Overcurrent protection device sizing

Overcurrent protection devices (OCPDs) protect conductors and equipment from excessive current that could cause overheating or fire. NEC sizing requirements differ for standard loads versus motor loads.

Standard vs motor load OCPD sizing

Standard loads (lighting, receptacles, general equipment):

• OCPD sized to protect the conductor
• Must not exceed conductor ampacity
• 125% factor for continuous loads
• Governed by NEC 240.4 and 210.19

Motor loads (motors, motor-driven equipment):

• OCPD sized based on motor full-load current (FLC)
• Multipliers depend on OCPD type (breaker vs fuse)
• Much larger than conductor protection
• Allows motor starting currents (locked rotor)
• Governed by NEC 430.52

The sizing method depends on load type, not component type. A breaker protecting a motor circuit uses motor sizing rules even though it's the same physical breaker as one protecting a lighting circuit.

Standard load OCPD sizing

For feeders and branch circuits serving general loads:

Step 1: Determine load current

• Standard-load current uses load.ratedCurrent when provided, otherwise it is derived from active power, rated voltage, and power factor.
• Simple branch OCPD sizing uses connected/design load current. load.demandFactor is retained for operating/load-flow and aggregate sizing, not to shrink the branch fuse or breaker.
• If a breaker or fuse protects a downstream bus, ekx sizes it from aggregate downstream demand on that protected bus,

including when the bus is reached through a cable or pass-through component.

• If a branch fuse/breaker is intentionally limited below connected load capability, use the branch design pin/lock path so the limit is explicit.
• Motor circuits use FLC-based sizing (NEC 430 path) described below.

Step 2: Apply continuous load factor

• For continuous loads (operating ≥3 hours): multiply by 125%
• Required OCPD rating = Load Current × 1.25
• Per NEC 210.19(A)(1) and 215.2(A)(1)

Step 3: Verify against conductor ampacity

• OCPD must not exceed effective conductor ampacity (after applicable derating and parallel-run treatment)
• Exception: Next standard size up per NEC 240.4(B)

Step 4: Select standard OCPD size

• Choose from NEC 240.6(A) standard ratings
• Common sizes: 15, 20, 25, 30, 40, 50, 60, 70, 80, 90, 100, 110, 125, 150, 175, 200, 225, 250, 300, 350, 400, 450, 500, 600, 700, 800A

Example:

A continuous 90 A load on a conductor with 120 A ampacity:

• Required OCPD: 90 A × 1.25 = 112.5 A
• Next standard size: 125 A (from NEC 240.6)
• Verify: 125 A > 120 A conductor ampacity (exceeds!)
• Must use: 110 A or 100 A OCPD (at or below conductor ampacity)
• Or upsize conductor to ≥125 A ampacity

NEC 240.6 standard OCPD sizes

Standard ampere ratings for overcurrent devices:

Up to 100A: 15, 20, 25, 30, 35, 40, 45, 50, 60, 70, 80, 90, 100

101A to 800A: 110, 125, 150, 175, 200, 225, 250, 300, 350, 400, 450, 500, 600, 700, 800

Above 800A: 1000, 1200, 1600, 2000, 2500, 3000, 4000, 5000, 6000

Additional standard sizes exist but these are most common in distribution systems.

Next standard size up rule (NEC 240.4(B))

When conductor ampacity does not match a standard OCPD size, you may use the next standard size up if:

• Conductor ampacity is ≥800 A, OR
• Conductors are not part of a multioutlet branch circuit

Limitations:

• Next size up maximum: 800 A
• Above 800 A, must use OCPD ≤ conductor ampacity
• Does not apply to motor circuits (different rules)

Example 1 (next size up allowed):

Conductor: 380 A ampacity

• No standard 380 A OCPD exists
• Next size up: 400 A (allowed, below 800 A threshold)
• Use 400 A OCPD

Example 2 (next size up not allowed):

Conductor: 750 A ampacity

• No standard 750 A OCPD exists
• Next size up: 800 A (allowed, equals 800 A threshold exactly)

Example 3 (next size up prohibited):

Conductor: 900 A ampacity

• No standard 900 A OCPD exists
• Next size up: 1000 A (not allowed, exceeds 800 A threshold)
• Must use 800 A OCPD or upsize conductor to 1000 A

Motor full-load current lookup (NEC 430.248 and 430.250)

For motors, use FLC values from NEC tables instead of nameplate current. For general motor applications, branch short-circuit and ground-fault protection uses the applicable NEC table value ahead of entered fullLoadAmps when horsepower and voltage are available. See NEC 2026 motor current filing.

NEC 430.248 - Single-Phase Motors:

NEC 430.250 - Three-Phase Motors (460V):

Complete tables in NEC include all voltage levels (115V, 208V, 230V, 460V, 575V) and additional horsepower ratings.

