SIZING OF SERVICE-ENTRANCE CONDUCTORS

The most important point to remember when you are determining the size of service-entrance conductors is to ensure they are large enough to carry the load. A conductor that is too small may become overheated and create a fire hazard. On the other hand, sizing the conductors far too large to supply the load can be expensive and unnecessary.

The NEC^R (Section 230-42(b)) has established the minimum size for service-entrance conductors as the following:

1. 100-ampere for a three-wire service to a single-family dwelling with six or more two-wire branch circuits.

2. 100-ampere for a three-wire service to a single-family dwelling when the total load has been determined to be 10 kVA or more.

3. 60-ampere service for other loads.

Determine the size of the service-entrance conductors in the following example.

Given:

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

Two small appliance loads and one laundry load

Special appliance loads

12-kVA range

6.0-kVA dryer

15-kVA central heater

9.0-kVA air conditioner

4.8-kVA water heater

1.2-kVA dishwasher

0.8-kVA disposal

1.6-kVA attic fan

0.4-kVA vent fan

Solution:

General lighting and receptacle loads

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

Small appliance loads

The demand factor (table 3-2) for general lighting and small appliance loads is as follows:

First 3,000 VA 100 percent = 3,000 VA

Remaining 6,300 VA 35 percent = 2,205 VA

Total demand load = 5,205 VA

12-kVA range

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

Total demand load = 8,000 VA

6.0-kVA dryer

The demand factor (table 3-3) for one 6-kVA dryer is 100 percent.

Total demand load = 6,000 VA

Heating and air conditioning

The smallest load (air conditioning) can be disregarded. The demand factor for heating load is 100 percent.

Total demand load = 15,000 VA

Fixed appliances

The demand factor for four or more fixed appliances is 75 percent.

8,800 VA 75 percent of demand factor = total demand load of 6,600 VA