Motor Cable Size Calculator

Enter motor FLA (prefer NEC Table FLC for code work), voltage, phase, one-way length, and VD limit to size branch conductors per NEC 430.22 (ampacity ≥ 125% FLA). 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 and conductor material (copper/aluminum). Ampacity still uses 125% FLA (NEC 430.22).

Voltage drop limit (%)

Conductor material

About this calculator

See all protection calculators on the hub. This sizes motor branch conductors: ampacity must be ≥ 125% of FLA (NEC 430.22), then checks voltage drop against your limit (typically ≤3% branch / ≤5% feeder+branch). Example: 14 A FLA, 480 V 3φ, 100 ft copper → 12 AWG at ~1% VD. 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.

Recommended conductor size

Result explanation

Recommended conductor: 12 AWG Copper

Assumptions: NEC 430.22 minimum ampacity = 125% FLA. Voltage drop is a recommendation (FPN), not a code requirement, but good design practice. Verify actual conductor conditions, derating factors, and terminal temperature ratings before installation.

Continue Your Calculation

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

Breaker size →

NEC 310.15(B)(16) — Ampacity, 75°C column

Use 75°C column for motor circuits per NEC 110.14(C) (equipment terminations rated 75°C for circuits >100A or #1 AWG and larger; 60°C for smaller). Motor conductor ampacity must be ≥ 125% FLA per NEC 430.22.

AWG/kcmil	Copper (A)	Aluminum (A)	Typical Motor FLA @ 125%
14	20	—	≤ 16 A FLA (small motors)
12	25	20	≤ 20 A FLA (5-7.5 HP @ 480V)
10	35	30	≤ 28 A FLA (10 HP @ 480V)
8	50	40	≤ 40 A FLA (15 HP @ 480V)
6	65	50	≤ 52 A FLA (25 HP @ 480V)
4	85	65	≤ 68 A FLA (30 HP @ 480V)
2	115	90	≤ 92 A FLA (50 HP @ 480V)
1/0	150	120	≤ 120 A FLA (75 HP @ 480V)
3/0	200	155	≤ 160 A FLA (100 HP @ 480V)
250	255	205	≤ 204 A FLA (125 HP @ 480V)

Source: NEC Table 310.15(B)(16), 75°C column, not more than 3 current-carrying conductors in raceway, ambient 30°C. Apply derating factors for more conductors (NEC 310.15(B)(3)(a)) or higher ambient (NEC 310.15(B)(1)).

Motor cable sizing formula

Step 1 — NEC 430.22 minimum ampacity: Min conductor ampacity = FLA × 1.25 (125% for continuous motor loads).

Step 2 — Voltage drop check:

Three-phase: Vd = √3 × L × I × R ÷ 1000
Single-phase: Vd = 2 × L × I × R ÷ 1000

Where: L = one-way cable length (ft), I = FLA (A), R = conductor resistance (Ω/1000ft from NEC Table 8).

Step 3 — Select conductor: Choose the smallest conductor that satisfies both ampacity (Step 1) and voltage drop (Step 2). If voltage drop exceeds limit, increase conductor size until within limit.

Example: 10 HP motor, 14 A FLA (NEC 430.250), 480V 3φ, 100 ft, 3% VD limit, copper.

Step 1: 14 × 1.25 = 17.5 A → minimum 12 AWG (25 A ampacity).
Step 2: Vd = 1.732 × 100 × 14 × 1.98 ÷ 1000 = 4.81 V = 1.00% (well within 3%).
Step 3: 12 AWG copper satisfies both. If length were 300 ft: Vd = 14.4 V = 3.0% (at limit); 350 ft would require 10 AWG.

Worked motor cable sizing examples

Example 1 — 10 HP pump (480V, 3φ, 100 ft)

Given: 10 HP · 14 A FLA (NEC 430.250) · 480V · 3φ · 100 ft · copper

Step 1 — Ampacity: 14 × 1.25 = 17.5 A → 12 AWG (25 A)

Step 2 — Voltage drop: 1.732 × 100 × 14 × 1.98 ÷ 1000 = 4.81 V = 1.00%

Result: 12 AWG copper (THHN/THWN-2). OCPD: inverse-time breaker ≤ 250% = 35 A → 30 A frame (NEC 430.52). Conduit: ½" EMT for 2 conductors.

Example 2 — 5 HP air compressor (240V, 1φ, 75 ft)

Given: 5 HP · 28 A FLA (NEC 430.248) · 240V · 1φ · 75 ft · copper

Step 1 — Ampacity: 28 × 1.25 = 35 A → 10 AWG (35 A)

Step 2 — Voltage drop: 2 × 75 × 28 × 1.24 ÷ 1000 = 5.21 V = 2.17%

Result: 10 AWG copper. Note: single-phase motors have higher voltage drop (2× factor vs √3). If length exceeds 100 ft, upgrade to 8 AWG. OCPD: inverse-time ≤ 250% = 70 A → 60 A frame. Use time-delay fuse for high inrush.

Example 3 — 50 HP conveyor (460V, 3φ, 150 ft, aluminum)

Given: 50 HP · 65 A FLA (NEC 430.250 @ 460V) · 3φ · 150 ft · aluminum · 3% VD limit

Step 1 — Ampacity: 65 × 1.25 = 81.25 A → 2 AWG aluminum (90 A)

Step 2 — Voltage drop (2 AWG Al): 1.732 × 150 × 65 × 0.319 ÷ 1000 = 5.39 V = 1.17%

Result: 2 AWG aluminum (XHHW-2). Aluminum is cost-effective for larger motor circuits but requires larger size and proper torque (NEC 110.14(B)). OCPD: inverse-time ≤ 250% = 162.5 A → 150 A frame. Conduit: 1¼" EMT. Note: verify aluminum conductor terminations are rated for aluminum (CU/AL).

Frequently asked questions

Why 125% of FLA for motor conductors?

NEC 430.22 requires motor branch circuit conductors to have ampacity of at least 125% of the motor full-load current. This accounts for motor heating during continuous operation and allows for brief overloads without conductor overheating. The 125% factor also applies to feeders supplying multiple motors (NEC 430.24).

Should I use nameplate FLA or NEC Table 430.250?

For conductor sizing and overcurrent protection, NEC 430.6(A)(1) requires using the FLA values from Tables 430.248 (single-phase) and 430.250 (three-phase), not the motor nameplate FLA. Nameplate FLA is used for overload relay sizing per NEC 430.32(A)(1) (typically 115-125% of nameplate FLA).

What is the maximum voltage drop for motor circuits?

NEC 210.19(A) FPN No. 4 recommends maximum 3% voltage drop on branch circuits and 5% total (feeder + branch). For motor circuits, excessive voltage drop can cause increased current, overheating, reduced torque, and difficulty starting. A good design target is 2-3% for branch circuits. Long motor runs may require larger conductors.

Can I use aluminum conductors for motor circuits?

Yes, aluminum is acceptable per NEC and often more cost-effective for larger circuits (#2 AWG and above). Key requirements: use aluminum-rated terminations (CU/AL), apply proper installation torque per NEC 110.14(B), and use anti-oxidant compound on aluminum connections. Aluminum conductors are typically one size larger than equivalent copper for the same ampacity.

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.