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.

Sizing recommendations and current selection checks are separate. The recommendation describes the standard rating or setting produced by the latest sizing run. Recheck evaluates the installed device rating, frame, and applicable settings from the current project and scenario facts. Manual and locked devices remain checked; a difference from the recommendation is not itself a failure. A device below a sizing margin or the medium-voltage minimum reads Below your target and comes with a suggestion, not a failed check. A limit you typed, such as a branch design limit, still fails.

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

Capacitor banks:

• EKX proposes the smallest standard rating at or above 135% of the bank's rated current. This is EKX's default, not a Code minimum: NEC 460.9 and CEC Rule 26-210 require only a rating "as low as practicable"
• A lower rating you select is shown as below EKX's default, not as a Code failure
• Under CEC, a rating above 250% of the bank's rated current is a Code violation (Rule 26-210)
• Not auto-sized on an NEC project above 1000 V
• See Capacitor banks

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.

Breaker and fuse auto-sizing uses the effective Allowed Breaker Ratings and Allowed Fuse Ratings under Settings → Sizing Rules. Excluding a rating never relaxes conductor, motor, transformer, protected-bus, or other limits. If the next allowed rating exceeds a hard limit, EKX blocks the change instead of skipping past the limit. Manual ratings and existing devices remain available and remain included in current selection checks.

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 one breaker or fuse feeds multiple terminal loads, model the distribution point as a bus or panel. Without that modeled bus/panel, ekx skips branch OCPD auto-sizing instead of guessing from the first load or silently aggregating loads as a branch device.
• 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 210.20(A) for branch circuits and 215.5 for feeders

Step 3: Verify against conductor ampacity

• OCPD must not exceed effective conductor ampacity (after applicable derating and parallel-run treatment)
• Exception: the conditional next-standard-size rule in NEC 240.4(B). If the load requirement already exceeds conductor ampacity, ekx blocks the OCPD proposal instead of rounding the requirement down.

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
• For breakers and switches above 1 kV, ekx first applies the configured Minimum MV/HV Breaker and Switch Rating. The system default is 600 A, and setting it to 0 disables this voltage-class floor.

Main and aggregate breaker sizing

A breaker or fuse is treated as aggregate bus protection when its downstream side protects a bus, including when that bus is reached through a short pass-through path such as breaker -> cable -> bus.

ekx applies these rules in order:

1. Calculate aggregate downstream demand on the protected bus. With motors on the bus, the hard minimum is the running load: every motor at full-load current plus the other loads.
2. Apply continuous-duty/nameplate requirements once to get the hard minimum breaker rating.
3. For a feeder that serves motors and other loads under NEC, apply 430.63: the rating must be not less than the other loads plus the largest motor's permitted 430.52 rating, plus the other motors' full-load current.
4. Apply the configured Minimum MV/HV Breaker and Switch Rating for breakers and switches above 1 kV.
5. Prefer the motor feeder conductor basis (125% of the largest motor plus the rest), then apply Main Breaker Sizing Margin when configured. The default margin is 0%. Both give way to the hard maximums below.
6. Check hard maximums from protected equipment and conductors. When the breaker is the only secondary overcurrent device of a transformer, the transformer's permitted secondary rating is also a hard maximum. With motors on the bus, CEC Rule 28-204 and NEC 430.62(A) (motor-only feeders) add a motor feeder maximum.
7. Select the nearest standard breaker or fuse size that satisfies the minimum without exceeding hard limits.

The motor feeder rules value the largest motor at the maximum branch protection its full-load current permits for the main's own device type, not at the branch fuse or breaker you installed. For example, a 27 A motor behind a CEC inverse-time main breaker counts as 27 A × 250% = 67.5 A. Under NEC the value rounds up to the next standard rating (430.52(C)(1)(a)), so the same motor counts as 70 A. The installed branch device does not affect the main. Changing a branch fuse does not change the main breaker's recommendation. Under NEC 430.62(B), a motor-only feeder whose cable is larger than 430.24 requires can have a main up to that cable's ampacity.

If no standard size fits between the minimum and the motor feeder maximum, the breaker is blocked and the edit panel states both values. Starting allowances for motors that will not start, simultaneous starting, and motor control center feeders are not modelled.

If the minimum is above the transformer's permitted secondary rating and the transformer is in Auto mode, ekx raises the transformer to the smallest allowed capacity that can accept the breaker and sizes the breaker, its cable, and its bus against that proposed capacity as one group. If the transformer is Manual or Locked, ekx blocks the breaker itself and states both values, for example a 193.8 A minimum under NEC 430.63 against a 175 A limit for an unsupervised location. The message lists what can change the outcome: mark the transformer as a supervised location when that is true, or choose a larger transformer. ekx does not draft the secondary cable or bus around a rating it already knows is blocked.

The MV/HV breaker frame floor is separate from the MV/HV bus rating floor. If both the main breaker and protected bus are in Auto mode, the bus can still receive a linked proposal so its rating follows the selected breaker frame.

