Should I use nameplate FLA or NEC Table 430.250?

Where the adopted NEC edition follows 430.6(A)(1), conductor and short-circuit/ground-fault protection sizing generally uses Table 430.248/430.250 FLC values rather than nameplate FLA. Nameplate FLA is typically used for overload relay sizing under 430.32. Confirm the locally adopted edition.

What is the maximum voltage drop for motor circuits?

Many NEC editions include informational notes about 3% branch and 5% feeder+branch voltage-drop planning targets; they are not automatic ampacity pass/fail rules. Excessive motor-circuit drop can raise current, cut torque and hurt starting; long runs often need upsizing or a detailed impedance study.

Can I use aluminum conductors for motor circuits?

Aluminum is allowed under NEC when terminations and installation methods permit it. Use AL/CU-rated terminals, manufacturer torque and appropriate joint compound where required. Ampacity and size steps still follow the adopted edition tables; do not assume a fixed one-size-larger rule without checking the table.

How does motor starting current affect conductor sizing?

Conductor sizing is based on FLA (×125%), not starting current (LRA). Conductors can handle brief starting inrush (5-7× FLA for NEMA B) without damage because the duration is short (seconds). However, voltage drop during starting should be checked — excessive starting voltage drop can cause contactor drop-out or failure to start. If starting VD is problematic, consider larger conductors, soft starter, or VFD.

Motor Cable Size Calculator

Enter motor FLA (prefer NEC Table FLC for code work), voltage, phase, one-way length, and VD limit to screen branch conductors against NEC 2023 430.22 (ampacity ≥ 125% FLA; adopted edition may differ). Default: 14 A · 480 V · 3φ · 100 ft · 3% VD · copper.

Quick calculator

Conductor ampacity checked against NEC 430.22 (125% FLA). Voltage drop checked against your limit. Results update instantly.

Motor FLA (A)

Voltage (V)

Phase

Cable length (ft, one-way)

Advanced calculator

Set voltage-drop limit, conductor material and termination basis. Ampacity still uses 125% FLA (NEC 430.22).

Voltage drop limit (%)

Conductor material

Termination temperature basis

Choose 75°C only when the equipment listing/marking and adopted code permit that termination basis.

About this calculator

See all protection calculators on the hub. This screens motor branch conductors: ampacity must be ≥ 125% of FLA (NEC 430.22), then checks voltage drop against your limit. Example: 14 A FLA, 480 V 3φ, 100 ft copper → 12 AWG at about 1.00% VD using the conservative 60°C termination basis. Verify terminal markings, installation corrections, adopted-code small-conductor rules and the actual motor branch requirements. Get FLA first with the Motor FLA Calculator, then size OCPD with the Breaker Size Calculator. For non-motor circuits use the general Cable Size Calculator.

Screening conductor size

Result explanation

Screening conductor: 12 AWG Copper (60°C basis)

Assumptions: NEC 2023 Table 310.16 and NEC 430.22 minimum ampacity = 125% FLA; 60°C is the conservative default termination basis. Verify equipment markings, small-conductor rules, installation corrections and voltage-drop policy before installation.

Continue Your Calculation

With conductor size selected, screen overcurrent protection and verify end-voltage drop on the run.

Breaker size →

NEC 2023 Table 310.16 ampacity basis

The calculator includes 60°C and 75°C column screens. It defaults to 60°C; use 75°C only when the equipment listing/marking and adopted NEC rules permit that termination basis. Values assume no more than three current-carrying conductors and 30°C ambient before adjustment/correction. Motor conductor ampacity is screened at 125% of the applicable motor FLC under NEC 430.22; final design must also apply the adopted edition, terminal limits, conductor insulation, ambient/grouping corrections and applicable small-conductor/protection rules.

Motor cable sizing formula

Step 1 — NEC 430.22 ampacity screen: minimum conductor ampacity = FLC × 1.25 using the FLC basis required by the adopted code.

Step 2 — Resistive voltage-drop screen: three-phase Vd = √3 × L × I × R ÷ 1000; single-phase Vd = 2 × L × I × R ÷ 1000. L is one-way feet and R is Ω/1000 ft.

Step 3 — Select the first table row that clears both checks. For the default 14 A, 480 V, three-phase, 100 ft copper case, 125% FLC is 17.5 A. The conservative 60°C basis selects 12 AWG and the resistive drop is about 1.00%. This remains a screening result: equipment markings, adopted-code small-conductor/protection rules, installation corrections, conductor count and short-circuit withstand can require a different conductor.

Worked motor cable sizing examples

Default production case: 14 A × 1.25 = 17.5 A. At 480 V three-phase and 100 ft, the 60°C copper basis returns 12 AWG with about 1.00% resistive drop.

Single-phase comparison: 28 A × 1.25 = 35 A. At 230 V and 75 ft, the 60°C copper basis returns 8 AWG with about 1.42% resistive drop.

Aluminum comparison: 70 A × 1.25 = 87.5 A. At 460 V three-phase and 150 ft, the 60°C aluminum basis returns 1/0 AWG with about 0.79% resistive drop.

These examples reproduce the production screening tables. They do not choose OCPD, raceway, final terminal rating, derating, parallel-conductor arrangements or short-circuit withstand.

Frequently asked questions

Why use 125% of motor FLC?

NEC 430.22 generally bases a single motor branch-circuit conductor ampacity on 125% of the applicable motor full-load current. Use the FLC source and exceptions required by the locally adopted edition.

Can I use the 75°C column?

Only when the equipment listing/marking and adopted termination-temperature rules permit it. This tool defaults to the 60°C column and requires an explicit selection for 75°C.

Does this choose the breaker or overload setting?

No. Conductor ampacity, short-circuit/ground-fault protection and overload protection are separate checks with different NEC motor rules.

How should starting current be handled?

Use motor or driven-equipment manufacturer inrush/locked-rotor data and a source-impedance study for starting voltage drop. This steady-state resistive screen does not predict motor-starting performance.