83%

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pedro1200

Senior Member
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Electrician
In the 2020 handbook p.267 it discusses the 83% rule. It says that the 83% is based on the disconnect rating and not based on the rating of the main feeder ocpd rating. So the ocpd isnt a factor?
 
I don't have the Handbook and haven't read it, but I would guess what you're reading is an attempt to clarify the 'Service or Feeder Rating'. This section and it's predecessors have always relied on this ambiguous and possibly circular notion of the service rating, which is undefined and unclarified. In my opinion claiming that it's the rating of the disconnect has no more support in the actual code language than using any other component of the service or feeder. I have usually taken the reasonable position that the rating of a service or feeder is it's lowest rated component, or 'weakest link'. But that's just the most reasonable interpretation IMO.

For my full analysis of everything that's wrong with 310.12, see here:

What Exactly Is The "Service Rating"?

Service to a single family dwelling with a 3/0 AL service entrance that hits a wireway then feeds two 125A disconnects with 125A fuses? The disconnects each feed a 125A MLO subpanel What is the rating of that service? 75C ampacity of 3/0 Al = 155A, so the rating is 155A / 0.83 = 186A. Please...
forums.mikeholt.com

One thing I think I missed there is that the changes since 2014 would now allow the table to be applied to 60C rated conductors such as NM cable. 🙄
 
jaggedben said:
One thing I think I missed there is that the changes since 2014 would now allow the table to be applied to 60C rated conductors such as NM cable. 🙄
Click to expand...
That part of 310.12(A) got fixed in the 2026 NEC, it now specifies that the table applies to conductors rated at least 75C when ampacity adjustment and correction are not required.

Cheers, Wayne
 
Feeders are affected by section 225.39. This clearly states the disco

225.39 Rating of Disconnect.

The feeder or branch-circuit disconnecting means shall have a rating of not less than the calculated load to be supplied, determined in accordance with Parts I and II of Article 220 for branch circuits, Part III or IV of Article 220 for feeders, or Part V of Article 220 for farm loads. Where the branch circuit or feeder disconnecting means consists of more than one switch or circuit breaker, as permitted by 225.33, combining the ratings of all the switches or circuit breakers for determining the rating of the disconnecting means shall be permitted. In no case shall the rating be lower than specified in 225.39(A), (B), (C), or (D).

(C) One-Family Dwelling.
For a one-family dwelling, the feeder disconnecting means shall have a rating of not less than 100 amperes, 3-wire.
Click to expand...

What is the real life issue?
 
I guess someone could install a 100 amp main disconnect panel with no other spaces for breaker and feed an interior panel with 60 amps as long as the disconnect is 100 amps--disconnect NOT overcurrent protective device.

I tried to change this rule as it makes no sense but the code making panel said the wording is as they want it.....duh
 
Dennis Alwon said:
Feeders are affected by section 225.39. ...
Click to expand...

Only outside feeders.

Dennis Alwon said:
I guess someone could install a 100 amp main disconnect panel with no other spaces for breaker and feed an interior panel with 60 amps as long as the disconnect is 100 amps--disconnect NOT overcurrent protective device.

I tried to change this rule as it makes no sense but the code making panel said the wording is as they want it.....duh
Click to expand...

I've always felt that with a fused disconnect the languange allows lower rated fuses without restriction as long as the disconnect rating complies.
 
What we call this 83% rule started out in the 1956 NEC that was the first edition of the code to provide a 'up in ampacity adjustment' for single phase service and ‘sub service’ or feeder conductors like 310.12 today. It was a note in the 3 conductors in a raceway or cable ampacity table for a long time.
My take is it was based on testing and there being greatly reduced heating from only 2 cc's in a raceway or cable,
it probably could be expanded beyond dwellings, I think it was focused on dwellings because of the housing boom at the time.
I dont think any more research has gone into 2-cc's in a raceway or cable since the 1950's.
At some point circa 2008 they mistakenly added 208V 3-wire services, which technically is not two cc's.
Adding a 2 cc table might be more relevant than ever but it would take research that would likely yield lower sales and who wants to fund that?
 
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