General-Purpose Receptacle Loads

All receptacles used in industrial areas have to be computed at 180 VA per outlet. One exception to this rule involves the use of multioutlet assemblies (prefabricated, wall-mounted outlet strips). The load for outlet strips is to be computed at 180 VA for every 5 feet of assembly. If the outlet strip will be heavily loaded with portable tools, such as bench grinders, drill presses, soldering irons, and so forth, a load of 180 VA per foot is to be computed.

All receptacles identified as continuous duty are required to be computed at 180 VA times 125 percent to obtain the total load rating. The NECR allows the use of a demand factor to all noncontinuous duty receptacles. Table 3-7 states that the first 10 kVA has to be figured at 100 percent and the remaining VA at 50 percent. This reduction of load is based on the concept that not all receptacles in a building are used at the same time.

Special Appliance Loads

The branch circuit rating to supply special appliances is computed using the nameplate rating of the appliance. The NECR permits the use of demand factors for appliances, such as galley equipment and arc welders. To determine the correct demand factor for each application, consult the NECR.

Table 3-7.\Demand Factors for Nondwelling Receptacle Loads

Reprinted with permission from NPFA 70-1990, the National Electrical CodeR, Copyrightc1989, National Fire Protection Association, Quincy, MA 02269. This reprinted material is not the complete and official position of the National Fire Protection Association, on the referenced subject which is represented only by the standard in its entirety.

Motor Loads

As we discussed in the dwelling units section of this chapter, all motors are classified as continuous duty. The branch circuit conductors supplying one motor must be sized to carry 125 percent of the full-load current rating. If conductors are feeding a group of motors, they are required to be large enough to carry 125 percent of the full-load current of the largest motor plus the sum of the full-load current of the remaining motors.

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

VOLTAGE DROP CALCULATION

Voltage drop becomes important in industrial areas in which long runs of conductors are supplying large (ampacity) loads. Excessive voltage drop can cause overheating of breakers, conductors, and appliances, creating a safety hazard.

Conductors for a branch circuit should be sized to prevent a voltage drop exceeding 3 percent at the farthest outlet of power, heating, or lighting load. Conductors supplying a feeder circuit should also be sized to prevent a voltage drop exceeding 3 percent at the farthest outlet.

Total voltage drop consists of the voltage drop in the feeder plus the voltage drop in the branch circuit. The maximum voltage drop of a combination feeder/branch circuit should not exceed 5 percent. The conductors of the feeder should be sized to prevent a voltage drop of more than 2 percent, and the conductors of the branch circuit should be sized to prevent a voltage drop exceeding 3 percent.

The basic formula for determining voltage drop in a circuit is as follows:

where:

VD = voltage drop

r = resistivity for conductor material:

Aluminum = 18 ohms per CM-ft

Copper = 12 ohms per CM-ft

L = one-way length of circuit conductor in feet

I = current in conductor in amperes

CM = conductor area in circular mils (See table 8 in chapter 9 of the NECR)

Sample problem: Determine the voltage drop in a 230-volt, two-wire heating circuit. The load is 50 amps. The conductor size is No. 6 AWG THW copper, and the one-way circuit length is 150 feet.

The maximum voltage drop is 5 percent of 240 volts, or 12 volts. A 6.86-volt drop is within the acceptable percentage. If the voltage drop had exceeded 5 percent, a larger size conductor would have to be used or the circuit length shortened.