Service Entrance Cable Size Calculator: 100A, 200A & 400A Services

Reading time: ~10 min read

"What size wire for a 200-amp service?" is the single most-asked wire question in the trade, and it has two correct answers depending on which NEC table applies. For dwelling units, NEC 310.12 (the old 310.15(B)(7)) recognizes that residential services never actually carry their full rating and lets you size at 83% — that's why 2/0 copper and 4/0 aluminum are everywhere. Everything else sizes off Table 310.16 at the 75°C column. The calculator below handles both paths: pick the service rating, material, and method, and it returns the conductor size, then runs a voltage-drop check if you give it a one-way run length.

Building a service package? Our electrical supplies collection stocks SER, USE-2, THHN/XHHW-2, meter bases, and panels from 169 authorized brands — and our generators collection pairs naturally with transfer equipment for backup-ready services.

Service Entrance Cable Calculator

Service Entrance Size Chart (Dwelling vs. Standard)

The two code paths side by side. The dwelling column assumes NEC 310.12 applies — service or feeder carries the entire load of a dwelling unit. Anything else (commercial, detached-building feeders that don't carry the whole dwelling, services feeding multiple units) uses the standard column:

Service rating	Dwelling 310.12 — Cu	Dwelling 310.12 — Al	Standard 310.16 (75°C) — Cu	Standard 310.16 — Al
100 A	4 AWG	2 AWG	3 AWG	1 AWG
125 A	2 AWG	1/0 AWG	1/0 AWG	2/0 AWG
150 A	1 AWG	2/0 AWG	1/0 AWG	3/0 AWG
200 A	2/0 AWG	4/0 AWG	3/0 AWG	250 kcmil
225 A	3/0 AWG	250 kcmil	4/0 AWG	300 kcmil
320 A (400A class)	350 kcmil	500 kcmil	400 kcmil	600 kcmil
400 A	400 kcmil	600 kcmil	600 kcmil	800 kcmil (or parallel 250 kcmil Al)

The 400A row deserves a footnote: most residential "400A" services are actually 320A continuous-class meter bases feeding two 200A panels, which is why 350 kcmil Cu / 500 kcmil Al is the honest dwelling answer. True 400A services at the 75°C column land at 600 kcmil Cu or parallel runs — at which point most shops run parallel 250 kcmil Al or a CT cabinet with utility-approved conductors.

The 83% Rule (NEC 310.12), Explained Without Folklore

310.12 isn't a discount and it isn't optional voodoo — it's the NEC formalizing diversity. Dwelling loads are wildly non-coincident: the range, dryer, AC, and water heater never peak together. So for a service or feeder carrying the entire load of a dwelling unit (one-family, two-family, or an individual unit of a multifamily), the conductors only need an ampacity of 83% of the service rating. 200 × 0.83 = 166A, and 2/0 Cu at 75°C (175A) or 4/0 Al (180A) covers it.

Where contractors get burned is applying 310.12 to things it doesn't cover:

• Subpanel feeders that carry only part of the dwelling load — a 100A feeder to a garage subpanel fed from the main panel is not the entire dwelling load; it's straight 310.16 (though 2023 NEC added 310.12(B) covering feeders serving the entire load of a dwelling unit, e.g., a downstream main panel after a service disconnect).
• Commercial and mixed-use services — no 83%, ever. 310.16 at the termination column.
• Services with no capacity margin — the conductor must still carry the calculated NEC 220 load; if the load calc exceeds 83% of rating, the calc governs.

SER vs. SEU vs. THHN vs. USE-2: Picking the Assembly

Once the size is settled, the assembly question is mostly about the route:

• SER (service entrance, round): the residential default overhead/interior answer — two or three insulated conductors plus a bare neutral/ground in an overall jacket, sunlight-resistant, run exposed or in raceway. 4/0-4/0-4/0-2/0 Al SER is the classic 200A dwelling assembly.
• SEU (U-style): flat twin-hot with concentric neutral — older stock, still listed, less common on new work.
• THHN/XHHW-2 in conduit: required when the AHJ wants raceway end-to-end, in commercial work, and whenever you want the neutral insulated and full-size. Costs more in labor, wins on flexibility and repairability.
• USE-2/RHW direct burial: the underground lateral answer — single conductors, no overall assembly, sunlight-resistant and rated for direct earth burial. Triplex/quadplex URD assemblies are the utility-side cousin.

One trap: NM-B and UF are not service-entrance wiring methods (338.10/340.10 restrictions aside, 338.12 limits SE use), and SER used indoors must still respect the 60°C NM-style rules only when the AHJ treats it as a cable assembly — most read SER at 75°C because the conductors inside are XHHW-2. Ask before you pull.

Voltage Drop on Long Service Runs

Rural lots, detached shops, and deep-set panels turn an ampacity answer into a voltage-drop problem fast. The math at 240V single-phase: VD = 2·K·I·L ÷ cmil, with K=12.9 for copper and 21.2 for aluminum. A 200A service run 300 feet on 4/0 Al drops about 15.4V — 6.4%, double the 3% design target — and the fix is 300 kcmil Al or stepping to copper. The NEC's 3% figures are informational notes for services (design practice, not a hard limit), but 210.19/215.2 make 3% branch/feeder numbers enforceable-adjacent in many jurisdictions, and every inspector has seen a dimming-light callback from a skinny lateral. Run the calculator above with your actual length before ordering wire; the cost of one size up is trivial next to a re-pull.

Related Resources

• Electrical supplies: SER, THHN, panels, and meter bases
• Generators and transfer switches
• Solar panels and interconnection hardware
• EV chargers and dedicated circuits