Estimates only. These results may be wrong for your specific installation, and they are not engineering or code-compliance advice. Check every result against the code that applies to you and with your local inspector (AHJ) before you install anything. Terms of Service
Branch-circuit protection for motors and breakers for general loads. Results update as you type. Motor circuit builder
Motor
How this is worked out
- Maximums: inverse-time breaker 250% of full-load current, dual-element time-delay fuse 175%. Exception 1 allows the next standard size up.
- Recommended breaker: the standard ordinary-service selection for across-the-line starting, which allows for inrush. It's never shown above the code maximum.
- If the motor won't start on these sizes, the code allows more: up to 400% of full-load current when it's 100 A or less, 300% when it's over 100 A, and 225% for time-delay fuses.
- Disconnect: at least 115% of full-load current.
Load
Breaker
How the motor breaker calculator works
Pick a motor by horsepower, phase and voltage and the calculator shows its full-load current from a standard table, the maximum breaker and fuse sizes, and the minimum disconnect rating. A second tab sizes a breaker for a general load from its current or power.
Steps
- Choose Motor or General load.
- For a motor, choose phase, voltage and horsepower. The full-load current comes from the standard table by default. Switch to Enter it to use a nameplate value.
- Read the maximums: inverse-time breaker at 250% of full-load current, dual-element time-delay fuse at 175%, and a disconnect of at least 115%.
- If the motor will not start on the recommended size, the page explains the larger sizes the code allows in that case.
- For a general load, enter current or power, voltage and phase, and say whether the load is continuous.
Example
A motor with a full-load current of 14 A has a maximum inverse-time breaker of 14 × 250% = 35 A before rounding to a standard size, a time-delay fuse maximum of 14 × 175% = 24.5 A before rounding, and a disconnect of at least 14 × 115% = 16.1 A.
Questions
- Why does it use the table current instead of the nameplate?
- Branch-circuit protection for motors is normally based on the full-load current in the standard tables, not the nameplate. The nameplate value is used for overload protection. Tell the tool to use your own number when you need to.
- What is the recommended size versus the maximum?
- The maximum is the most the rule allows. The recommended size is the usual selection for across-the-line starting, and the tool never shows one above the maximum.
- What about 208 V motors?
- A 208 V motor is looked up in the breaker maker's 200 V row for its horsepower, because that table is written for 200 V motors used on 208 V systems.
- Does it size the overload relay?
- No. This tool covers branch-circuit protection and the disconnect. Overload settings come from the nameplate and the manufacturer's instructions.
- Can I trust the result for a VFD or soft starter?
- Those circuits often follow the drive or starter manufacturer's rating tables. Use the Motor circuit builder for a starting point and the equipment instructions for the final answer.
The code references on this page are pointers, not a substitute for the code. NEC® and National Electrical Code® are registered trademarks of the National Fire Protection Association (NFPA). WireAndWidgets is not affiliated with or endorsed by NFPA. Rules differ by edition and by local amendment, so check the code your inspector (AHJ) enforces before you build or install anything.