Pro tips for sizing service entrance

Start with the load calc, not the panel

Sizing service entrance conductors begins at Article 220, not the supply house counter. Run the standard calc per 220.40 or the optional method under 220.82 for a dwelling. Optional almost always yields a smaller service for single-family homes with electric heat or AC, so do both and take the lower number when code allows.

Write the calc on paper and leave it in the job folder. Inspectors ask, and memory fades. If you are feeding a 200A panel, you still need the math to prove 200A is right, not just convenient.

Field tip: on a remodel with an added heat pump and EV charger, rerun the optional calc before assuming the existing 200A service holds. A 4.5 kW dryer plus a 48A EVSE plus a 5-ton heat pump routinely pushes older homes past 200A on paper.

Know when 310.12 applies

For one-family dwellings and the individual units of two-family and multifamily dwellings, 310.12 lets you size service and main feeder conductors at 83 percent of the ungrounded service rating. This is the rule behind the familiar 4/0 aluminum SER on a 200A residential service.

310.12 applies only to the service conductors and the main power feeder, defined as the feeder between the main disconnect and the panelboard that supplies all branch circuits. Subfeeds to detached structures or subpanels fed past the main panel do not get the 83 percent treatment. Size those off the standard ampacity tables in 310.16.

• 200A service, dwelling: 2/0 Cu or 4/0 Al per 310.12
• 400A service, dwelling: 400 kcmil Cu or 600 kcmil Al per 310.12
• 200A feeder to a detached garage: size from 310.16, not 310.12

Temperature termination, not just conductor ampacity

110.14(C) governs the terminations. Most residential panels and meter bases are rated 75C, and you size from the 75C column in Table 310.16 unless the equipment label says otherwise. A 100A breaker with 60C-only lugs forces you to the 60C column, and that is still legal on equipment rated 100A or less under 110.14(C)(1)(a).

Check both ends. A 75C rated panel paired with a meter socket stamped 60C/75C is fine, but if the POCO-supplied meter base is marked 75C only and the service disconnect has 60C lugs, you have just lost ampacity. Read the stickers before you pull wire.

Parallel conductors and the size floor

Above 400A, parallel sets usually win on price and bending radius. Parallel conductors are allowed at 1/0 AWG and larger per 310.10(G). All conductors in each parallel set must be the same length, material, size, insulation, and terminated the same way.

For a 400A service, two sets of 3/0 Cu or 250 kcmil Al in parallel handle it cleanly under 310.12 for a dwelling. For commercial at 400A, you are back in 310.16 territory: two sets of 250 kcmil Cu at 75C gives 510A, plenty of headroom. Undersizing one leg of a parallel set is a rewire, so measure twice.

1. Verify 1/0 minimum before splitting into parallel sets.
2. Cut all phase conductors to the same length, even if one side of the can is closer.
3. Keep each parallel set in its own raceway, or follow 300.20 for single-conductor runs in ferrous raceway.
4. Torque every lug with a calibrated driver, 110.14(D).

Grounded and grounding conductors size independently

The neutral is sized per 220.61 based on the maximum unbalanced load, but it cannot be smaller than the grounding electrode conductor required by 250.66 when it also serves as the bonding means at the service. On a 200A dwelling with a typical mix of 240V and 120V loads, a 2/0 Cu or 4/0 Al neutral tracks the ungrounded conductors and satisfies 250.24(C)(1) for the minimum grounded conductor at the service.

The grounding electrode conductor is separate. For 4/0 Al or smaller service conductors, 250.66 calls for a 4 AWG Cu GEC to a concrete-encased electrode, or 6 AWG Cu to a ground rod regardless of the service size, per 250.66(A). Do not upsize the ground rod conductor past 6 Cu thinking bigger is better, it just costs more and changes nothing.

Field tip: if you upsize the ungrounded service conductors for voltage drop on a long lateral, 250.122(B) and 250.102(C)(1) force a proportional upsize on the grounded and bonding jumpers. Skip this and you fail the rough.

Lateral length, voltage drop, and the POCO

NEC does not mandate a voltage drop limit for service conductors, but 210.19 and 215.2 informational notes recommend 3 percent on branch circuits and 5 percent total. On a 300 foot underground lateral feeding a 200A panel, 4/0 Al drops roughly 3 percent at full load. Bump to 250 kcmil Al if the run is longer or the load is steady.

Coordinate with the utility before you trench. Many POCOs specify conduit size, burial depth, and minimum conductor based on their own standards, not just the NEC. Their rules can be stricter, and they win.