Service Size Determination
The service size is determined by the NEC 220 load calculation. Standard residential sizes: 100A (small homes with gas appliances), 200A (most modern homes), 320A (large homes with electric heat + EV charging), 400A (estate homes or heavy electrical loads).
For commercial: 400A, 600A, 800A, 1200A, 1600A, 2000A, 3000A, and 4000A are standard switchboard ratings. Size is determined by the NEC 220 commercial load calculation.
NEC 230.79 minimum service size: 100A for single-family dwelling, 100A for other than single-family (measured per individual dwelling unit), 60A for loads limited to two 2-wire branch circuits.
Service Entrance Conductors
NEC 230.42 requires service entrance conductors to have ampacity ≥ the calculated load. NEC 310.12 provides specific allowances for service conductors that may differ from standard Table 310.16 ampacity.
Common residential service conductor sizes (aluminum, which is standard for service): 100A = 1/0 AWG aluminum, 200A = 4/0 AWG aluminum, 320A = 500 kcmil aluminum, 400A = 2 parallel sets of 4/0 AWG aluminum.
Parallel service conductors: Permitted for 1/0 AWG and larger per NEC 310.10(G). For 400A residential service: typically two parallel sets of 4/0 AWG aluminum + associated neutrals and ground.
Grounding and Bonding
Grounding Electrode Conductor (GEC): Sized from NEC Table 250.66 based on the largest service conductor. For 4/0 AWG aluminum service: minimum 4 AWG copper GEC. For 500 kcmil aluminum: minimum 2 AWG copper GEC.
Main Bonding Jumper: Sized per NEC 250.28 using Table 250.66 based on service conductor size. This jumper connects the neutral bus to the equipment ground bus in the main panel — the only panel where this bond is allowed.
Grounding Electrode System: Must include all available electrodes per NEC 250.50 — water pipe, building steel, Ufer (concrete-encased), and ground rods. Two ground rods minimum unless single rod achieves ≤ 25Ω.
Meter Base and Service Equipment
The meter base ampere rating must match the service rating. 200A service = 200A meter base. The utility will specify meter base type (socket vs. CT metered).
For services over 200A residential: many utilities require CT (current transformer) metering. CT cabinets are typically rated 320A or 400A and are larger than standard meter bases.
Main breaker: Must match service rating per NEC 230.90. 200A service = 200A main breaker. The main breaker also serves as the service disconnecting means per NEC 230.71.
NEC 230.71 allows up to 6 switches or breakers as the service disconnecting means (the '6-handle rule'). However, NEC 2023 effectively requires a single main disconnect at the service per 230.71(B) for emergency responder access.
Common Mistakes to Avoid
Sizing service on connected load instead of demand load — Always apply NEC 220 demand factors. A 60 kW connected load typically calculates to 30-40 kVA demand.
Using copper for service conductors — While not wrong, aluminum is standard for service entrance conductors and costs 50-60% less. Equipment terminals at panels and meter bases are rated for aluminum.
Not accounting for future load growth — NEC requires sizing for current load, but adding EV charging (40-80A), heat pumps, or pool equipment later can push a 200A service to its limit. Consider 320A or 400A if future loads are anticipated.
Improper weather head height — NEC 230.26 requires service entrance cables or raceways to terminate at a minimum of 10 feet above finished grade for the attachment point (weather head).