How to Size Every Motor Circuit with NEC 430

Reading the NEC Motor FLA Tables

NEC Tables 430.248 (single-phase) and 430.250 (three-phase) list FLA values for standard AC induction motors at common voltages. These are used for circuit sizing calculations when the motor nameplate FLA is unavailable, and always for branch circuit protection sizing per NEC 430.6(A) — even when you have the nameplate.

📋 NEC Tables 430.248 & 430.250 — Motor FLA Quick Reference

The Four Components of Every Motor Circuit

NEC Article 430 treats motor circuit protection as four distinct elements, each with its own code section and sizing method. They all start from the same FLA value but use different multipliers for different reasons.

You install a 5 HP compressor motor, size the breaker at 125% of FLA like you would a standard load, and it trips every time the motor starts. So you put in a bigger breaker. Now you have a code violation. Motor circuits look like standard branch circuits but they play by completely different rules — four separate protection elements all sized from the same FLA number, each with its own NEC section and multiplier. Here’s exactly how to get all four right.

What Full-Load Amps (FLA) Actually Is

Full-Load Amps (FLA) is the current a motor draws when operating at its nameplate rated horsepower under rated voltage conditions. It’s the baseline number that every motor circuit calculation starts from — conductor size, breaker size, overload relay setting, and disconnect rating all derive from FLA.

The NEC provides FLA values in two tables. NEC Table 430.248 covers single-phase AC motors. NEC Table 430.250 covers three-phase AC motors. These values are used when the motor nameplate is unavailable, and as the basis for branch circuit protection sizing per NEC 430.6(A).

Here’s what makes motor circuits different from everything else in the NEC: a motor doesn’t just draw its FLA. At the moment of startup, before the rotor is moving, the motor presents itself to the circuit as essentially a short circuit. That initial draw — called Locked Rotor Current (LRC) — is typically 600–800% of FLA and lasts for a fraction of a second until the rotor accelerates to running speed. Every piece of motor circuit design accounts for this inrush.

Worked Example — 7.5 HP Single-Phase 230V Motor

A 7.5 HP single-phase 230V motor — the kind you see on large air compressors, woodworking machinery, and shop equipment. Let’s size the entire circuit step by step.

Notice something: the branch circuit wire (6 AWG, 65A ampacity) is protected by a 100A breaker. That seems backwards — a 100A breaker on 65A wire. This is intentional and explicitly permitted under NEC 240.4(G), which defers motor conductor protection to NEC Article 430. The oversized breaker handles inrush. The overload relay at 50A protects the motor. The wire is protected by the fact that the motor’s overload relay will trip before the wire reaches its thermal limit under sustained overload conditions.

Service Factor — What It Means for Overload Sizing

Service factor (SF) is a multiplier printed on the motor nameplate that indicates how much above rated horsepower the motor can operate continuously without damage. SF 1.15 means the motor can produce 115% of its rated output indefinitely. SF 1.0 means the motor is rated exactly at its nameplate horsepower with no thermal margin.

NEC 430.32(A)(1) uses SF to determine overload relay sizing:

• Motor with SF 1.15 or higher: Overload relay set at 125% of nameplate FLA
• Motor with SF less than 1.15 or no marked SF: Overload relay set at 115% of nameplate FLA
• Motor with temperature rise 40°C or less: Also qualifies for the 125% setting

In practice, most standard NEMA motors are SF 1.15 and use the 125% setting. Premium efficiency motors (NEMA Premium, IE3) often still carry SF 1.15 but may have lower nameplate FLA than NEC table values — use the nameplate FLA for overload relay setting to get tighter, more accurate thermal protection.

Tools for Motor Circuit Work

🔧 What You Need for Motor Circuit Installation

Strip the conductors, make the connections, verify the voltage — in that order

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Klein Tools 11047 Wire Stripper/Cutter 10–18 AWG

• Strips 10 AWG — covers most motor circuit conductors
• Precision stripping holes — no nicks in the wire
• Cuts and loops in the same tool
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Connections

Ideal Industries 30-1010 Lever Connector Kit

• Tool-free lever connections for control wiring
• Handles solid and stranded wire
• Assortment kit — covers multiple wire sizes
• Reusable — open the lever, adjust if needed

