differing ampacity of mhf
- Thread starter Srv52761
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- Location
- New Jersey
- Occupation
- Journeyman Electrician
Yes in most cases it's true. Look at 310.15(B)(7) for when you can use the "83%" rule.
kwired
Electron manager
- Location
- NE Nebraska
- Occupation
- EC
As infinity mentioned it depends if you can use (B)(7) for your application. Isn't service vs feeder so much as it depends if it is a conductor supplying the entire load of a dwelling, or if a subfeeder within the dwelling it can apply if the main service/feeder is same size.
For general use cases the ampacity of #2 aluminum is 90 amps, (B)(7) is a conditional allowance.
winnie
Senior Member
- Location
- Springfield, MA, USA
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- Electric motor research
Correct. A garage is not a residence.
A way to make sense of this: The cable is always rated at 90A 'ampacity'. This means that the cable can safely carry 90A on a continuous basis without overheating.
A 100A residential service, with a calculated load of 100A or less, protected by a 100A breaker, is permitted to use conductors of 83A or greater ampacity.
The 90A cable doesn't magically become a 100A cable, rather a 100A service is unlikely to see 100A of loading for an extended period of time.
-Jon
A way to make sense of this: The cable is always rated at 90A 'ampacity'. This means that the cable can safely carry 90A on a continuous basis without overheating.
A 100A residential service, with a calculated load of 100A or less, protected by a 100A breaker, is permitted to use conductors of 83A or greater ampacity.
The 90A cable doesn't magically become a 100A cable, rather a 100A service is unlikely to see 100A of loading for an extended period of time.
-Jon
- Location
- Illinois
- Occupation
- retired electrician
No, the ampacity of the conductor does not change...you are just permitted to use a conductor at a rating greater than its ampacity for dwelling unit service conductors.
The following code language in 310.12(A) in the 2020 code makes it clear that the ampacity of the conductors is not changing...just the permitted use is changing.
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