Derating wire in conduit for branch circuits

JoeNorm

Senior Member
Location
WA
Hello, I understand how to use the table to derate multiple wires in a single raceway. What I am not sure of is how to do this for branch circuits where the load is variable. Should I use the breaker rating for the calc? Or, maybe I would use 80% of the breaker rating?

thanks for the help
 

infinity

Moderator
Staff member
Location
New Jersey
Occupation
Journeyman Electrician
Hello, I understand how to use the table to derate multiple wires in a single raceway. What I am not sure of is how to do this for branch circuits where the load is variable. Should I use the breaker rating for the calc? Or, maybe I would use 80% of the breaker rating?

thanks for the help
You start with the number of current carrying conductors in the raceway and then look up the adjustment % in the table. You are derating the conductor ampacity once you find that number then you use it to size the OCPD. If the circuit ampacity after derating is too small then you may need to increase the condcutor size.
 

kwired

Electron manager
Location
NE Nebraska
Yes, receptacle is the example I was getting at.
For multiple receptacles on a circuit you pretty much have to use the branch circuit rating as the base load for calculations.

For circuits supplying fixed loads you only need to base calculations on actual load.
 

Dennis Alwon

Moderator
Staff member
Location
Chapel Hill, NC
Occupation
Retired Electrical Contractor
Unless the receptacles are for specific pieces of equipment then I would not consider the receptacles as continuous, therefore if you buy into the thought that 13 receptacles per 20 amp circuit then you would load the breaker up to 100%. 80% is only required for continuous loads.

180 va/recep x 13 recep = 2340 va

20 amp circuit --- 20 amps x 120V= 2400 va
 

augie47

Moderator
Staff member
Location
Tennessee
Occupation
State Electrical Inspector (Retired)
If you are feeding receptacles, you might want to keep a Section of 240.4 in mind once you calculate your adjusted ampacity:
(B) Overcurrent Devices Rated 800 Amperes or Less.
The next higher standard overcurrent device rating (above the ampacity of the conductors being protected) shall be permitted
to be used, provided all of the following conditions are met:

(1) The conductors being protected are not part of a branch circuit supplying more than one receptacle for cord-and plug-connected portable loads.
 

texie

Senior Member
Location
Fort Collins, Colorado
Occupation
Electrician, Contractor, Inspector
As others stated, and the reason I asked if these are receptacle loads in post #3, you have to figure the load at the value of the OCPD.
 

kwired

Electron manager
Location
NE Nebraska
Unless the receptacles are for specific pieces of equipment then I would not consider the receptacles as continuous, therefore if you buy into the thought that 13 receptacles per 20 amp circuit then you would load the breaker up to 100%. 80% is only required for continuous loads.

180 va/recep x 13 recep = 2340 va

20 amp circuit --- 20 amps x 120V= 2400 va
The 80% doesn't apply to ampacity of conductors within raceway or cable, it applies to termination ampacity unless you have a 100% rated device.

Plus unless you dealing with older conductors you generally are able to use the 90C ampacity when selecting for what can be run in the raceway after any adjustments are applied.

Otherwise I have to look up where it is, (Texie mentioned it also but never gave a code section either) but you must use 20 amp as the load base for multi receptacle circuit of a circuit with 20 amp ocpd. supply a fixed 13 amp load from 20 amp circuit however and you only need to use 13 amps as the load base before figuring adjustments. (still needs to be 20 amp conductor minimum unless a 430 or 440 application or a few other articles that have different allowances here.
 

infinity

Moderator
Staff member
Location
New Jersey
Occupation
Journeyman Electrician
This thread seems to be off on a tangent. Isn't the question how do you derate for more than 3 CCC's in a raceway when the conductors feed receptacles?
 

kwired

Electron manager
Location
NE Nebraska
This thread seems to be off on a tangent. Isn't the question how do you derate for more than 3 CCC's in a raceway when the conductors feed receptacles?
I think the bulk of the posts are on the right tangent. You adjust ampacity of a particular conductor based on the applicable ampacity table column, but the load assigned to the conductor can vary, and when it is multiple receptacle outlets with no specifically known load you have to assume it can draw up to what the OCPD is.
 

