Last month, we looked at how to calculate various lighting loads. This month, we'll look at how to calculate receptacle and appliance loads. Let's tackle receptacle loads first.
Receptacle loads. A receptacle is a contact device installed at an outlet for the connection of an attachment plug. To calculate the total VA of receptacle loads, calculate the VA for each type of occupancy and total the results. For continuous loads - loads where the maximum current is expected to continue for 3 hours or more - you'll need to use a multiplier of 125%.
If the receptacle is a part of a multioutlet assembly, such as a surface, flush, or freestanding raceway, you will have to follow a slightly different path to arrive at the total VA. Let's try to calculate the VA for a general-purpose receptacle load.
First, examine the use for each receptacle. Count the number of receptacles that will serve continuous-operation loads, and multiply that number by 125% for your new total. Then, count the number of receptacles that will serve noncontinuous-operation loads and add this to the previous total. Multiply the results of that addition by 180VA to get your answer.
Example: What is the load in VA for 55 general-purpose receptacles that serve noncontinuous loads and 55 more that serve continuous loads?
Step 1: Calculate the continuous load, per Secs. 220-3(b)(9), 215-2(a), and 230-42(a)(1). 55 receptacles x 125% x 180VA = 12,375VA
Step 2: Calculate noncontinuous load, per Secs. 220-3(b)(9), 215-2(a), and 230-42(a)(1). 55 receptacles x 180VA = 9900VA
Solution: The continuous load is 12,375VA and the noncontinuous load is 9900VA. Therefore, your total load is 12,375VA + 9900VA = 22,275VA.
You can't apply the demand factor in Table 220-13 to the continuous load of 12,375VA, even though this value exceeds 10,000VA. Let's look at how you can apply demand factors.
Applying demand factors. You calculate general-purpose receptacle outlets for cord-and-plug connected loads (noncontinuous operation), per Sec. 220-3(b)(9) and Table 220-13. If the VA rating of the receptacle load exceeds 10,000VA, you can apply a demand factor of 50% to all VA exceeding 10,000VA, per Table 220-13. Don't make the mistake of applying the demand factor to the first 10,000VA as well.
Example: What is the VA rating for 225 general-purpose receptacle outlets to cord-and-plug connected loads used for noncontinuous operation?
Step 1: Calculate the load, per Sec. 220-3(b)(9). 225 x 180VA = 40,500VA
Step 2: Apply demand factors, per Table 220-13. The first 10,000VA x 100% = 10,000VA. The remaining 30,500VA x 50% = 15,250VA
Solution: The total load is 25,250VA.
Fixed multioutlet assemblies. As we mentioned above, calculating the load for multioutlet assemblies is a bit different from calculating the load for general-purpose receptacles. For connected loads not operating simultaneously, divide the length of the multioutlet assembly by 5 ft and then multiply the result by 180VA. For connected loads operating simultaneously, multiply each foot of the multioutlet assembly by 180VA. You can add the multioutlet assembly load to the noncontinuous receptacle load and demand factors applied, per Table 220-13.
Example 1: What is the load in VA for 200 ft of multioutlet assembly used to cord-and-plug connected loads not used simultaneously? Calculate load not used simultaneously, per Sec. 220-3(b)(8)(a).
VA = (length/5 ft) x 180VA. VA = (200 ft/5 ft)x 180VA. VA = 7,200VA
Example 2: What is the load in VA for 200 ft of multioutlet assembly used to cord-and-plug connected loads used simultaneously? Calculate load used simultaneously, per Sec. 220-3(b)(8)(b).
VA = length x 180VA. VA = 200 ft x 180VA. VA = 36,000VA
Now let's look at a slightly more complex example.
Example 3: What's the total load when you have 200 ft of multioutlet assembly (for cord-and-plug connected loads used simultaneously) and 225 general-purpose receptacle outlets (for cord-and-plug connected loads used in noncontinuous operation)?
Step 1: Compute load used simultaneously, per Sec. 220-3(b)(8)(b). Load = length x 180VA. Load = 200 ft x 180VA. Load = 36,000VA
Step 2: Compute load for receptacles, per Sec. 220-3(b)(9). 225 outlets x 180VA = 40,500VA
Step 3: Add multioutlet assembly and receptacle loads. Multioutlet assembly = 36,000VA. Receptacle load = 40,500VA. Total load = 76,500VA
Step 4: Apply demand factors, per Table 220-13. Because you have no continuous loads, your entire load is factorable.
Factorable load = 76,500VA. The first 10,000VA x 100% = 10,000VA. The remaining 66,500VA x 50% = 33,250VA. Solution: The total load = 33,250VA + 10,000VA = 43,250VA
Let's add another twist to the calculation.
Example 4: If the multioutlet loads in the previous example were continuous, you would have to arrive at the answer differently. You would exclude the 36,000VA of continuous load from the demand factor calculation.
Factorable load = 40,500VA. The first 10,000VA x 100% = 10,000VA. The remaining factorable 30,500VA x 50% = 15,250VA. Therefore, total load = 36,000VA + 10,000VA + 15,250VA = 61,250VA
Appliance loads. You calculate appliance loads by multiplying the VA rating of each load by 100% for noncontinuous operation and 125% for continuous operation. To determine the classification, appliance loads operating for less than 3 hr are noncontinuous operation loads. An appliance load operating for 3 hr or more is a continuous operation load. Let's run through a couple of examples to familiarize ourselves with these types of calculations.
Example 1: What is the VA rating for a 208V, 3-phase, 165A appliance load operating for 10 hr and supplied by an individual branch-circuit?
Step 1: Compute VA, per Sec. 220-2. VA = V x I x 1.732
VA = 208V x 1.732 x 165A
VA = 59,400
Step 2: Compute continuous load, per Secs. 215-2(a) and 230-42(a)(1). 59,400VA x 125% = 74,250VA
Solution: The load at continuous operation is 74,250VA. Why did 1.732 enter into the calculation? When doing 3-phase calculations, you must multiply by the square root of 3, which is 1.732.
Example 2: What is the VA rating for an appliance load of 82A operating at 480V, 3-phase, for 2 1/2 hr every 4 hr?
Step 1: Compute VA, per Sec. 220-2. VA = V x 1.732. VA = 480V x 1.732 x 82A. VA = 68,142
Because this is a noncontinuous load, you have your answer.
Secs. 215-2(a) and 230-42(a)(1) require you to multiply noncontinuous loads by 100%, which won't change your results. If you were working with continuous loads, you would multiply by 125% to arrive at 85,178VA.
When doing any of these calculations, always determine which portions of your total load is continuous or noncontinuous. Calculate all loads as though they were noncontinuous, and then multiply the continuous loads by 125% and add your noncontinuous loads and continuous loads together. Remember: You can't apply demand factors to continuous loads - by definition, they are always in demand.