Breaker size for dwelling unit vs. conductor size

Please help me understand how it's ok to use a 100A main breaker with minimum conductor ampacity of 91A and 1 AWG AL rated 100A. This is in Question 37 of Unit 9 (Dwelling Unit Calculations) ElectricalExam Preparation book. I have always though the breaker should be the weakest link? A little confused; please help.
 

Carultch

Senior Member
Location
Massachusetts
Please help me understand how it's ok to use a 100A main breaker with minimum conductor ampacity of 91A and 1 AWG AL rated 100A. This is in Question 37 of Unit 9 (Dwelling Unit Calculations) ElectricalExam Preparation book. I have always though the breaker should be the weakest link? A little confused; please help.
I don't understand the physical basis for it either, but it is what we are allowed to do in practice, and it is what governs the correct answer on exams. It seems to me that if load diversity is the justification for it, then the load calculation algorithm should capture that factor, and make the service OCPD accurately reflect what is realistic for the loads. Another factor I've heard is that 310.16 allowable ampacities are conservative, but this doesn't justify the double standard for why it only applies to a whole dwelling unit circuit, as opposed to feeders in general. I agree with you, that an OCPD should be the weak link, based on the theory for why we use an OCPD in the first place. I get that you would want to understand the physical basis for NEC rules to help see the big picture, but it is not a requirement to pass the exam or to pass your inspection in practice.

In any case, it is 310.15(B)(12) formerly 310.15(B)(7), that allows entire dwelling unit conductors to either be sized at 83% of the service OCPD, or per the corresponding table if no derate factors apply, provided that it is between 100A and 400A.
 

winnie

Senior Member
Location
Springfield, MA, USA
Occupation
Electric motor research
IMHO the '83% rule' is a response to having conservative assumption piled on top of conservative assumption, but it doesn't itself have a good direct physics basis.

Article 220 dwelling calculations are known to be very conservative. Just look at the conductor and transformer sizes used by power companies.

Conductor ampacity tables are known to be conservative.

Most houses with 200A service (nominal 48kW capacity) operate at an average power level of 1-2kW.

On the other hand, breakers have a certain tolerance. A 100A breaker might never trip at 125A and be perfectly in spec.

In any case there are several analogous situations in the code, where the breaker is _not_ the weak link, and the conductors are considered protected by the combination of the breaker and the load characteristics. Look at the protection for motor feeders, for example, where the breaker has a trip rating much greater than the conductor ampacity and provides short circuit protection, and the motor overload device also provides overload protection for the wires feeding the motor. Or look at the 'next size up' rule, where you can use a breaker that is larger than the conductor ampacity as long as the calculated load is less than the conductor ampacity.

-Jon
 

kwired

Electron manager
Location
NE Nebraska
Please help me understand how it's ok to use a 100A main breaker with minimum conductor ampacity of 91A and 1 AWG AL rated 100A. This is in Question 37 of Unit 9 (Dwelling Unit Calculations) ElectricalExam Preparation book. I have always though the breaker should be the weakest link? A little confused; please help.
Don't quite understand your question as worded. Disregarding the dwelling 83% rules for the moment, 91 amps is still under the ampacity of 1 AWG AL, which is 100, and a 100 amp conductor can be protected at 100 amps.

Or is your conductor in question in need of ampacity adjustments and is now a 91 amp conductor? If so it can still be protected at next standard size up which is 100 amps, but the calculated load on it can not exceed the adjusted ampacity of 91.
 
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