Code CalculationsFeb 1, 2001James Stallcup Sr. | Electrical Construction and Maintenance

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.

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.

Code CalculationsMar 1, 2

As we've discussed in the last two Code Calculations articles, when calculating the total load on your system, you must

first identify specific types of loads. Then, you can calculate each load. For all loads, remember to determine if they are

continuous or noncontinuous. For continuous loads — loads where the maximum current is expected to continue for three

hours or more — multiply the VA of the load by 125%, per Sec 215-2(a). In this installment, we'll take a look at how to

calculate motor and noncoincident loads.

Motor loads (Secs. 430-24, 430-25, and 430-26)

When calculating motor loads, you need to know how to convert a motor's current rating (given in amps) to a VA rating.

To do this, multiply the motor's nameplate amperage by the supply voltage. If you don't have nameplate data, then

multiply the supply voltage by the current rating shown in the appropriate table in 430, Part N. Use the tables

appropriate to your power supply. Table 147 is for DC, 148 is for single-phase AC, 149 is for 2-phase AC, and 150 is for 3-

phase AC.

Example: What is the VA rating for a group of 480V, 3-phase motors rated at 125 hp, 40 hp, and 30 hp? Note: Your 480V

supply will require you to use a motor rated at 260V.

Step 1: Find FLA per Table 430-150.

125 hp = 156A

40 hp = 52A

30 hp = 40A

Step 2: Calculate total VA per Sec. 220-2.

VA = V x I x 1.732 (where 1.732 is the square root of 3)

VA= 480V x 156A x 1.732 = 129,692VA

VA = 480V x 52A x 1.732 = 43,231VA

VA = 480V x 40Ax 1.732 = 33,254VA

Solution: You can find the total load for the motors by adding the three individual motor loads. Total load = 206,177VA

Multiple motor loads (Secs. 430-6(a) and 430-24)

Suppose you have several motor loads on the same circuit. The Code requires you to size the conductors at least equal to

the sum of all the loads, plus 25% of the highest-rated motor in the group.

Example: Calculate the load for the following:

First load = motor of 40A

Second load = motor of 52A

Third load = motor of 65A

Fourth load = HVAC unit of 23A

Fifth load = lighting of 40A

Step 1: Add up all the loads, rounding up to the nearest whole number.

40A + 52A + 65A + 23A + 40A = 220A

Step 2: Select the largest motor load, and add 25% of it to the total.

25% of 65A = 16.25A

Solution: The total load = 220A + 16.25A = 236.25A. Round up to get 237A.

Noncoincident loads (Sec. 220-21)

You typically don't run heaters and air conditioning simultaneously on the same feeder — though you may do so in some

circumstances.

Some chiller-based systems run the chillers at full capacity and use local electric heating in the duct to adjust the

temperature upward, but these are unusual applications. In most cases, you will run either heat or A/C. Therefore, heat

and A/C are normally examples of noncoincident loads. Calculate this kind of load by choosing the larger of the VA ratings

and disregard the other.

Example: Which is the larger load: 240kW heating unit or a 97A A/C unit? The supply is a 480V 3-phase system.

Step 1: Select larger load per Sec. 220-21.

Heating load: 240kW x 1000 x 100% = 240,000VA

A/C load: 480V x 97A x 1.732 = 80,642VA

Solution: The 240,000VA-heating unit is the larger load. Thus, you will use this load in your overall load calculations and

not use the other load.