DrSparks

The Everlasting Know-it-all!
Location
Madison, WI, USA
Occupation
Master Electrician and General Contractor
Derating for more than 3 CCC has to do with conductor allowable ampacity. It has nothing to do with OCPD size. The OCPD you ultimately select is a result of the calculation, not the cause.

Sent from my BE2028 using Tapatalk
 

JoeNorm

Senior Member
Location
WA
I think the bulk of the posts are on the right tangent. You adjust ampacity of a particular conductor based on the applicable ampacity table column, but the load assigned to the conductor can vary, and when it is multiple receptacle outlets with no specifically known load you have to assume it can draw up to what the OCPD is.
This is the answer I was looking for with the OP. Thank you.
 

winnie

Senior Member
Location
Springfield, MA, USA
Occupation
Electric motor research
A separate tangent is 'how many current carrying conductors are counted when one circuit feeds multiple loads?'

Say 1 circuit feeds multiple loads via switches, so that you have say 6 switched hots, 1 unswitched hot, and 1 neutral in the same pipe from 1 circuit. I'd claim that this should count as 2 CCCs.

Jon
 

retirede

Senior Member
Location
Illinois
A separate tangent is 'how many current carrying conductors are counted when one circuit feeds multiple loads?'

Say 1 circuit feeds multiple loads via switches, so that you have say 6 switched hots, 1 unswitched hot, and 1 neutral in the same pipe from 1 circuit. I'd claim that this should count as 2 CCCs.

Jon

Could be three depending on which end the switches are on.
 

infinity

Moderator
Staff member
Location
New Jersey
Occupation
Journeyman Electrician
A separate tangent is 'how many current carrying conductors are counted when one circuit feeds multiple loads?'

Say 1 circuit feeds multiple loads via switches, so that you have say 6 switched hots, 1 unswitched hot, and 1 neutral in the same pipe from 1 circuit. I'd claim that this should count as 2 CCCs.

Jon
All in one raceway it counts as 8 CCC's. The fact that they're on one circuit is irrelevant. Something I've been complaining about for decades.
 

Dennis Alwon

Moderator
Staff member
Location
Chapel Hill, NC
Occupation
Retired Electrical Contractor
All in one raceway it counts as 8 CCC's. The fact that they're on one circuit is irrelevant. Something I've been complaining about for decades.

Wouldn't the note cover it

1Number of conductors is the total number of conductors in the
raceway or cable, including spare conductors. The count shall be
adjusted in accordance with 310.15(B)(5) and (6). The count shall not
include conductors that are connected to electrical components that
cannot be simultaneously energized
 

infinity

Moderator
Staff member
Location
New Jersey
Occupation
Journeyman Electrician
Wouldn't the note cover it
Yes that would apply but think about 8 CCC's in one raceway on a 15 amp circuit. They're all energized at the same time so you have no more than 15 amps spread out through 8 conductors how is that going to generate more heat than if there were 2 CCC's? Yes they take up more air space within the raceway but the fact that they require derating is silly.
 

wwhitney

Senior Member
Location
Berkeley, CA
Occupation
Retired
Wouldn't the note cover it
I made a PI for 2023 that would add the following sentence to the end of the note you quoted: "Where two or more conductors are protected by the same overcurrent device, and their currents are additive at the overcurrent device, they may be counted as a single conductor." Which seem logical to me, 20A spread over 5 conductors generates less heat than 20A on 1 conductor.

The panel response was "Loading and overcurrent protection could be changed in the future based on installed conductor sizes." To which my PC basically says "sure it could, but the possibility of future changes applies to any rule in the NEC, so give me a substantive technical reason to reject this particular proposal." I'm not holding my breath.

Cheers, Wayne
 
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