Heating and Air-Conditioning Loads

Where it is unlikely that two loads will be in use at the same time, the NECR allows the smaller of the two loads to be omitted. Since the electric heating load is usually larger than the airconditioning load, the air-conditioning load is dropped when the service-entrance conductor size is figured.

In determining branch circuit conductor size for heating equipment, use 125-percent demand factor. Also, figure the fan motor load, if applicable, at 125-percent nameplate rating.

Water Heater Load

In determining the size of the service-entrance conductor, apply the nameplate full-load current rating (100 percent) of all fixed-storage type of water heaters rated for 120 gallons or less.

The branch circuit conductors supplying water heaters are required to be large enough to carry 125 percent of the nameplate full-load rating.

Motor Loads

Motors are classified as continuous duty, and the branch circuit conductors supplying motors have to be sized to carry 125 percent of the full-load current rating. Conductors that are feeding a group of motors have to be large enough to carry 125 percent of all rated current of the largest motor plus the sum of the full-load running current of the remaining motors.

Branch circuit conductors supplying a combination of motor loads and lighting or appliance loads have to have an ampacity of 125 percent of the largest motor plus the sum of the other loads.

DETERMINATION OF NUMBER AND SIZE OF BRANCH CIRCUITS

Determining the number of branch circuits required to serve a dwelling unit is a relatively easy process after you identify the number and type of appliances that will be installed. You can determine the general lighting and receptacle load by multiplying the area of the dwelling by 3 VA. The size of the branch circuits supplying the remaining loads is based on the nameplate rating of the device and is computed by using the applicable demand factors. Solving this sample problem will assist you in determining the size and number of branch circuits for a dwelling unit.

Given:

1,600-sq-ft dwelling with 120-/240-V, single-phase service

Two small appliance circuits and one laundry circuit

Special appliance circuits

One 12-kVA range

One 1.2-kVA dishwasher

One 0.8-kVA compactor

One 6.0-kVA water heater

One 10-kVA central heater

Solution:

General lighting and receptacle load

1,600 sq ft 3 VA = 4,800 VA

120 V 15 amps (circuit breaker capacity) = 1,800 VA

The number of 15-amp circuits is determined as follows:

Small appliance and laundry circuits

Three 20-amp circuits are required.

12-kVA range

The demand factor (table 3-4) for a 12-kVA range is 8kVA.

One 35-amp circuit is required.

1.2-kVA dishwasher

One 15-amp circuit is required. 0.8-kVA compactor

One 15-amp circuit is required.

6.0-kVA water heater

The demand factor for a water heater is 125 percent.

One 35-amp circuit is required.

10-kVA central heater

The demand factor for space heating is 125 percent.

One 60-amp circuit is required.

The total number and size of branch circuits required to serve the sample dwelling are as follows:

Five 15-amp circuits

Three 20-amp circuits

Two 35-amp circuits

One 60-amp circuit