What Size Wire for a 25 Amp Breaker?
Conductor size, aluminium equivalent, derating and maximum run length for a 25 amp circuit — all derived from published conductor geometry and the voltage-drop formula.
Quick answer
Copper @ 75 °C
10 AWG
Copper @ 60 °C
10 AWG
Aluminium @ 75 °C
10 AWG
3% drop @ 240 V
115ft
Conductor options for 25 A
Estimated ampacity of each candidate size, so you can see the margin you are buying.
| Material / rating | Minimum size | Estimated ampacity | Margin over 25 A |
|---|---|---|---|
| Copper, 60 °C | 10 AWG | 27 A | +2 A |
| Copper, 75 °C | 10 AWG | 32 A | +7 A |
| Copper, 90 °C | 12 AWG | 27 A | +2 A |
| Aluminium, 75 °C | 10 AWG | 25 A | +0 A |
| Aluminium, 90 °C | 10 AWG | 28 A | +3 A |
Source & method: Conductor areas come from the ASTM B258 relationship d = 5 × 92^((36 − n) / 39). Ampacity is a fitted physical model, not a reproduction of any code table, and runs within roughly ±8% of published values.
Maximum run length on 10 AWG copper
One-way distance before voltage drop passes the 3% and 5% guidelines.
| System | 3% limit | 5% limit | Drop @ 100 ft |
|---|---|---|---|
| 120 V 1Ø | 57 ft | 96 ft | 5.18% |
| 240 V 1Ø | 115 ft | 193 ft | 2.59% |
| 208 V 3Ø | 115 ft | 193 ft | 2.59% |
| 480 V 3Ø | 267 ft | 446 ft | 1.12% |
Source & method: VD = 2 × K × I × L / cmil single phase, 1.732 × K × I × L / cmil three phase, with K = 12.9 for copper. 10 AWG is 10,383 circular mils and 0.9988 Ω per 1000 ft at 20 °C.
Wire size by run length for 25 A
Ampacity sets the floor; distance is what actually forces the upsize on long runs. Smallest copper conductor that carries the load and still holds 3% drop.
| One-way run | 120 V 1Ø | 240 V 1Ø | 480 V 3Ø |
|---|---|---|---|
| 50 ft | 10 AWG | 10 AWG | 10 AWG |
| 75 ft | 8 AWG | 10 AWG | 10 AWG |
| 100 ft | 6 AWG | 10 AWG | 10 AWG |
| 150 ft | 4 AWG | 8 AWG | 10 AWG |
| 200 ft | 4 AWG | 6 AWG | 10 AWG |
| 250 ft | 3 AWG | 6 AWG | 10 AWG |
| 300 ft | 2 AWG | 4 AWG | 8 AWG |
Source & method: A dash means no listed size holds 3% at that distance and voltage — split the load, raise the system voltage, or move the panel closer.
Hot attics, conduit runs and bundling
What a 25 A circuit actually needs once ambient temperature and conductor count are applied.
| Ambient | 3 conductors | 4–6 conductors | 7–9 conductors |
|---|---|---|---|
| 30 °C | 10 AWG | 10 AWG | 8 AWG |
| 40 °C | 10 AWG | 8 AWG | 8 AWG |
| 50 °C | 8 AWG | 8 AWG | 6 AWG |
| 60 °C | 8 AWG | 6 AWG | 4 AWG |
Source & method: Ambient correction F = √((Tc − Ta) / (Tc − 30)) with Tc = 75 °C; bundling adjustment uses the standard stepped schedule (1.00 / 0.80 / 0.70). The conductor shown is the smallest copper size whose corrected ampacity still carries 25 A.
Where 25 A circuits are actually used
25 A is an uncommon residential rating and a routine commercial one: fractional-horsepower motor branch circuits, small package HVAC units, some ductless mini-split condensers, and equipment whose nameplate maximum overcurrent protection is printed as 25 A. It rarely appears as a general-purpose circuit because no standard receptacle configuration matches it.
When you see 25 A, check the equipment nameplate before checking any chart. Air-conditioning and refrigeration equipment is sized from its minimum circuit ampacity and its maximum overcurrent device, both stamped on the unit, and those two numbers override the ordinary conductor-plus-breaker pairing. The conductor follows the MCA; the breaker follows the MOCP and may be larger than the conductor's ampacity would otherwise allow.
For the installer, 25 A is mostly a stocking and device question. Two-pole 25 A breakers are less commonly on the shelf than 20 A or 30 A, and substituting the next size up is not permitted where the nameplate prints a maximum. Conductor choice usually ends up one size larger than the current alone suggests because the terminals on small HVAC equipment are listed at 75 degrees Celsius.
Continuous loads
A continuous load — one expected to run three hours or more, like commercial lighting or an EV charger — is designed at 125% of the load current. 25 A continuous becomes 31.3 A of design current, which lands on a 35 A overcurrent device and 10 AWG copper.
Frequently asked
- What size wire do I need for a 25 amp breaker?
- 10 AWG copper is the smallest conductor our model estimates at or above 25 A using a 75 °C insulation rating; 10 AWG is required if the terminations are limited to 60 °C. In aluminium the equivalent is 10 AWG. Verify against the code edition adopted locally before installing.
- Can I use aluminium wire on a 25 amp circuit?
- Yes, where the terminations are rated for aluminium. Because aluminium carries roughly 78% of the current of the same copper size, a 25 A circuit steps up to 10 AWG and uses K = 21.2 in the voltage-drop formula instead of 12.9.
- How far can a 25 amp circuit run?
- On 10 AWG copper the 3% voltage-drop guideline is reached near 57 ft one-way at 120 V and 115 ft at 240 V. Longer runs need the next size or two up — length, not ampacity, decides most feeder sizes.
- Is a 25 amp breaker sized at 100% of the load?
- Only for non-continuous loads. A continuous load is designed at 125%, so 25 A of continuous current becomes 31.3 A of design current and lands on a 35 A device with conductors sized to match.
- What is typically on a 25 amp circuit?
- Commonly small wall ovens, some mini-split condensers. The load type also drives the receptacle and disconnect ratings, not just the conductor size.
Other breaker sizes
More on 10 AWG
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