You are absolutely correct in your understanding of the NEC and your approach to feeder sizing. Your colleagues are misinterpreting the "80% breaker standard."Here's a breakdown to clarify: Your Calculation

You correctly calculated the required feeder current as 284.5 amps after applying the appropriate demand factors for non-continuous loads and the 125% factor for continuous loads. This is the crucial step outlined in 215.2 and 210.19(A)(1) Exception No. 1.

The Misunderstanding of the "80% Breaker Standard"

The "80% breaker standard" refers to the listing requirements for circuit breakers and specifically addresses the maximum continuous load allowed on the breaker, per 210.20(A) and 215.
3. It's not about arbitrarily increasing the feeder size beyond what is required by the calculated load. It's about selecting a breaker that can handle the already calculated continuous load. The breaker must be rated not less than 125% of the continuous load. In your example, the 125% of the continuous load is already taken into account within your feeder calculation.

215.2(A)(1) & 210.19(A)(1) Exception No. 1

Explicitly states to increase the continuous load by 125%. You've already done that.

Breaker Selection

To elaborate, this means if you have a 20A circuit breaker in a panel, and if you plan to run it at a continuous load (anything running for 3 hours or more), you cannot load it beyond 80% of its capacity (16A).

Minimum Feeder Size

Since your calculation already accounts for the 125% factor for continuous loads, the minimum feeder size is indeed the next standard size up from the calculated demand of 284.5A, which is 300A. Note that 310.16 has to be followed for ampacity ratings.

Good Design Practice vs. Code Minimum

You're right to distinguish between meeting the minimum code requirements and good design practice. While the minimum feeder size is 300A, as calculated, adding a 20-25% buffer for future expansion is a prudent engineering decision, but it's not strictly required by the code.

In Summary

Your colleagues are incorrectly applying the 125% factor to the entire calculated load instead of only applying it to the continuous loads during the load calculation process. You are correct. You've already accounted for the continuous loads in your initial calculations.

How to Explain to Your Colleagues


Refer to 215.2(A)(1) and 210.19(A)(1) Exception No. 1

Point out that the code explicitly instructs increasing only the continuous load by 125% during the initial load calculation.2.

Explain the Purpose of the 80% Rule

Explain that the "80% rule" (or the 125% factor in code language) is about the breaker's ability to handle continuous loads and that this has already been factored into the load calculations.3.

Walk Through an Example

Use your example and clearly show how you applied the 125% factor to the continuous load only.4.

Clarify Breaker Selection vs. Feeder Sizing: Emphasize that the minimum required ampacity of the conductors is determined by the load calculations (including the 125% factor for continuous loads), while the breaker selection must also consider the continuous load requirements.By clearly explaining these points and referring to the relevant sections of the NEC, you should be able to clear up the confusion and ensure everyone is following the correct procedures for feeder sizing. Flag for review