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 NECR (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.
1,600-sq-ft dwelling with 120-/240-V, single-phase service
Two small appliance loads and one laundry load
Special appliance loads
15-kVA central heater
9.0-kVA air conditioner
4.8-kVA water heater
1.6-kVA attic fan
0.4-kVA vent fan
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
The demand factor (table 3-4) for a 12-kVA range is 8 kVA.
Total demand load = 8,000 VA
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
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