Mike Dolt Enterprises
Mike Dolt Enterprises
User: infinity, Location: New Jersey
Asked on: Monday, December 23, 2024 at 11:28 pm EST
Answered on: Thursday, March 6, 2025 at 6:37 am EST
Question: Someone asked me today about the branch circuit for a 10kw, 240 volt, thermostatically controlled electric garage heater. I immediately said 60 amp circuit. Then when I looked in the codebook I thought well maybe not so fast. 424.4(B) says the branch circuit conductors shall be sized at 125% of the rating of the heater load. What about the OCPD? Can a 50 amp OCPD be used with #6 conductors? 424.4(A) seems to say that the OCPD can be 50 amps for this load. Obviously if you've already run 65 amp conductors it would be easy to simply use a 60 amp OCPD but is it required if the load is non-continuous? Also for this installation no NM cable will be used.424.4(B) Branch-Circuit Sizing.The branch-circuit conductors for fixed electric space-heating equipment and any associated motors shall be sized not smaller than 125 percent of the load.424.4 Branch Circuits.424.4(A) Branch-Circuit Requirements.An individual branch circuit shall be permitted to supply any volt-ampere or wattage rating of fixed electric space-heating equipment for which the branch circuit is rated.
You're right to pause and analyze this situation carefully. Your initial instinct of 60 amps is a good starting point, but let's break down the code requirements to ensure compliance
Calculations:
Heater Current
10,000 Watts / 240 Volts = 41.67 Amps
125% of Heater Current (Conductor Sizing)
41.67 Amps 1.25 = 52.09 Amps
Conductor Sizing
Based on 424.4(B), you need conductors rated for at least 52.09 amps. #6 AWG copper conductors, THHN/THWN for example, have an ampacity of 55 amps in the 75°C column according to Table 310.16. This is sufficient.
Overcurrent Protection (OCPD)
This is where it gets a bit nuanced. Let's analyze the code sections and their interaction:
424.4(A)
"An individual branch circuit shall be permitted to supply any volt-ampere or wattage rating of fixed electric space-heating equipment for which the branch circuit is rated." This allows you to use a branch circuit rated for the equipment's load. However, it doesn't override the other requirements.
240.4(B) - Next Higher Overcurrent Protective Device Rating
When conductors have an ampacity that does not correspond with a standard size OCPD you are permitted to use the next standard size OCPD.
Analysis
Conductor Ampacity
Your #6 conductors are rated for 55 amps, exceeding the 52.09 amp requirement derived from 424.4(B).
2.
OCPD Selection
50 amp OCPD does not violate the NEC since the conductors have been sized to 125% of the load and the load is 41.67 amps.
Therefore, a 50 amp OCPD is permitted, given the calculations and assumptions made above.Important Considerations
75°C Terminations
Make sure the heater and the breaker are rated for 75°C terminations. If not, you must use the 60°C ampacity of the wire, which is lower.
Motor Load
You mentioned "any associated motors." If the heater has a significant motor load (e.g., a fan), you must factor that into the calculations as per Article 430. This might necessitate a larger OCPD.
Voltage Drop
While the ampacity is code-compliant, calculate the voltage drop, especially if the run is long. Excessive voltage drop can damage the heater and cause inefficient operation.
Disclaimer: I am an AI chatbot and cannot provide electrical advice. This analysis is based on my understanding of the NEC, but local codes and interpretations may vary. Always consult with a licensed electrician for code-compliant electrical installations.* They can assess the specific situation and ensure the installation meets all applicable requirements. Flag for review