Mike Dolt Enterprises
Mike Dolt Enterprises
User: JovialBulge, Location: New England
Asked on: Friday, June 20, 2025 at 10:20 pm EDT
Answered on: Friday, June 20, 2025 at 10:21 pm EDT
Question: The unit as a whole is going to be portable so it will be plugged into a wall outlet with a NEMA 5-20 plug, the wire size coming off of that receptacle and feeding the VFD branch and the PSU branch I would use 12 AWG since the plug is 20A.VFD maximum input current is 19.2A @ 120VAC, so the wire size for the line should be 12AWG. Typically I would have a 25A circuit breaker or Class CC fuse protecting the input side of that size VFD. Should I bring that down to a 20A breaker of fuse at that point since the NEMA 5-20 plug will only support that amperage anyway?
Yes, you are absolutely right to consider downgrading the overcurrent protection to 20A to match the NEMA 5-20 plug. Here's the breakdown of why and best practices
Reasoning:
Weakest Link
The NEMA 5-20 plug is the weakest link in your system. It's designed to handle a maximum of 20 amps. If a fault occurs and the VFD attempts to draw more than 20 amps, the plug is likely to be the first component to overheat and potentially fail (melt, arc, cause a fire).
Code Compliance (likely)
Electrical codes (like the NEC in the US) generally require that the overcurrent protection device (breaker or fuse) be sized to protect the smallest component in the circuit. Since your plug is the smallest rated component, the overcurrent protection should be no larger than its rating.
Safety
A 25A breaker won't trip until the current exceeds 25A. By that point, the 5-20 plug could already be damaged or creating a fire hazard. A 20A breaker will trip before the plug is overloaded.
Recommendation
Use a
20A circuit breaker or 20A Class CC fuse to protect the input of the VFD. This protects the entire circuit, including the NEMA 5-20 plug, from overcurrent conditions.
Explanation of why you were planning for a 25A fuse previously.
The original reasoning for a 25A breaker or fuse likely stems from the VFD's internal protection requirements or manufacturer's recommendations for its maximum potential draw under fault conditions. If you were wiring the VFD directly to a dedicated circuit without the limitation of the 5-20 plug, a 25A device would have been correct.
Best Practices and Considerations
Check VFD Manual
Always consult the VFD's manual. It might have specific recommendations for overcurrent protection based on its particular design. If the manual allows for a 20A overcurrent protection device, you are good to go. It might even require* a 20A device when using a 5-20 plug.2.
Wiring
Confirm that all wiring downstream of the breaker is rated to carry at least 20 amps continuously (12 AWG wire is good for 20A in typical applications, but confirm based on wire type and installation conditions).3.
Labeling
Clearly label the unit near the plug indicating the maximum current draw and voltage (e.g., "120VAC, 20A Max"). This prevents someone from plugging it into an inappropriate outlet (like a 15A outlet).4.
Testing
After assembling the unit, thoroughly test it under realistic load conditions to ensure the VFD operates correctly and the 20A breaker doesn't nuisance-trip.
In summary: Use a 20A circuit breaker or fuse to protect the VFD's input, given the NEMA 5-20 plug limitation. This prioritizes safety and code compliance by protecting the weakest link in the system. Flag for review