Code Calculations

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

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

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.