What Size Wire for a 45 Amp Breaker?
Conductor size, aluminium equivalent, derating and maximum run length for a 45 amp circuit — all derived from published conductor geometry and the voltage-drop formula.
Quick answer
Copper @ 75 °C
6 AWG
Copper @ 60 °C
6 AWG
Aluminium @ 75 °C
6 AWG
3% drop @ 240 V
162ft
Conductor options for 45 A
Estimated ampacity of each candidate size, so you can see the margin you are buying.
| Material / rating | Minimum size | Estimated ampacity | Margin over 45 A |
|---|---|---|---|
| Copper, 60 °C | 6 AWG | 50 A | +5 A |
| Copper, 75 °C | 6 AWG | 59 A | +14 A |
| Copper, 90 °C | 8 AWG | 49 A | +4 A |
| Aluminium, 75 °C | 6 AWG | 46 A | +1 A |
| Aluminium, 90 °C | 6 AWG | 52 A | +7 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 6 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 | 81 ft | 135 ft | 3.69% |
| 240 V 1Ø | 162 ft | 271 ft | 1.84% |
| 208 V 3Ø | 162 ft | 271 ft | 1.84% |
| 480 V 3Ø | 375 ft | 626 ft | 0.8% |
Source & method: VD = 2 × K × I × L / cmil single phase, 1.732 × K × I × L / cmil three phase, with K = 12.9 for copper. 6 AWG is 26,251 circular mils and 0.3951 Ω per 1000 ft at 20 °C.
Wire size by run length for 45 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 | 6 AWG | 6 AWG | 6 AWG |
| 75 ft | 6 AWG | 6 AWG | 6 AWG |
| 100 ft | 4 AWG | 6 AWG | 6 AWG |
| 150 ft | 3 AWG | 6 AWG | 6 AWG |
| 200 ft | 2 AWG | 4 AWG | 6 AWG |
| 250 ft | 1 AWG | 4 AWG | 6 AWG |
| 300 ft | 1/0 AWG | 3 AWG | 6 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 45 A circuit actually needs once ambient temperature and conductor count are applied.
| Ambient | 3 conductors | 4–6 conductors | 7–9 conductors |
|---|---|---|---|
| 30 °C | 6 AWG | 6 AWG | 4 AWG |
| 40 °C | 6 AWG | 4 AWG | 4 AWG |
| 50 °C | 4 AWG | 4 AWG | 3 AWG |
| 60 °C | 4 AWG | 2 AWG | 1 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 45 A.
Where 45 A circuits are actually used
45 A shows up on electric heat, larger air handlers with multiple heat-strip stages, and equipment nameplates that specify a 45 A maximum device. As a standard overcurrent device size it exists, but as a designed branch circuit it is rare outside HVAC.
Multi-stage electric heat is the case to watch. Strip heat is a continuous load and is staged, so the design current is 125% of the total energised at once, not of the whole bank. Getting that wrong in either direction either trips on the coldest night of the year or leaves a conductor protected above its rating.
Aluminium is uncommon at 45 A: the equipment terminals on air handlers and heat kits are frequently listed for copper only, and the conductor lands in a compact junction box with little bending room. Check the equipment label for conductor material and terminal temperature before pricing the feeder.
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. 45 A continuous becomes 56.3 A of design current, which lands on a 60 A overcurrent device and 6 AWG copper.
Frequently asked
- What size wire do I need for a 45 amp breaker?
- 6 AWG copper is the smallest conductor our model estimates at or above 45 A using a 75 °C insulation rating; 6 AWG is required if the terminations are limited to 60 °C. In aluminium the equivalent is 6 AWG. Verify against the code edition adopted locally before installing.
- Can I use aluminium wire on a 45 amp circuit?
- Yes, where the terminations are rated for aluminium. Because aluminium carries roughly 78% of the current of the same copper size, a 45 A circuit steps up to 6 AWG and uses K = 21.2 in the voltage-drop formula instead of 12.9.
- How far can a 45 amp circuit run?
- On 6 AWG copper the 3% voltage-drop guideline is reached near 81 ft one-way at 120 V and 162 ft at 240 V. Longer runs need the next size or two up — length, not ampacity, decides most feeder sizes.
- Is a 45 amp breaker sized at 100% of the load?
- Only for non-continuous loads. A continuous load is designed at 125%, so 45 A of continuous current becomes 56.3 A of design current and lands on a 60 A device with conductors sized to match.
- What is typically on a 45 amp circuit?
- Commonly cooktops, mid-size heat pumps. The load type also drives the receptacle and disconnect ratings, not just the conductor size.
Other breaker sizes
More on 6 AWG
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