Disclaimer

Every AWG recommendation, ampacity value, voltage-drop percentage, and breaker suggestion on wirecalculator.com is computed from published National Electrical Code table data — NEC Table 310.15(B)(16)/(17) ampacities, conductor resistance figures, the 240.4(D) small-conductor rule, and 240.6(A) standard overcurrent ratings. These outputs are planning and educational figures to help you budget a circuit, buy the right spool of wire, or cross-check a quote. They are not a stamped design, a load calculation, or an approved submittal.

This tool does not replace a licensed electrician, a professional engineer (PE), or your local electrical inspector / Authority Having Jurisdiction (AHJ). For service entrances, feeder and subpanel designs, continuous or harmonic loads, hazardous locations, multi-family or commercial work, or anything requiring a permit, retain a qualified professional who can perform the load calculation, evaluate terminal temperature limits, and sign off on the installation before energizing.

The NEC (NFPA 70) is revised every three years, and each state, province, and municipality adopts its own edition with local amendments — some jurisdictions still enforce the 2017 or even 2014 edition, and Canada uses the CEC (CSA C22.1) entirely. Conductor sizing, permitted insulation types, GFCI/AFCI requirements, and burial depth rules differ between editions and jurisdictions. You are responsible for confirming that your installation complies with the code edition adopted in your area before you pull a single conductor.

A load entered as 30 A when the appliance draws 32 A, a run measured as 50 feet when the cable path is actually 85 feet around obstructions, or a three-phase load entered as single-phase — any of these changes the answer significantly. Measure the actual cable route (not the straight-line room distance), read the nameplate or continuous-duty rating rather than the breaker size, and double-check whether the circuit is 120 V, 240 V, or a 120/240 V split before trusting any result.

The calculator's ampacity baseline assumes three current-carrying conductors at a 30°C (86°F) ambient. Real installations frequently require additional correction and adjustment factors that this tool does not apply: ambient-temperature correction (NEC 310.15(B)(2)), more-than-three-conductor bundling (NEC 310.15(C)(1)), conduit fill and rooftop ambient adders, thermal insulation contact, and equipment terminal temperature limits that cap conductors at 60°C or 75°C regardless of insulation rating. Your electrician must evaluate these before finalizing any size.

EV charging equipment, water heaters, space heaters, and many other loads are continuous by definition (three hours or more). NEC 210.20(A) and 215.3 require the branch-circuit conductors and overcurrent device to be rated at 125% of the continuous load — a 48 A EVSE needs a 60 A circuit, not 48 A worth of conductor. The calculator sizes for the amperage you enter; entering the continuous load without the 125% multiplier is the single most common user-side error we see.

The commonly cited 3% branch-circuit and 5% total-drop targets come from informational notes to NEC 210.19(A) and 215.2(A)(3) — they are recommendations, not mandatory rules. Sensitive loads (motors under high starting torque, medical equipment, long PV runs) may need tighter limits; a garage door opener on a 150-foot run may tolerate more. Choosing 1–5% in the calculator is an engineering decision, and the tool simply enforces whatever ceiling you select.

Where aluminum conductors are permitted and selected, installation practice matters as much as sizing: joint oxidation must be removed and inhibitor applied, terminations must be rated AL/CU, lugs must be torqued to specification, and small-gauge aluminum branch wiring (legacy 15–20 A circuits) is a known fire risk in older homes. The calculator applies aluminum ampacity and resistance data, but it cannot evaluate the termination hardware on your specific equipment.

wirecalculator.com is provided "as is" without warranties of accuracy, fitness for a particular circuit, or merchantability. Always verify conductor sizes against the current NEC adopted by your jurisdiction, the manufacturer's instructions for the equipment you are feeding, and your AHJ's interpretation. Follow the electrician's rule: measure the actual route, de-rate for the real conditions, and never energize a circuit that hasn't been inspected where inspection is required.