Mike Dolt Enterprises
Mike Dolt Enterprises
User: Larry Bohn, Location: ashland, oregon
Asked on: Wednesday, February 12, 2025 at 7:17 pm EST
Answered on: Thursday, March 6, 2025 at 8:34 am EST
Question: Given a 45 amp continuous load, 240V, single phase, (two current carrying conductors). This circuit will be pulled through a rigid metal conduit which already has eight current carrying conductors. THHN will be used for wire. Total of 10 current carrying conductors will be in the conduit. Here's my calculation.210.19(A)1, 210.20(A), and 240.4(B) -- 45 X 1.25 =56.25 amps , choose 60 amp breaker and #4 THHN to comply with 110.14(C) for the breaker terminations.210.19(A)2 and T.310.15(C)1 -- 45 / .5 = 90 amps, choose #4 THHN for wires when in the raceway.Now we have a 60 amp breaker with #4 THHN wires. However, 240.4(B) also says the wires shall be protected at their ampacity, with the option of going up to the next breaker size above the ampacity of the wires.#4 THHN in the conduit, after derating, has an ampacity of 47.5 amps, meaning the maximum overcurrent device cannot be any greater than 50 amps (240.4(B).Upsizing the wire to a #3 THHN we have 115A x 50% = 57.5 amps, choosing the 60 amp breaker, (which is required by 210.20), is OK.Final answer, 60 amp breaker (with 60 degree terminations), and #3 THHN. Am I correct? Nobody that I've been around ever checks, after calculating the ampacity of the wire required in the conduit, to see if the breaker size is still OK. They'll stay with the 60 amp breaker and #4 THHN.What do you think?
You are absolutely correct in your analysis and final answer. Your methodical approach and attention to detail regarding conductor ampacity, derating, and overcurrent protection are commendable. Let's break down why your reasoning is sound
Understanding the Conflict:
The issue arises from the interplay between: Load Calculation and Initial Sizing
You correctly calculated the continuous load at 45A 1.25 = 56.25A. This mandates a minimum 60A overcurrent device (breaker) according to 210.20(A).
Conductor Ampacity and Derating
You accurately identified that #4 THHN, after derating for 10 current-carrying conductors (50% per Table 310.15(C)(1)), has a reduced ampacity of 47.5A.
Overcurrent Protection Rules
While 210.20(A) requires the OCPD to be rated not less than 125% of the continuous load, 240.4(B) states that, unless specifically permitted otherwise (none here apply), overcurrent protection must protect the conductors at their ampacity. This is where the conflict arises - you need a 60A breaker to satisfy 210.20(A) but #4 THHN derated to 47.5A requires a smaller breaker per 240.4(B).
Your Solution and Justification
Your solution of upsizing to #3 THHN is the correct one. Here's why:1.
#3 THHN Ampacity
#3 THHN has an ampacity of 115A (before derating).2.
Derating
With 10 current-carrying conductors, the derating factor is 50% (0.5). This brings the ampacity of #3 THHN in your conduit down to 115A 0.5 = 57.5A.
3.
Compliance with 240.4(B)
#3 THHN's derated ampacity is 57.5A. 240.4(B) allows for the use of the next higher standard overcurrent device rating if the conductor ampacity doesn't correspond to a standard size. Since 57.5A is not a standard breaker size, you can indeed use the next higher standard overcurrent device, which is a 60A breaker.
Why Staying with #4 THHN and a 60A Breaker is Incorrect (and Potentially Dangerous)
If you kept the #4 THHN and the 60A breaker, you would be violating 240.4(B) and potentially creating an unsafe situation. The breaker would be set to allow more current than the derated conductor is rated to handle continuously. This could lead to overheating, insulation breakdown, and potentially a fire.
Important Considerations (that you addressed)
110.14(C) Terminal Temperature Rating
You correctly considered this by noting the 60-degree terminations. This is important because the initial ampacity of the wire MUST be based on the temperature rating of the terminations, which are often rated at 60 degrees.*
Documentation
It's essential to document your calculations and reasoning. This helps ensure consistency, makes future maintenance easier, and demonstrates compliance with the NEC.
In summary, your final answer is correct: Use a 60A breaker (with 60 degree terminations) and #3 THHN conductors. You have a solid understanding of the code and a commitment to safe and compliant installations. It's important to advocate for these best practices, even if others don't routinely follow them. Flag for review