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Wire size calculator / Subpanel wire size

Wire size for a 100 A or 200 A subpanel

100 A and 200 A are entered currents. The rows show the calculator’s single-conductor estimate under each assumption list, including a short run and a 60°C terminal case.

A subpanel feeder is a load calculation, a terminal rating, a metal the equipment lists, a grounding and bonding scheme, and a length. This calculator does not do that design. The numbers 100 A and 200 A on this page are inputs, the same way any other current is an input. They are not standard answers.

Length is why a cell that matches the load can still be the wrong conductor. The 25 ft and 80 ft rows at 200 A and 75°C terminals produce the same copper and aluminum estimates (3/0 AWG (Ampacity governs) copper, 250 kcmil (Ampacity governs) aluminum). Between those lengths, voltage drop has not replaced the governing limit. A longer one-way run can still do that. A 60°C terminal row is included because the general unmarked rule for smaller equipment is not something to ignore just because the panel nameplate is large. Read the assumptions in the row before you read the gauge.

These rows assume the entire entered current is noncontinuous. A panel’s 100 A or 200 A rating is not a load calculation. NEC 215.2(A)(1) requires the applicable continuous-load allowance before correction and adjustment, and adequate ampacity for actual load after those factors. NEC 215.3 and Article 240 separately govern overcurrent protection. A mixed feeder needs its actual continuous and noncontinuous loads evaluated.

For a new 120/240 V feeder using a wire equipment grounding conductor, the usual arrangement is two ungrounded conductors, a neutral and an equipment grounding conductor. Keep neutral isolated from the subpanel enclosure and equipment grounding conductors downstream of the service disconnect (250.24(A)(5), 250.142(B)). A feeder to a separate building also requires the grounding electrode system in 250.32(A) and the equipment grounding path in 250.32(B); earth is not a substitute for that fault-return path. Neutral sizing, grounding conductor sizing, building disconnects and existing-installation exceptions are outside these estimates.

The whole-dwelling feeder allowance in NEC 310.12(B) requires a feeder supplying the entire load associated with an individual dwelling unit, along with the section’s other conditions. It is not a blanket reduction for an ordinary subpanel feeder. The separate feeder ampacity provision in 310.12(C) also needs review for an actual dwelling installation; neither provision is applied here. The older source edition calls these rules 310.15(B)(7). No dwelling reduction is applied in these rows.

Single-conductor estimates

The first row is the calculator’s 100 A example preset. Later rows change the terminal rating, the current, or the length and leave the other checks on their stated values. No row applies dwelling service options, demand factors, or parallel conductors.

Hypothetical 240 V single-phase currents. Not a subpanel, service, or feeder design. Table 8 resistance at 75°C. A “No size” cell means no single conductor in the table met both checks under that row’s assumptions.
Example Assumptions Copper estimate Aluminum estimate
100 A, 60°C terminals 240 V single-phase, 100 A noncontinuous, 80 ft one way, 3% drop limit, 75°C insulation, 60°C terminals, 30°C ambient, 2 current-carrying conductors, small-conductor check on, Table 8 resistance only 1 AWG (Ampacity governs) 1/0 AWG (Ampacity governs)
100 A, 75°C terminals 240 V single-phase, 100 A noncontinuous, 80 ft one way, 3% drop limit, 75°C insulation, 75°C terminals, 30°C ambient, 2 current-carrying conductors, small-conductor check on, Table 8 resistance only 3 AWG (Ampacity governs) 1 AWG (Ampacity governs)
200 A, 75°C terminals, 80 ft 240 V single-phase, 200 A noncontinuous, 80 ft one way, 3% drop limit, 75°C insulation, 75°C terminals, 30°C ambient, 2 current-carrying conductors, small-conductor check on, Table 8 resistance only 3/0 AWG (Ampacity governs) 250 kcmil (Ampacity governs)
200 A, 75°C terminals, 25 ft 240 V single-phase, 200 A noncontinuous, 25 ft one way, 3% drop limit, 75°C insulation, 75°C terminals, 30°C ambient, 2 current-carrying conductors, small-conductor check on, Table 8 resistance only 3/0 AWG (Ampacity governs) 250 kcmil (Ampacity governs)
200 A, 60°C terminals, 80 ft 240 V single-phase, 200 A noncontinuous, 80 ft one way, 3% drop limit, 75°C insulation, 60°C terminals, 30°C ambient, 2 current-carrying conductors, small-conductor check on, Table 8 resistance only 250 kcmil (Ampacity governs) 350 kcmil (Ampacity governs)

100 A, 60°C terminals, copper. Voltage drop would allow 4 AWG. Ampacity requires 1 AWG, so ampacity governs.

100 A, 60°C terminals, aluminum. Voltage drop would allow 3 AWG. Ampacity requires 1/0 AWG, so ampacity governs.

100 A, 75°C terminals, copper. Voltage drop would allow 4 AWG. Ampacity requires 3 AWG, so ampacity governs.

100 A, 75°C terminals, aluminum. Voltage drop would allow 3 AWG. Ampacity requires 1 AWG, so ampacity governs.

200 A, 75°C terminals, 80 ft, copper. Voltage drop would allow 2 AWG. Ampacity requires 3/0 AWG, so ampacity governs.

200 A, 75°C terminals, 80 ft, aluminum. Voltage drop would allow 1/0 AWG. Ampacity requires 250 kcmil, so ampacity governs.