Motor conductor sizing (NEC 430.22)

Motor branch circuit conductors must carry 125% of motor FLC:

Required conductor ampacity = Motor FLC × 1.25

This 125% factor accounts for:

• Continuous motor operation (most motors run >3 hours)
• Service factor when motor operates above rated horsepower
• Margin to prevent conductor overheating

Example:

50 HP motor at 460V:

• FLC from NEC 430.250: 65 A
• Required conductor ampacity: 65 A × 1.25 = 81.25 A
• Select conductor: 4 AWG copper (85 A at 75°C)

Then apply temperature and grouping derating as with standard conductors.

Motor OCPD sizing multipliers (NEC 430.52)

Motor branch circuit protection devices are sized much larger than conductor protection to allow motor starting (locked rotor current can be 6-8× FLC):

Sizing formula:

Motor OCPD = Motor FLC × Multiplier

Round up to next standard OCPD size.

Example:

50 HP motor at 460V with inverse time breaker:

• FLC: 65 A (from NEC 430.250)
• Multiplier: 2.5 (inverse time breaker)
• Required OCPD: 65 A × 2.5 = 162.5 A
• Next standard size: 175 A breaker

If motor fails to start, NEC 430.52(C)(1) Exception allows increasing to the next standard size, but not exceeding:

• Inverse time breakers: 4.0 × FLC maximum
• Instantaneous trip: 13.0 × FLC maximum
• Fuses: as-calculated

OCPD type selection

Choose OCPD type based on application and coordination:

Circuit breakers (thermal-magnetic):

• Resettable (no replacement needed)
• Adjustable trip settings on some models
• Lower interrupting capacity than fuses
• Common for motor circuits (inverse time)

Fuses (current-limiting):

• Higher interrupting capacity
• Better current limiting (less arc flash energy)
• Must be replaced after operation
• Dual-element fuses best for motors

Instantaneous trip breakers:

• Motor starting only (require separate overload protection)
• Not suitable for continuous duty loads
• Very high multiplier (8.0×) for motor starting

For most applications, inverse time breakers (2.5×) or time-delay fuses (1.75×) provide best protection coordination.

Maximum OCPD for conductor protection

When sizing OCPD for conductor protection (not motor circuits), the OCPD must not exceed conductor ampacity except as allowed by next size up rule:

Standard conductors (NEC 240.4(D)):

*Exceptions apply for specific applications

Larger conductors:

• OCPD ≤ conductor ampacity
• Or next size up per NEC 240.4(B) if ≤800 A

OCPD margin policy in NEC mode

For OCPD sizing, the engine follows NEC section-based rules directly and does not apply an extra user sizing margin. Breakers and fuses are selected from the NEC-required ampacity path (load/current basis, motor multiplier where applicable, conductor constraints, and standard-size rounding).

Switches are different: they are sized as equipment current ratings, not as OCPDs protecting a conductor. Disconnect and load-break switches use a 15% system default Switch Sizing Margin, with project and per-switch overrides available. The margin is applied after the switch required-current basis is calculated, then the switch rating rounds up to the next standard ampere size.

For motor switches, the required-current basis is 115% of motor FLC per NEC 430.110(A), then any switch sizing margin is applied. For standard loads or aggregate bus protection switches, the basis is the connected/design current or aggregate downstream demand, then the margin is applied.

Branch devices vs bus protection

Use the device's downstream topology to interpret the recommendation:

• Branch device: protects one terminal load or motor. The recommendation is based on the connected branch current and

applicable conductor limit.

• Bus-protection device: protects a downstream distribution bus. The recommendation is based on aggregate downstream

demand from that bus, so it can be larger than a branch device serving one load.

This prevents a feeder or main breaker from accidentally sizing itself from the first downstream branch load when it is actually protecting a bus with multiple downstream loads.

Interpreting sizing warnings

"OCPD size limited by conductor ampacity":

• Selected OCPD would exceed conductor capacity
• Either upsize conductor or reduce OCPD size

"OCPD exceeds next size up threshold":

• Cannot use next standard size up (>800 A)
• Must upsize conductor to match OCPD

"Motor locked rotor protection may be inadequate":

• Selected motor OCPD may not allow motor starting
• Consider using fuse instead of breaker, or increase size per 430.52 exception

"No standard OCPD size matches required ampacity":

• Rare edge case
• Verify load calculation and conductor sizing

Related topics

• Sizing overview - Introduction to NEC sizing engine
• Auto-sizing - How the sizing cascade works
• Cable sizing - Conductor ampacity calculations
• Auto-parallel sizing - Automatic parallel conductor configuration
• Sizing warnings and errors - Resolving sizing issues
• Protection devices component guide - Breakers and fuses in SLD editor