Motor circuits use different steps for wire size, overcurrent protection, and the equipment grounding conductor.
A quick example makes it easier to see how the process works.
Quick Motor Circuit Example
Example: a 5 HP, 230V, single-phase motor using copper conductors, 75°C-rated terminals, and a time-delay fuse.
First, get the motor full-load current from NEC Table 430.248.
5 HP, 230V, single-phase motor = 28 amps
Step 1: Calculate the Minimum Wire Ampacity
For a single motor, use NEC 430.22 and size the branch-circuit conductors at 125% of the motor full-load current.
28 amps × 125% = 35 amps
The conductor needs an ampacity of at least 35 amps.
Using the 75°C column, #10 copper has a 35-amp ampacity, so #10 copper works for this example.
Step 2: Calculate the OCPD
For a time-delay fuse, use NEC Table 430.52.
28 amps × 175% = 49 amps
The next standard fuse size is 50 amps.
For this example:
OCPD = 50-amp time-delay fuse
Step 3: Size the EGC
Size the equipment grounding conductor from NEC Table 250.122 using the rating of the branch-circuit OCPD.
For a 50-amp OCPD:
EGC = #10 copper
Final Results for This Example
- Motor full-load current: 28 amps
- Minimum conductor ampacity: 35 amps
- Wire size: #10 AWG copper
- OCPD: 50A time-delay fuse
- EGC: #10 AWG copper
How Sparky Toolbox Helps
Instead of looking up the motor current, calculating the conductor ampacity, checking the OCPD, and then finding the EGC size separately, enter the motor horsepower, voltage, phase, and protection type into Sparky Toolbox.
The Motor Calculator shows:
- Recommended wire size
- Fuse size
- Breaker size
- EGC size
That gives you a quick starting point when planning a motor circuit or building a material list.
Important Reminder
This is one simple example. Motor circuits can change based on terminal ratings, conductor type, motor duty, starting conditions, disconnects, overload protection, and other job details.
Always verify the current Code, equipment nameplate, manufacturer instructions, project requirements, approved plans, and local inspection requirements before installation.