30A Electric Furnace — Wire Size Guide
30-amp circuit for an electric furnace or air handler with supplemental electric heat strips. Small electric furnaces with 5-10 kW of heating capacity operate on a single 30-amp, 240-volt circuit. Larger units may require multiple circuits or higher amperage. The circuit must match the nameplate minimum circuit ampacity.
Copper Wire Size by Distance
The table below shows the recommended copper wire gauge for a 30A Electric Furnace circuit (240V, 30A, Single-Phase) at various one-way distances from the panel. Two voltage drop limits are shown: 3% for branch circuits and 5% for feeder-plus-branch combined. The ampacity column is based on NEC Table 310.16 at 75 degrees C. When the 3% and 5% columns show different gauges, the distance is long enough that voltage drop — not ampacity — is the controlling factor, and you must use the gauge appropriate for your circuit type.
| Distance | 3% Drop Gauge | 3% Drop (V) | 3% Drop (%) | 5% Drop Gauge | 5% Drop (V) | 5% Drop (%) |
|---|---|---|---|---|---|---|
| 25 ft | 10 AWG | 1.86 | 0.78% | 10 AWG | 1.86 | 0.78% |
| 50 ft | 10 AWG | 3.72 | 1.55% | 10 AWG | 3.72 | 1.55% |
| 75 ft | 10 AWG | 5.58 | 2.33% | 10 AWG | 5.58 | 2.33% |
| 100 ft | 8 AWG | 4.67 | 1.95% | 10 AWG | 7.44 | 3.10% |
| 125 ft | 8 AWG | 5.84 | 2.43% | 10 AWG | 9.30 | 3.88% |
| 150 ft | 8 AWG | 7.00 | 2.92% | 10 AWG | 11.16 | 4.65% |
| 200 ft | 6 AWG | 5.89 | 2.46% | 8 AWG | 9.34 | 3.89% |
| 250 ft | 4 AWG | 4.62 | 1.93% | 8 AWG | 11.67 | 4.86% |
| 300 ft | 4 AWG | 5.54 | 2.31% | 6 AWG | 8.84 | 3.68% |
| 400 ft | 3 AWG | 5.88 | 2.45% | 6 AWG | 11.78 | 4.91% |
| 500 ft | 2 AWG | 5.82 | 2.43% | 4 AWG | 9.24 | 3.85% |
Aluminum Wire Size by Distance
Aluminum conductors cost less and weigh less than copper but have higher resistance, requiring larger gauge sizes to carry the same current. The table below shows the recommended aluminum wire gauge for this 30A Electric Furnace circuit at various distances. Aluminum is a practical choice for longer feeder runs and service entrance conductors where the material savings justify the larger conductor size. All aluminum terminations must be rated for aluminum use (AL-CU marking) and treated with anti-oxidant compound.
| Distance | 3% Drop Gauge | 3% Drop (V) | 3% Drop (%) | 5% Drop Gauge | 5% Drop (V) | 5% Drop (%) |
|---|---|---|---|---|---|---|
| 25 ft | 8 AWG | 1.92 | 0.80% | 8 AWG | 1.92 | 0.80% |
| 50 ft | 8 AWG | 3.84 | 1.60% | 8 AWG | 3.84 | 1.60% |
| 75 ft | 8 AWG | 5.76 | 2.40% | 8 AWG | 5.76 | 2.40% |
| 100 ft | 6 AWG | 4.85 | 2.02% | 8 AWG | 7.68 | 3.20% |
| 125 ft | 6 AWG | 6.06 | 2.53% | 8 AWG | 9.60 | 4.00% |
| 150 ft | 4 AWG | 4.57 | 1.91% | 8 AWG | 11.52 | 4.80% |
| 200 ft | 4 AWG | 6.10 | 2.54% | 6 AWG | 9.70 | 4.04% |
| 250 ft | 3 AWG | 6.05 | 2.52% | 4 AWG | 7.62 | 3.18% |
| 300 ft | 2 AWG | 5.74 | 2.39% | 4 AWG | 9.14 | 3.81% |
| 400 ft | 1 AWG | 6.07 | 2.53% | 3 AWG | 9.67 | 4.03% |
| 500 ft | 1/0 AWG | 6.03 | 2.51% | 2 AWG | 9.57 | 3.99% |
Installation Tips for 30A Electric Furnace
Proper installation is just as important as selecting the correct wire size. The following tips address the key NEC requirements and best practices specific to 30a electric furnace circuits. Always verify these requirements against the current edition of the NEC and your local code amendments before beginning work.
- This is a 240-volt circuit requiring two hot conductors, a neutral (if needed), and an equipment grounding conductor. Use a two-pole breaker rated for the circuit amperage.
- For a 30-amp circuit, use a cable or conduit rated for the conductor size. The equipment grounding conductor must be sized per NEC Table 250.122.
- All wire connections must be made with listed connectors and torqued to manufacturer specifications. Use anti-oxidant compound on all aluminum connections.
- Verify local code amendments, as your jurisdiction may have requirements beyond the NEC minimum. Always pull a permit and schedule inspections for electrical work.
NEC Code References
The 30A Electric Furnace circuit is governed by NEC 424.3, 424.19, 424.22. These NEC sections specify the minimum requirements for circuit sizing, overcurrent protection, grounding, and installation methods. The wire sizing in this guide is based on the following NEC tables and articles:
- NEC Table 310.16 — Ampacity of insulated conductors rated 0-2000V in raceways, cables, or direct buried. Provides the maximum current each gauge can safely carry at 60, 75, and 90 degrees C.
- NEC Chapter 9, Table 8 — Conductor properties including DC resistance per 1000 feet for uncoated copper and aluminum. These resistance values are used in the voltage drop formula.
- NEC 210.19(A) Informational Note No. 4 — Recommends that branch circuit conductors be sized so that the voltage drop does not exceed 3%, and the combined feeder plus branch circuit drop does not exceed 5%.
- NEC 215.2(A)(1) Informational Note No. 2 — Same voltage drop recommendation applied to feeder conductors.
- NEC 424.3, 424.19, 424.22 — Specific requirements for this application, including circuit sizing, overcurrent protection, and installation methods.
Other Applications
Browse wire sizing guides for other common electrical circuits. Each page includes pre-computed wire sizes at multiple distances, installation tips, and NEC code references.