OCPD Sizing: Fuse & Breaker

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About this app

Size overcurrent protective devices with confidence, based on NEC (NFPA 70) and CEC (CSA C22.1) requirements.
OCPD Sizing is a calculator for electricians, designers and engineers who need the right fuse or breaker rating and a clear reason for it. Describe the load, and get a standard device rating with the calculated value, the interrupting-rating check and the conductor-protection check behind it.
What it handles:
- General, motor, transformer, listed-load, capacitor, PV, energy-storage (ESS) and DC-DC converter loads
- Motor loads: polyphase, single-phase and wound-rotor/DC types, by horsepower or full-load current, with starting considerations
- Transformer primary and primary+secondary protection schemes
- PV source and output circuits, including string count and short-circuit current
- Fuse and breaker sized side by side, or either one alone, with time-delay fuse preference and breaker trip type (thermal-magnetic, electronic, instantaneous-only)
- Available fault current (AIC) entry with an interrupting-rating check against the selected device
- Conductor protection check against a chosen size, material and termination rating, including CEC PV-specific requirements
- Selective coordination notes for upstream/downstream device pairs
- Single-phase, three-phase and DC systems across standard NEC and CEC voltages, including medium voltage
- A full calculation trace with the applicable NEC/CEC references, available to review
Built for a phone: a short step-by-step flow, and a two-pane layout with inputs and results side by side on a tablet or in split-screen. The result leads with the device rating, then short rows for voltage rating, interrupting rating and conductor protection, with device details and full engineering notes available to expand.
Calculations run on your device. An internet connection is used occasionally to confirm your trial or purchase status, not to perform the calculation itself.
Access: every feature is unlocked for a 7-day free trial. After that, the app stays fully functional for up to 5 complete calculations per month at no cost. A one-time in-app purchase removes that monthly limit permanently — no subscription.
OCPD Sizing is not affiliated with or endorsed by NFPA or CSA Group. Results are an engineering aid intended to support, not replace, the professional judgement of the engineer or designer of record, and do not account for every local amendment or site condition. Always verify against the adopted code edition for your jurisdiction. No warranty of any kind is expressed or implied.

What’s new

- Tapping a number field selects its value, so you can type over the default.