-9000 procedures, production figures, job postings, Pareto charts of production problems, overtime schedules, and other information production management people want visible from the factory floor. Let's take a look at how to calculate these types of loads.

Again, we can refer to Sec. 215-2(a) and 230-42(a)(1). We have two methods available. The first method is to multiply the linear feet of the show window by 200VA per foot. You compute this load at 100% for noncontinuous operation and 125% for continuous operation.

Example: What is the lighting load in VA for a 40-ft show window used at noncontinuous or continuous operation?

Step 1: Compute noncontinuous load per Sec. 220-12(a), 215-2(a), and 230-42(a)(1).

40 ft x 200VA x 100% = 8000VA.

Step 2: Compute continuous load per Sec. 220-12(a), 215-2(a), and 230-42(a)(1).

40 ft x 200VA x 125% = 10,000VA

Solution: The noncontinuous load is 8000VA, and the continuous load is 10,000 VA.

If you know the individual loads, you can use the second method, but you can use the results of that method only if the calculated load is higher than the results of the first method. With this method, you size the show window load by multiplying the VA rating of each fixture by 125%. If you don't know the VA, assume 180VA for each outlet and multiply that by 125%.

Track lighting load. You may also find some track lighting in industrial settings. To calculate this load, assume 150VA for every 2 ft of lighting track. Round any leftover up to 2 ft, regardless of how little it is. You'll also need to multiply by 125% for continuous operation.

Example: What is the load in VA for 180 ft of lighting track used at noncontinuous or continuous operation?

Step 1: Compute noncontinuous load per Sec. 220-12(b), 215-2(a), and 230-42(a)(1).

180 ft/2 ft x 150VA x 100% = 13,500VA

Step 2: Compute continuous load per Sec. 220-12(b), 215-2(a), and 230-42(a)(1).

180 ft/2 ft x 150VA x 125% = 16,875VA

Solution: The noncontinuous load is 13,500VA, and the continuous load is 16,875VA.

Low-voltage lighting load. You may also run into some low-voltage lighting systems - those operating at 30V or less. To compute the load for these systems, multiply the full-load amps of the isolation transformer by 100% for noncontinuous operation and 125% for continuous operation.

Example: What is the load in VA for a low-voltage isolation transformer with an FLA of 150A used at noncontinuous or continuous operation?

Step 1: Compute noncontinuous load per Art. 411, Sec. 215-2(a), and Sec. 230-42(a)(1).

150A x 100% = 150A

Step 2: Compute continuous load per Art. 411, Sec. 215-2(a), and Sec. 230-42(a)(1).

150A x 125% = 187.5A

Solution: The noncontinuous load is 150A, and the continuous load is 187.5A.

Outside lighting load. To calculate outside lighting loads, multiply the VA rating of each lighting unit by 100% for noncontinuous operation and 125% for continuous operation.

Example: What is the lighting load for 110 noncontinuous operated lighting fixtures with each ballast having a rating of 175VA and 130 continuous operated lighting fixtures with a 175VA ballast in each unit?

Step 1: Compute noncontinuous load per Sec. 220-4(b), 215-2(a), and 230-42(a)(1).

175VA x 110 x 100% = 19,250VA

Step 2: Compute continuous load per Sec. 220-4(b), 215-2(a), and 230-42(a)(1).

175VA x 130 x 125% = 28,438VA

Solution: The noncontinuous load is 19,250VA, and the continuous load is 28,438VA.

Outside sign lighting load. Most industrial facilities have lighted outside signs. You must size these loads at a minimum of 1200VA. Multiply this VA rating by 125% for signs operating for 3 hr or more and 100% for those operating less than 3 hr.

Example: What is the lighting load in VA for a 1800VA sign that operates 10 hr continuously?

Step 1: Compute load per Sec. 600-5(b)(3), 215-2(a), and 230-42(a)(1).

1800VA x 125% = 2250VA

Solution: The total outside sign load is 2250VA.

Next month's article will focus on calculating receptacle loads and special loads.