Largest motor load

The demand factor for largest motor load is an additional 25 percent.

1,600 VA 25 percent = total demand load of 400 VA

Total connected load for phases A and B

Refer to table 3-5 (NEC^R table 310-16), column 2, to determine the size of the entrance conductor. Two 2/0 THW copper conductors are required.

SIZING OF NEUTRAL CONDUCTORS

The neutral conductor is required to be large enough to carry the maximum unbalanced load between the neutral conductor and any phase conductor. For example, if phase A on a 240-volt, single-phase service carried a 75-amp, 120-volt load and phase B carried a 50-amp, 120-volt load, the neutral would have to carry the difference, or 25 amps. However, the largest load between any phase and neutral is 75 amps, so the neutral conductor must be rated to that ampacity.

Since the neutral conductor has to carry only phase-to-neutral loads (120 volts), all single-phase or three-phase, 208-volt or 240-volt loads can be omitted. There is one exception to this rule, however. For a feeder supplying household electric ranges, wall-mounted ovens, countermounted cooking units, and electric dryers, the maximum unbalanced load must be computed at 70 percent of the demand load. Electric ranges and dryers cannot be considered phase-to-phase loads because they may have 120-volt heating elements or 120-volt motors.

As a sample problem, let's determine the size of the neutral conductor in the previous example.

General lighting and receptacle demand load = 5,205 VA

Table 3-5.\Ampacities of Insulated Conductors Rated 0-2000 Volts, 60 to 90C (140 to 194F) Not More Than Three Conductors in Raceway or Cable or Earth (Directly Buried), Based on Ambient Temperature of 30C (86F)

Unless otherwise specifically permitted elsewhere in this Code, the overcurrent protection for conductor types marked with an obelisk () shall not exceed 15 amperes for 14 AWG, 20 amperes for 12 AWG, and 30 amperes for 10 AWG copper; or 15 amperes for

12 AWG and 25 amperes for 10 AWG aluminum and copper-clad aluminum after any correction factors for ambient temperature and number of conductors have been applied.

Reprinted with permission from NPFA 70-1990, the National Electrical Code^R, Copyright^c1989, 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.

Fixed appliances (phase to neutral)

4,000 VA 75 percent demand factor = 3,000 VA

Range load

8,000 VA 70 percent = 5,600 VA

Dryer load

6,000 VA 70 percent = 4,200 VA

Largest motor load

1,600 VA 25 percent = 400 VA

Total neutral load

Refer to table 3-5, column 2, to determine the size of the neutral conductor.

One No. 4 THW copper conductor is required.

NOTE: The NEC^R states that the size of the neutral conductor (grounded conductor) shall not be less than the size of the grounding electrode conductor.

Table 3-6 (NECR, table 250-94) shows the minimum grounding electrode conductor size based on the size of the service-entrance conductor. In the previous example we determined that a No. 4 THW copper conductor was needed to carry the neutral current. Refer to table 3-6 to determine the neutral conductor size required for a 2/0 service-entrance conductor. A No.4 THW copper conductor is the correct size in this example but not in every case. Always consult table 3-6 after making the neutral conductor load calculations.