Wire Size Calculator

Ampacity is the maximum current a wire can safely carry without overheating (NEC Table 310.15(B)(16)). Voltage drop is the percentage of voltage lost over the length of the run. You must satisfy both, and the stricter requirement wins. For short runs under 50 feet, ampacity usually governs — a 14 AWG wire can carry 15A regardless of distance. For longer runs, voltage drop almost always forces you to a larger gauge. A 14 AWG wire at 15A and 200 feet will have over 5% voltage drop, well beyond the NEC 3% recommendation. This calculator checks both constraints and shows you which one is limiting your wire choice.

Fire alarm NAC circuits are one of the most common wire sizing challenges because they combine long distances with meaningful current loads. A typical horn/strobe draws 100-350mA, and a circuit with 10 devices might pull 2-3A total. At 500 feet on 18 AWG wire, you're looking at significant voltage drop from the 24VDC panel output. Set this calculator to Fire Alarm mode, enter your total current draw and one-way distance, and it will show you the minimum gauge. In most cases, 14 AWG or 12 AWG is required for runs over 300 feet — which is why NFPA 72 allows up to 10% voltage drop, accounting for the reality of large building circuits.

NEC ampacity ratings are based on 30°C ambient temperature. Above-ceiling spaces, attics, and sun-exposed conduit runs can reach 40-50°C or higher, which reduces the wire's safe current capacity by 10-20%. This calculator includes a temperature derating input — set the ambient temperature to your actual conditions and the tool adjusts resistance calculations and ampacity limits accordingly. In a 45°C attic, a 14 AWG THHN wire's ampacity drops from 20A to roughly 17.5A. If you're already at the edge of the wire's capacity, this derating could push you to the next gauge up.

High-power PoE devices (802.3bt Type 3 and 4) can draw 60-90W each. At 48V, that's 1.25-1.875A per device — well above the 350mA of a basic VoIP phone. Standard Cat6 cable (23 AWG conductors) has a DC resistance of about 93 ohms per 1000 feet per pair. Over a 250-foot run at 1.5A, that's a meaningful voltage drop — especially since 802.3bt uses all four pairs to distribute current. This calculator models the actual cable resistance for your selected cable type and distance, telling you whether the built-in conductors in your Ethernet cable are sufficient or whether you need to shorten the run, use a midspan injector, or switch to lower-resistance Cat6a cable.

Aluminum wire costs roughly 40-60% less than copper for the same ampacity, but it has about 61% of copper's conductivity — so you need to upsize approximately two AWG numbers (e.g., use 10 AWG aluminum instead of 12 AWG copper). This makes aluminum most cost-effective for longer, higher-amperage runs like feeders and service entrances where the material savings outweigh the cost of a slightly larger conduit. For low-voltage systems (PoE, fire alarm, access control), aluminum is rarely used because the wire gauges are already small and the connections are more prone to problems.

This calculator lets you toggle between copper and aluminum to see the exact gauge comparison for your specific circuit.

The wire from your power source to the first device must carry the total current for all devices on the circuit. From the first device to the second, it carries the current for all remaining downstream devices, and so on. You need to size the home-run wire for the full cumulative current and the full distance to the farthest device. Use this calculator to model the worst case: enter the total circuit current and the distance to the farthest device. If the gauge passes for the worst case, it passes for all intermediate points.

For circuits where the intermediate legs carry significantly less current, you may be able to use a smaller gauge for those legs — but the home-run gauge must handle the full load.