Motor Cable Sizing Calculator

Motor Cable Sizing for NEC Article 430 Planning

This page is built for the search intent behind queries such as motor cable size calculator, motor wire size calculator, and 3 phase motor cable size calculator. The key idea is simple: motor conductor sizing starts with NEC table full-load current, not with a generic power formula or a nameplate overload setting.

What the Calculator Actually Does

• Looks up NEC table FLC from horsepower, voltage, and phase count.
• Applies NEC 430.22 by screening conductor ampacity at 125% of table FLC for one motor branch circuits.
• Checks the selected termination basis so a 60°C or 75°C equipment limit can cap the usable ampacity.
• Reviews branch-circuit voltage drop using the entered one-way length and the NEC table running current.

Why the Calculator Uses Table FLC Instead of Motor Nameplate Current

NEC 430.6(A)(1) sends branch-circuit conductor sizing back to the standard ampere ratings in the NEC motor tables. For the AC motors modeled here, that means Table 430.248 for single-phase motors and Table 430.250 for three-phase motors. This is different from overload protection, which follows actual motor nameplate current under NEC 430.32.

Method reference after using the calculator

Use the calculator above for the actual motor HP, voltage, phase count, conductor material, insulation type, termination rating, ambient factor, conductor count, and run length. The reference below explains the method after you have a calculator output for the specific motor circuit.

Core Formula Used by the Conductor Screen

Minimum Motor Branch-Circuit Conductor Ampacity = NEC Table FLC × 1.25

That is the basic single-motor rule from NEC 430.22. This page then checks whether the selected conductor still clears that ampacity after the entered ambient-temperature and conductor-count adjustments, and whether the selected 60°C or 75°C termination column still permits the result.

Preset examples

Use the presets above to compare a standard branch-circuit run and a longer run with derating pressure. Enter the actual motor data, conductor details, termination basis, ambient condition, conductor count, and run length before carrying the calculator output to conductor schedules or downstream protection tools.

Why Termination Rating Still Matters on 90°C Insulation

Many field installations use 90°C insulation such as THHN/THWN-2 or XHHW-2. That higher column is often used as the starting point for ambient and conductor-count adjustments, but the final usable ampacity can still be limited by a 60°C or 75°C equipment termination basis. This page keeps that distinction visible so the result does not overstate usable ampacity.

How Voltage Drop Is Treated Here

Voltage drop is shown as a design screen. It is not presented as a blanket code pass/fail rule for every case, but many designers and installers still target about 3% on the branch circuit to keep motor starting and running performance from drifting too far. The calculator only uses running current for this check, so very long runs and difficult starts may still need a deeper review.

What This Page Does Not Claim

This page does not claim to model every motor conductor problem. It does not replace the adopted NEC edition, actual equipment lugs, field ampacity adjustments beyond the entered data, or manufacturer guidance for adjustable-speed drives and specialty motor installations. It is an honest branch-circuit wire-size screen for common AC motor work.