Mike Dolt Enterprises
Mike Dolt Enterprises
User: DST Engineer, Location: Long Island, NY
Asked on: Wednesday, June 25, 2025 at 3:10 pm EDT
Answered on: Wednesday, June 25, 2025 at 3:13 pm EDT
Question: I have a Make Up Air Unit that requires two feeds. (1) for the heating element for the space and (1) for the exhaust fan. The heating element is 48.1amps. This is a continuous load per Article 224 which gets us 60.125 amps (48.1*1.25).Can I use Article 220.5(B) to reduce the continuous load to 60 amps (round to the nearest whole amp) to size my breaker and conductors? Or do I have to size conductors and circuit breakers to ampacity of the unit and the continuous load (60.125A). I just used the example above as context, but curious what the community thinks.P.S. big thanks to Dennis Alwon, charlie b, and everyone in this forum. You've helped me so much in my career.
Okay, let's break this down. You're asking if you can use the rounding rules in 220.5(B) to round down after applying the 125% factor for a continuous load.
The short answer is: NO. You CANNOT round down in this scenario to reduce the calculated ampacity for sizing conductors and overcurrent protection.
Here's why, referencing the NEC: 210.19(A)(1): Conductors - Minimum Ampacity and Size
"Branch-circuit conductors shall have an ampacity not less than the maximum current to be served." This reinforces the idea that you need to account for the entire load.
210.20(A): Overcurrent Protection - Continuous Loads
"Where a branch circuit supplies continuous loads... the overcurrent device shall have an ampacity not less than the noncontinuous load plus 125 percent of the continuous load." This is similar to 224 but applied to the branch circuit.
220.5(B): Rounding
This section is for calculating load in general and permits rounding up to the next higher whole amp. It does NOT permit rounding down to reduce the load below the calculated value, especially after applying the 125% factor for continuous loads.
The 125% factor is not negotiable
The NEC requires that the overcurrent protection be sized at least 125% of the continuous load to protect the circuit from overheating. Rounding down after applying the 125% factor defeats the purpose of that requirement.
In your example
Heating element: 48.1 amps (continuous) Continuous load calculation: 48.1 amps 1.25 = 60.125 ampsYou cannot* round 60.125 amps down to 60 amps for conductor and overcurrent device sizing.
What you MUST do
Conductor Ampacity
The conductors feeding the heating element MUST have an ampacity of at least 60.125 amps. You'll need to select conductors based on the appropriate temperature rating and installation methods.
2.
Overcurrent Protection
The overcurrent device (breaker or fuses) protecting the heating element circuit MUST have a rating of at least 60.125 amps. The next standard size breaker available above 60.125 amps would be 65A.
In summary, you CANNOT use the rounding rule to circumvent the 125% requirement for continuous loads. Doing so would be a violation of the NEC and could create a safety hazard. Flag for review