Safety guide for sizing service entrance

Start with the Load Calc, Not the Panel

Sizing a service entrance begins with Article 220, not a guess at the main. Run the standard calculation per 220.40 through 220.61, or use the optional method in 220.82 for a dwelling. Pick one method and stay in it. Mixing Part III and Part IV in the same calc is how services end up undersized or bloated.

For a single-family dwelling under the optional method, apply 100% to the first 10 kVA of general load and 40% to the remainder, then add HVAC at the larger of heating or cooling per 220.82(C). Fastened-in-place appliances rated 1/4 HP or 500 W and above count toward the general load at nameplate. Ranges follow Table 220.55, dryers follow 220.54.

Document the calc on paper before you pull wire. AHJs ask for it, and you will need it when the homeowner adds a heat pump water heater two years from now.

Match Conductor to Service, Not to the Breaker Alone

For dwelling services and feeders supplying the main power to a dwelling, 310.12 allows the 83% rule: the ungrounded conductors can be sized to carry 83% of the service rating. A 200 A dwelling service runs fine on 2/0 copper or 4/0 aluminum THHN/THWN-2 at 75°C. Non-dwelling services do not get this allowance; size to the full load per 310.16 at the 75°C column for terminations, per 110.14(C)(1).

Derate for ambient and conduit fill before you commit. A 4/0 aluminum SER in a hot attic rated 90°C still terminates at 75°C. The 75°C ampacity is your ceiling at the lugs, regardless of insulation.

• 100 A dwelling: 4 AWG Cu or 2 AWG Al (310.12)
• 150 A dwelling: 1 AWG Cu or 2/0 AWG Al
• 200 A dwelling: 2/0 AWG Cu or 4/0 AWG Al
• 400 A dwelling: 400 kcmil Cu or 600 kcmil Al
• Non-dwelling 200 A: 3/0 Cu or 250 kcmil Al at 75°C

Grounded Conductor and the GEC

The grounded (neutral) service conductor sizes off the maximum unbalanced load per 220.61, but never smaller than 250.24(C)(1) requires, which points to Table 250.102(C)(1). On a 200 A service with 2/0 copper ungrounded, the minimum grounded conductor is 4 AWG copper based on that table. If the ungrounded conductors are upsized for voltage drop, the grounded conductor scales proportionally by circular mil area.

The grounding electrode conductor is sized from Table 250.66. For 2/0 copper service conductors, the GEC is 4 AWG copper. When the only electrode is a ground rod, 250.66(A) caps the GEC to that rod at 6 AWG copper. Concrete-encased electrodes cap at 4 AWG per 250.66(B). Metal water pipe gets the full table value.

Pull the GEC before you set the meter can. Threading 4 AWG solid through a finished exterior wall after drywall is nobody's idea of a good afternoon.

Service Disconnect Rules Changed, Read Them Again

Since the 2020 cycle, 230.67 requires surge protection on dwelling services, and 230.85 requires an emergency disconnect for one- and two-family dwellings, marked per the language in 230.85(E). The disconnect must be outside, or inside immediately adjacent to the point of entry when permitted, and rated for the service. Meter-main combos with integrated SPDs are the cleanest solution on new services.

The six-disconnect rule in 230.71 is gone for most applications. A single service disconnect is now the standard; grouped disconnects must be in separate enclosures or compartments, or they count as one listed assembly. Don't quote the old 2017 language to an inspector running the 2023 code.

1. Verify the AHJ's adopted code cycle before the rough
2. Confirm SPD requirement per 230.67 and whether the panel's integrated SPD is listed and accessible
3. Mark the emergency disconnect with the exact wording from 230.85(E)
4. Check working clearances per 110.26 at both the outside disconnect and the indoor panel

Mast, Drip Loop, and Clearances

Overhead service clearances live in 230.24. Ten feet above finished grade at the drip loop for 120/240 V residential, 12 feet over residential driveways, 18 feet over public roads. The mast must carry the service drop without deforming, which is why 1-1/4 inch rigid is the floor for most POCO specs, with 2 inch being common for 200 A. Check the utility handbook too, they often exceed NEC minimums on mast bracing and weatherhead height.

Underground lateral depth is in Table 300.5. Direct-buried service conductors need 24 inches of cover. PVC conduit drops to 18 inches. Rigid metal gets to 6 inches. The meter base height is usually set by the POCO, typically 5 to 6 feet to the center of the meter.

Call the POCO before the trench goes in. Their spec sheet overrides yours, and a failed meter set because of a 4 inch height miss is a full day lost.

Field Checklist Before You Energize

Before the POCO cuts in, walk the service one more time. Torque matters: every lug gets the manufacturer's value per 110.14(D), which is now a code requirement, not a best practice. A calibrated torque screwdriver pays for itself the first time an inspector asks.

• Load calc on paper, matches panel rating
• Conductors sized per 310.12 or 310.16, derated for conditions
• Grounded conductor per 220.61 and 250.24(C)
• GEC per Table 250.66, with correct taps at rods and Ufer
• Main bonding jumper installed, neutral-ground bond only at service
• SPD listed, installed, and accessible per 230.67
• Emergency disconnect labeled per 230.85(E)
• Working clearances per 110.26, no storage, no panel in a bathroom or clothes closet
• All lugs torqued and marked

Get the calc, the conductors, and the disconnect right, and the rest of the service almost sizes itself. The code gives you the numbers. The field gives you the judgment to apply them.