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Verify Voltage

Gardner Bender GMT-312 Digital Multimeter

• Verify motor supply voltage before startup
• AC/DC voltage, resistance, continuity
• Budget-friendly — solid for field verification
• Good backup meter for the job site bag

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Pre-Calculated Motor Circuit Sizing Chart (5–150 HP)

Need the answer without opening a calculator? These charts use NEC Table 430.250 full-load current for three-phase induction motors, size the branch conductor at 125% of table FLA (NEC 430.22), pick the smallest THWN-2 copper at 75°C that meets that ampacity (Table 310.16), and list the practical inverse-time breaker at the next standard size at or above 250% of table FLA (NEC 430.52). Dual-element fuse max is 175% of FLA — often one or two sizes smaller than the breaker column.

Wire sizes assume a single set of copper conductors, 75°C terminations, and no ambient/bundling derate. Long runs, aluminum, 90°C columns, and VFD output conductors change the answer — see the voltage-drop section below and verify against the nameplate for overloads.

📋 208 V three-phase — Table 430.250 FLA, 430.22 conductor (125%), THWN-2 Cu 75°C, inverse-time breaker (250% max)

📋 230 V three-phase — Table 430.250 FLA, 430.22 conductor (125%), THWN-2 Cu 75°C, inverse-time breaker (250% max)

📋 460 V three-phase — Table 430.250 FLA, 430.22 conductor (125%), THWN-2 Cu 75°C, inverse-time breaker (250% max)

* Breaker column = next standard rating ≥ FLA × 250%. Dual-element (time-delay) fuse max = FLA × 175% — select the next standard fuse ≤ that ceiling unless 430.52 exceptions apply. Overload relays still use nameplate FLA (430.32), not this chart.

Voltage Drop and Run Distance — When to Upsize the Wire

NEC Article 430 sizes motor conductors for ampacity and protection. It does not automatically fix voltage drop. A 460 V motor that is happy on 8 AWG for ampacity can still starve on a 300 ft feeder if you never check drop. Most shops target 3% or less on the branch (5% branch + feeder combined is the common design goal from NEC Informational Notes).

Three-phase voltage drop (copper):

VD = (1.732 × K × I × D) ÷ CM

• K ≈ 12.9 for copper (21.2 for aluminum) at DC resistance approximation used in field tables
• I = motor table FLA (A) — or the continuous current you expect
• D = one-way run length in feet
• CM = conductor circular mils (12 AWG = 6530, 10 AWG = 10380, 8 AWG = 16510, 6 AWG = 26240, 4 AWG = 41740, 2 AWG = 66360, 1/0 = 105600)

Percent drop = (VD ÷ system voltage) × 100. Example: 10 HP at 460 V (14 A table FLA), 200 ft of 12 AWG Cu: VD = (1.732 × 12.9 × 14 × 200) ÷ 6530 ≈ 9.6 V ≈ 2.1% — fine. Stretch that to 400 ft and you are past 4% on the same wire.

Several Motors on One Circuit (NEC 430.24 / 430.53)

Everything above is one motor, one branch circuit. The moment you put two compressors, a conveyor pack, or a machine tool with multiple motors on a shared feeder or a shared branch breaker, Article 430 switches rules:

• NEC 430.24 — feeder conductors supplying several motors: ampacity ≥ sum of all motor table FLAs plus 25% of the highest-rated motor FLA.
• NEC 430.53 — more than one motor on a single branch circuit is only allowed under specific group-protection conditions (430.53(A)/(B)/(C)). The common shop case is 430.53(C): one OCPD for the group, each motor still has its own overload, and the OCPD is capped at Table 430.52 for the largest motor plus the FLA of the others.

Do not size a multi-motor feeder by pretending it is a single motor at the total HP. Use the dedicated tool and walkthrough:

Get All Four Motor Circuit Values Instantly

Our free motor FLA calculator covers single-phase and three-phase motors from 1/6 HP to 50 HP at all standard voltages. Enter HP and voltage — get conductor size, maximum breaker, overload relay setting, and minimum disconnect in one calculation.

Data per NEC Tables 430.248 and 430.250. Sizing per NEC 430.6, 430.22, 430.32, 430.52, and 430.110. Always verify with current NEC edition, motor nameplate data, and local jurisdiction requirements. TestTalkHQ.com — free tools for tradespeople.