200 A, 75°C terminals, 25 ft, copper. Voltage drop would allow 6 AWG. Ampacity requires 3/0 AWG, so ampacity governs.

200 A, 75°C terminals, 25 ft, aluminum. Voltage drop would allow 4 AWG. Ampacity requires 250 kcmil, so ampacity governs.

200 A, 60°C terminals, 80 ft, copper. Voltage drop would allow 2 AWG. Ampacity requires 250 kcmil, so ampacity governs.

200 A, 60°C terminals, 80 ft, aluminum. Voltage drop would allow 1/0 AWG. Ampacity requires 350 kcmil, so ampacity governs.

Planning estimate only. Consult local code and a qualified electrician before buying wire or energizing a circuit. Presets are hypothetical loads; this tool does not design motor, PV, battery, appliance, or EV installations.

Sources for these cells: existing Table 310.16 numerical cells in NFPA’s public development record, pp. 142–143 (proposed changes are not treated as adopted code) and NFPA 70 (2017), Chapter 9 Table 8, printed p. 689, in a third-party-hosted copy. The same publication supplies 110.14 (terminals and aluminum), 210.19/215.2 (continuous loads), 240.4(D) (small conductors), 250.24/250.32 (grounding), and 625.41/625.42 (EV loads). Schneider Electric’s voltage-drop formulas support the arithmetic; Table 8 AC results here omit reactance and assume unity power factor. Resistance and geometry are construction- and temperature-specific. Adopted editions and local amendments can differ.

Related guides

Open the wire size calculator · Ampacity chart · Voltage drop calculator · Printable chart

Run the 100 A example

The form starts from this page’s example. The address stays on this guide until you edit a field. After an edit, a shared link includes those settings in the URL, and opening that link sends the URL to the host.

Circuit

Headline metal

Both metals are compared. Verify equipment compatibility; some equipment permits copper only.

System

General NEC notes suggest 3% on a branch or feeder and 5% combined. Special equipment and local codes may require other limits; upstream drop is not included.

Estimated size

Enter the load current and the one-way distance. The size appears here, with the limit that set it.

Temperature and bundling

These conditions affect the table ampacity used for the comparison.

Conductor insulation

Use the rating marked for wet or dry conditions. THHN is 90°C dry; XHHW is 90°C dry or 75°C wet. For NM-B use 90°C insulation and a 60°C cap below.

Terminal / cable ampacity cap

Use the lowest applicable rating at either end, including cable limits. The general unmarked default is 60°C for equipment rated ≤100 A or marked for 14–1 AWG. Confirm equipment markings before selecting 75°C; NM-B remains capped at 60°C.

Equipment grounding conductors are excluded. Verify neutrals, harmonic loads, and all other current-carrying conductors in the raceway or cable; the automatic count does not check these.

AC impedance (Chapter 9 Table 9)

At power factor 1, the reactance term is zero, but Table 9 AC resistance still applies. Unlisted sizes have no Table 9 result.

Planning estimate only. Consult local code and a qualified electrician before buying wire or energizing a circuit. Presets are hypothetical loads; this tool does not design motor, PV, battery, appliance, or EV installations.

Questions

Is there one wire size for a 100 A subpanel?

No. 100 A is the entered current on the preset, not a complete feeder. Length, terminal temperature, insulation, ambient, conductor count, and voltage drop all move the estimate. A load calculation, grounding, the disconnect, and the metal the lugs allow are separate. This page does not design a subpanel.

Does a 200 A cell in the ampacity table mean 200 A feeder wire?

No. A Table 310.16 cell that prints 200 A is the listing for that size and temperature under 30°C and at most three current-carrying conductors. The feeder still has to survive voltage drop at its length, and the terminals have to be rated for the column you used. The rows below are calculator output, not a feeder schedule. Parallel conductors are not proposed.

Should a 200 A panel use the 75°C column?

The ampacity chart on this site repeats the general terminal practice: equipment rated above 100 A, or marked for conductors larger than 1 AWG, generally uses 75°C, subject to the marking at both ends. A hotter lug alone does not raise the equipment. One row below keeps 60°C terminals on purpose so the difference is visible. Confirm the equipment. Dwelling-unit service options in the adopted code are not applied here.

Does this table size the neutral and equipment grounding conductor?

No. Each result is one ungrounded conductor per leg, not the complete feeder. A new 120/240 V feeder with an insulated wire equipment grounding conductor normally has two ungrounded conductors, a neutral, and an equipment grounding conductor. Keep the subpanel neutral isolated from the enclosure and equipment grounding conductors downstream of the service disconnect. A separate building also needs its grounding electrode system under 250.32(A); ground rods do not replace the equipment grounding path. Neutral and equipment grounding conductor sizing must be checked separately. Existing-installation exceptions are not evaluated.