The right AWG gauge depends on three things: how many amps the circuit carries, how long the wire run is, and what conductor material you use. Enter your load below and the calculator returns a recommended gauge based on NEC ampacity tables (NFPA 70, Table 310.16) plus a voltage-drop check.
Planning estimate. Verify with a licensed electrician and your local AHJ (authority having jurisdiction) before installation. NEC / NFPA 70 requirements.
Wire sizing has two independent constraints: ampacity and voltage drop. You must satisfy both. Ampacity is the maximum continuous current a wire can carry without overheating its insulation. Voltage drop is the voltage lost to resistance over the length of the run.
The calculator uses NEC Table 310.16 copper ampacities at 60 degrees C (the temperature rating used for residential wiring terminations per NEC 110.14(C)):
For continuous loads (running more than 3 hours), NEC 210.20(A) requires the circuit to be rated at 125% of the load, so the calculator divides your amps by 0.8 before looking up the ampacity table. For aluminum, ampacity is reduced roughly one size (aluminum carries about 61% of copper's current at the same gauge).
Voltage drop uses the standard DC/AC formula: Vdrop = (2 x length x amps x resistivity) / circular mils. Resistivity for copper is approximately 10.4 ohm-cmil/ft, and for aluminum approximately 17 ohm-cmil/ft. The calculator then checks whether the drop exceeds 3% of the system voltage and, if so, moves up to the next AWG until the run meets the limit.
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Solar panel wiring follows NEC Article 690 (Solar Photovoltaic Systems), which layers additional rules on top of the standard ampacity tables. PV source circuits must be sized for 125% of the short-circuit current (Isc) per panel string, and then that result is multiplied by another 125% for the continuous-load factor, so you end up sizing for 156% of Isc. That sounds like a lot, but it reflects real-world conditions: panels can briefly exceed their rated Isc under certain cloud-edge conditions.
For a typical residential 8 kW system with string inverters, the DC wire from the roof array to the combiner box commonly uses 10 AWG copper (good to about 30 amps at the distances involved). Runs longer than 50 feet at low voltages, such as 12V or 24V battery banks, often need 8 or 6 AWG to keep voltage drop under 3%.
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12 AWG copper is the standard for a 20-amp circuit under NEC Table 310.16. If the load is continuous (running more than 3 hours), NEC 210.20 requires the circuit to handle 125% of the load, so you would size for 25 amps, which still lands on 12 AWG copper. On a long run of 75 feet or more, check whether 10 AWG is needed to keep voltage drop under 3%.
10 AWG copper handles 30 amps per NEC Table 310.16. On runs longer than about 60 feet at 120V or 120 feet at 240V, the voltage drop can push past 3%, and 8 AWG may be needed. For aluminum at 30 amps, start at 8 AWG since aluminum requires one size larger than copper for the same ampacity.
American Wire Gauge numbers run backward: smaller numbers mean thicker wire. 14 AWG is thinner and lower ampacity than 12 AWG, which is lower than 10 AWG. Each step down roughly doubles the cross-sectional area and adds about 25% more ampacity. NEC Article 310 publishes full ampacity tables for every gauge by conductor type and insulation temperature rating.
Yes. Thinner wire has more resistance per foot, so voltage drops further over the same run length. Over long wire runs this wastes power as heat and can starve a load of the voltage it needs. NEC informational notes in 210.19 recommend keeping branch circuit voltage drop under 3%, which becomes critical for low-voltage DC solar systems.
Aluminum is code-compliant for service entrances, feeders, and some branch circuits when using devices and connectors rated for aluminum conductors (marked AL or CU/AL). Because aluminum conducts about 61% as well as copper, you need to go up one AWG size. Most electricians avoid aluminum for 15 and 20 amp branch circuits due to its history of connection failures from thermal expansion.
DC wiring in a solar PV system is governed by NEC Article 690. Size the wire for 156% of the panel string's short-circuit current (125% for PV source circuits x 125% for continuous load). At 24V or 48V battery banks, even modest amps can produce large voltage drops over 30 or 40 feet, so 8 or 6 AWG is common. Always verify with your inverter documentation and a licensed electrician.