The protected bus rating is a hard equipment limit. If the protected bus is in Auto mode and the selected main breaker is above the current bus rating, ekx proposes or applies a bus upsize so the bus rating follows the main breaker. If the bus is Manual or Locked, ekx will not exceed that bus rating; if the requested margin does not fit, the margin is blocked and the breaker falls back to the smallest standard size that still satisfies the hard minimum. If no standard size can satisfy the minimum without exceeding the protected bus, the sizing result is blocked and the edit panel explains the conflict. When the main breaker and its bus both come down, the bus change is applied only together with the main breaker's, so the bus never sits below the installed main.

The protected conductor path is treated the same way. A uniquely identified source-side cable caps the main breaker by its effective ampacity. A proven direct source or transformer connection can size without a cable cap. If the breaker-side source conductor passes through a simple one-in/one-out upstream bus before reaching the transformer, source, or upstream OCPD, ekx keeps the nearest breaker-side cable as the protected conductor and uses the upstream path only to prove the boundary. Ambiguous source-side paths, branched upstream buses, multiple possible sources, or missing conductors through non-source topology block the main-breaker upsize until the one-line is clarified.

For transformer secondary mains, ekx uses a conservative conductor-protection model: the secondary main must still be compatible with the protected secondary conductor ampacity unless that conductor is also resized. Detailed transformer secondary tap-rule exceptions are not assumed automatically.

Example:

A continuous 90 A load on a conductor with 110 A ampacity:

• Required OCPD: 90 A × 1.25 = 112.5 A
• Next standard size: 125 A (from NEC 240.6)
• Verify: required 112.5 A > 110 A conductor ampacity (the conductor is already inadequate)
• A 110 A or smaller OCPD would be below the calculated 112.5 A requirement, so it is not a compliant auto-sizing result
• Upsize the conductor to support the selected standard OCPD, or reduce/correct the load

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:

• The conductor ampacity does not correspond to a standard device rating
• The conductors are not part of a branch circuit supplying more than one receptacle for portable cord-and-plug loads
• The selected next standard rating does not exceed 800 A

The rule covers branch, feeder and main breakers and fuses alike. NEC 215.5 applies it to feeders and 230.90(A) Exception No. 2 to service conductors. ekx sizes and checks every one of them with the same limit, so a 400 A main on a 380 A feeder cable passes. In a CEC 2024 project, Rule 14-104(1)(a) and Table 13 permit the same rating within 800 A.

Limitations:

• Next size up maximum: 800 A
• Above 800 A, must use OCPD ≤ conductor ampacity (NEC 240.4(C))
• Does not apply to motor circuits (different rules)
• Not permitted for transformer secondary conductors (NEC 240.21(C))
• Above 1000 V, NEC Article 240 no longer applies, and NEC 245.27(A) permits higher feeder ratings. ekx keeps this limit there as its own design limit and labels the check as a design limit, not a Code requirement.

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 at the limit):

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 sections 430.6, 430.22, and 430.250.

NEC 430.248 - Single-Phase Motors:

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

EKX only uses combinations that have an actual value in the NEC table. A dash or a nonstandard horsepower is not silently replaced with an extrapolated or nearby value; entered nameplate current remains available as the fallback basis.

Motor overload protection is a separate nameplate-current calculation. For a continuous-duty motor over 1 hp, EKX shows 125% of nameplate FLA when the marked service factor is at least 1.15, and otherwise shows the conservative 115% limit. The recommendation is contextual information beside the service-factor field; it does not interrupt ordinary motor modeling.

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 × selected device/mode multiplier

The Tight, Standard, and Maximum choices stay within the applicable device-specific range. Instantaneous-trip breakers and the two fuse types do not reuse the inverse-time-breaker range.

NEC 240.4(G) routes motor branch conductor protection to Article 430. The motor short-circuit and ground-fault OCPD can therefore be larger than the NEC 430.22 conductor ampacity; motor overload protection is a separate function. If a user-entered maximum conflicts with the selected mode or its rounded standard rating, ekx blocks the proposal instead of lowering the calculated requirement.

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 the motor will not start at the selected setting, treat that as an engineering review of starting method, device type, trip settings, and the applicable 430.52 permissions. Auto-sizing does not silently exceed the selected device's modeled table range.

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 branch 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, configured voltage-class breaker frame floor where applicable, and standard-size rounding).

Aggregate/main breaker sizing can use the separate Main Breaker Sizing Margin setting. That margin is optional and defaults to 0%. The Minimum MV/HV Breaker and Switch Rating is not a margin; it is a configurable equipment floor for breakers and switches above 1 kV.

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.

If a device reaches multiple terminal loads before a modeled bus or panel, ekx treats the branch topology as ambiguous and skips the OCPD recommendation. Add a downstream bus/panel for aggregate protection, or add separate branch devices for each load.

Interpreting sizing warnings

"OCPD size limited by conductor ampacity":

• Required OCPD exceeds conductor capacity
• Upsize the conductor or reduce/correct the connected load; ekx does not create an undersized breaker proposal

"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

The installed-selection check does not use a motor-feeder sizing recommendation as a minimum allowable rating. Where the existing planner does not expose a separate limit for the installed branch protective settings, the check reports Needs review and identifies that unchecked scope.

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