Can a new commercial or industrial load be added to existing service equipment without performing an extensive standard calculation?

For many years designers have struggled with doing standard load calculations in all types of existing commercial and industrial occupancies when confronted with requests to add loads to existing feeders and services. Effective with the 1981 NEC, however, there is a new approach that allows you to sidestep the more complicated standard calculation in cases where there are accurate electrical demand records going back over the last year.

Standard calculation

The standard calculation is a more detailed and comprehensive method and takes a great deal of time to complete. Not only do loads (which may number in the thousands) have to be determined and evaluated based upon their conditions of use, but they have to be classified as continuous or noncontinuous in operation. This is because the continuous loads need to be calculated at 125% for the purposes of sizing overcurrent protective devices. Finally, the Code allows you to apply demand factors to some or all of these loads. It helps to sort the loads into one of the following groups:

Load groupings:

* General lighting load.

* Receptacle load.

* Special appliance load.

* Largest noncoincident load.

* Motor load.

* Largest motor load.


Suppose you would like to add a 42A 3-phase load to an existing 208Y/120V service for a small office building with a restaurant on the first floor. The service is rated 350A and is wired with 400kcmil THWN service conductors. The existing load schedule consists of 80A of lighting load, 80A of receptacle load, and 150A of cooking load spread over ten appliances.

Calculating continuous load:

The lighting load will be continuous.

Step 1. Calculating load

Sec. 220-3(b); -10(b); Sec. 230-90(a) 80A x 125% = 100A

Solution: The calculated load is 100A.

Calculating noncontinuous load:

The receptacle load will usually be considered noncontinuous.

Step 1: Calculating load

Sec. 220-3(c)(6); -10(b); Sec. 230-90(a) 80A x 100% = 80A

Solution: The calculated load is 80A.

Calculating loads with demand factors:

The cooking equipment:

Step 1: Finding demand factor

Sec. 220-20; Table 220-20 10 units allows 65%

Step 2: Applying demand factors 150A x 65% = 97.5A

Solution: The demand load is 97.5A


Step 1: Finding the existing total load Sec. 220-10(a)

100A + 80A + 97.5A = 277.5A Step 2: Add the new load 277.5A + 42A = 319.5A Step 3: Rate the service elements Art. 310, Table 310-16

400kcmil ampacity is 335A; overcurrent device given as 350A.

Solution: The new load can be added.

Optional calculation

For the purposes of allowing additional load to be connected to an existing service or feeder, you can also use actual maximum kVA demand figures to determine the existing load instead of using the standard calculation. You must, however, comply with all of the following conditions:

* The maximum demand data is available for a minimum period of one year.

* The existing demand at 125% plus the new load is not greater than the ampacity rating of the service conductors.

* The service-entrance conductors have suitable overcurrent protection.


If that small office/restaurant building has good demand records, you can use the optional calculation. If the highest demand recorded over the past year is 78.4kVA, could the new load be added?

Step 1: Calculating demand per NEC Sec. 220-35 78.4kVA x 125% = 98kVA

Step 2: Conversion to amperes

98,000VA/208V + [square root of 3] = 272A

Step 4: Calculating new total load

272A + 42A = 314A

Solution: The new load can be added. This approach, when possible, is much faster and easier.

Next issue

When can the service overcurrent protective device be sized higher than the ampacity of the service conductors?

James G. Stallcup is the owner of Grayboy & Associates in Fort Worth, Tex., a consulting firm specializing in the NEC.