PC: 120.82(A) Dwelling Unit Optional Load Calc

wwhitney

Senior Member
Location
Berkeley, CA
Occupation
Retired
Here's what I came up with to eliminate any ambiguity as to the question "for the service to a duplex, can I calculate the load just by adding together the optional load calcs for each dwelling unit?"

Do you think the single phrase added is sufficiently clear? I could try something even more direct, like adding a whole sentence stating "A feeder or service supplying multiple such dwelling units shall be permitted to have its load calculated as the sum of the individual loads of those dwelling units."

Or I could further change the first sentence to say "This section applies to any number of dwelling units, each having theits total connected load served by a single 120/240-volt or 208Y/120-volt set of 3-wire service or feeder conductors with an ampacity of 100 or greater"

Cheers, Wayne


120.82(A) Feeder and Service Load.

This section applies to a dwelling unit having the total connected load served by a single 120/240-volt or 208Y/120-volt set of 3-wire service or feeder conductors with an ampacity of 100 or greater. It shall be permissible to calculate the feeder and service loads in accordance with this section instead of the method specified in Article 120, Part III. The calculated load shall be the result of adding the loads from 120.82(B), 120.82(C), and 120.82(D) for each dwelling unit individually. Feeder and service-entrance conductors whose calculated load is determined by this optional calculation shall be permitted to have the neutral load determined by 120.61.

Statement of Problem and Substantiation for Public Comment

Consider a duplex with a single service that supplies two sets of service entrance conductors to two service disconnects, one for each dwelling unit. The service entrance conductors supplying just one dwelling unit can have their load computed using 120.82.

There is, however, some uncertainty as to whether the service conductors supplying both dwelling units may have their load calculated as the sum of those two 120.82 calculations. The current wording of 120.82 leads some AHJs to the interpretation that 120.82 only applies to conductors carrying the load of exactly one dwelling unit, and that the results of that computation can not be used for determining the load on any upstream conductors that supply more than one dwelling unit. This point of view is supported by the observation that 120.84 is available for conductors supplying 3 or more dwelling units, and that 120.85 is available for conductors supplying exactly 2 dwelling units.

However, it makes no sense to limit the calculation results from 120.82 to conductors that supply exactly one dwelling unit. Where a set of conductors supplies multiple other sets of conductors, it is is physically impossible for the load on those conductors to exceed the sum of the loads on the multiple other conductors supplied. Summing those individual loads is always a conservative overestimate of the load on the common supply conductors.

Therefore this change clarifies that conductors that supply multiple dwelling units may have their load determined by calculating the 120.82 load of each dwelling unit separately, and then adding those results.
 

ramsy

Roger Ruhle dba NoFixNoPay
Location
LA basin, CA
Occupation
Service Electrician 2020 NEC
In the first draft report for the 2026 NEC, the CMPs renumbered Article 220 to be Article 120.

Cheers, Wayne
Building officials typically wont acknowledge code before adopted in their jurisdictions, and need time to adjust to changes once adopted.

Describing Article 220 as 120 may be losing your AHJ audience on the forum, if averse to suffering NEC changes before needed.
 

tortuga

Code Historian
Location
Oregon
Occupation
Electrical Design
Building officials typically wont acknowledge code before adopted in their jurisdictions...
Well then they are in the wrong sub forum as were discussing the 2026 NEC here in "Proposals/Comments for the next NEC cycle".
:)
Regarding the proposal I wonder if there are any past load studies that would shed some light on this topic?

It would be interesting to see a plot comparing all the optional calc options with 1 - 10 units using alternating size condo units say 600 SQFT with 1ton hvac 10.6 FLA, 1200 SQFT 1.5 ton hvac 13.9 FLA, where all the units have; 12kw electric cooking , 1kw dishwasher and full laundry (5.3kw dryers), 1.5kw electric resistance heater.
 

don_resqcapt19

Moderator
Staff member
Location
Illinois
Occupation
retired electrician
Building officials typically wont acknowledge code before adopted in their jurisdictions, and need time to adjust to changes once adopted.

Describing Article 220 as 120 may be losing your AHJ audience on the forum, if averse to suffering NEC changes before needed.
You have to make the the Public Comment against the sections that appear in the First Draft Report.
 

ramsy

Roger Ruhle dba NoFixNoPay
Location
LA basin, CA
Occupation
Service Electrician 2020 NEC
You have to make the Public Comment against the sections that appear in the First Draft Repot.
Thanks Don,

Did not check which forum Wayne's question took me to.

Like most people on the forum who ignore these P.I. questions, I did not intend to give NFPA a piece of my mind without charge.

Just read Bob's PI guide from 2008.

Will need to check how much free advice NFPA got from me, since 2008, and send them my bill.
 
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