Mike Dolt Enterprises
Mike Dolt Enterprises
User: mburtis, Location: Wyoming
Asked on: Saturday, May 29, 2021 at 11:38 pm EDT
Answered on: Thursday, June 20, 2024 at 11:14 am EDT
Question: First post here, I work on the industrial maintenance side as a plant electrician for a water treatment plant. Some of you may know me and this post from electriciantalk. 310.12 is tripping me up a little. I get that if a service/feeder is supplying the entire residential load then the wire can be downsized. Part (c) is tripping me up. The way I read it no feeder in a single dwelling needs to be bigger than 83 percent of the service rating. For example if you have a 200 amp service fed with 2/0 copper there is no need to use larger than 2/0 in any other feeders. That doesn't seem to be the consensus with more knowledgeable poeple. Can I gather some opinions on this.
Interpretation of NEC 310.12(C)
NEC 310.12(C) states that "Feeder conductors for one-family dwellings shall have an ampacity not less than 83 percent of the service entrance conductor rating."Common Interpretation
The common interpretation of this rule is that all feeders in a single-family dwelling should be sized at least 83% of the service ampacity. This is based on the understanding that the maximum load on the service at any given time is limited to 83% of its rated ampacity.
Alternative Interpretation
Your interpretation is that the rule only applies to feeders that supply the entire residential load. However, this interpretation is not supported by the actual wording of the rule, which does not specify that the feeder must supply the entire load.
Reasoning for Common Interpretation
The reasoning behind the common interpretation is to ensure that all feeders can safely handle their portion of the maximum load on the service. If some feeders were sized smaller than 83% of the service ampacity, they could potentially overload and become a fire hazard.
Practical Application
In practice, most electricians size all feeders in single-family dwellings at least 83% of the service ampacity. This ensures compliance with the NEC and provides a margin of safety.
Conclusion
Based on the wording of NEC 310.12(C) and the common interpretation of the rule, it is generally recommended to size all feeders in single-family dwellings at least 83% of the service ampacity. Flag for review