Reason for sizing OCPD 125% for continuous load?

G

Genepires

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Washington state
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Electrician
Kinda stuck/confused on the reasoning why the OCPD for a continuous load needs to be 125%. I get why the wire needs to be over sized to minimize the heat generation from continuous current. Is this same reasoning (heat generation) apply to OCPD too? Or just cause the wire is oversized, then the breaker must also be oversized? That just doesn’t seem right.
 
Both the conductor and the OCPD are sized to 125%. The combination of the two keeps excess heat from building up in the panel. If the panel and breakers were listed for 100% continuous operation the design would have already accounted for this excess heat. Standard breakers and panels are designed for 80% continuous.

Welcome to the Forum. :)
 
My understanding is circuit breakers are tested in open air. When you have a bunch of them crammed side by side in a panel board, under full loading they would generate excessive heat and trip prematurely. Hence the "derating". So with the breaker upsized, the conductors now need to be upsized.
 
It is a myth that the OCPD limitations affect the conductor.
Conductors are sized by the load and the environment they are installed in.
Breakers and fuses are sized and tested to protect the selected conductors.
 
electrofelon said:
Do you mean the common "conductor is a heat sink to pull more heat away from the breaker" theory?
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It is not a theory.
UL listing testing requires a length of conductor to be installed in the lugs of both breakers and fusible switches. The conductor length is present during temperature heat rise tests as well as conductor pullout/damage during short circuit tests. These tests prove that breakers and fuses do actually protect all conductors selected and applied following the NEC.
 
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jim dungar said:
It is not a theory.
UL listing testing requires a length of conductor to be installed in the lugs of both breakers and fusible switches. The conductor length is present during temperature heat rise tests as well as conductor pullout/damage during short circuit tests. These tests prove that breakers and fuses do actually protect all conductors selected and applied following the NEC.
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What I mean is, it is commonly stated that the conductors are oversized on a continuous load to act as a better heat sink and pull more heat out of the breaker.
 
What I have always found confusing is the structure of these requirements in the NEC. The continuous load derating is found in the conductor articles which essentially require 125% for the conductor sizing and the overcurrent protection. Why not just require 125% for continuous loads in article 240 and let the conductor size follow automatically? Is there something I'm missing that would make this not work?
 
jim dungar said:
It is a myth that the OCPD limitations affect the conductor.
Conductors are sized by the load and the environment they are installed in.
Breakers and fuses are sized and tested to protect the selected conductors.
Click to expand...
So it's the OCPD limitations themselves that are the reason for the 125% multiplier, right? And the conductors are just upsized to match the OCPD? That's been my understanding.
 
electrofelon said:
What I mean is, it is commonly stated that the conductors are oversized on a continuous load to act as a better heat sink and pull more heat out of the breaker.
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Which is the myth.
The breaker and fuses are oversjzed because the NEC requires the conductor to be oversized.

The breaker and fusibles testing are all conducted at 100% loading yet the UL requirements are that they carry 133% of the rating for an extended period of time.
 
electrofelon said:
Why not just require 125% for continuous loads in article 240 and let the conductor size follow automatically? Is there something I'm missing that would make this not work?
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It would work, but then you'd get to use 240.4(B), so conductors would be slightly downsized in some cases. So to the extent that the OCPD requires the larger conductor for heatsinking to avoid nuisance trips, in addition to the 125% upsizing, that could be a problem.

Cheers, Wayne
 
jaggedben said:
So it's the OCPD limitations themselves that are the reason for the 125% multiplier, right? And the conductors are just upsized to match the OCPD? That's been my understanding.
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No.
The OCPD is upsized to match the conductor.

Look at the NEC sizing requirements.
Start with the load.
Select the conductor and adjust for its installation and environment.
Finally select the OCPD.
 
jim dungar said:
Which is the myth.
The breaker and fuses are oversjzed because the NEC requires the conductor to be oversized.
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No, the conductors are oversized because the NEC requires the OCPD to be oversized.

The existence of the exception on conductor size for 100% rated OCPD shows that is correct, rather than what you stated. If the conductor size increase were required independent of OCPD, there would be no such exception.

Cheers, Wayne
 
jim dungar said:
No.
The OCPD is upsized to match the conductor.
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It's sorta a chicken vs egg thing. My understanding is THE ONLY REASON for the 125% sizing of any component is because of the OCPD in cramped panel boards with little air flow. So if you increase the OCPD size, as a consequence you now must increase the conductor size for the conductors to remain protected. If I am misunderstanding and there is another reason that the conductors be increased in size, by all means let me know.

I think there's much confusion on this issue because the way the rules are structured in the NEC
 
wwhitney said:
No, the conductors are oversized because the NEC requires the OCPD to be oversized.

The existence of the exception on conductor size for 100% rated OCPD shows that is correct, rather than what you stated. If the conductor size increase were required independent of OCPD, there would be no such exception.

Cheers, Wayne
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Using the 2017 edition.
210.19(A)(1) says, "Branch-circuit conductors shall have an ampacity not less than the load..."
210.19(A)(1)(a) says the conductors need to be oversized when there is a continuous load.
210.19(A)(1)(b) says the conductors ampacity need to be adjusted/corrected as applicable.
210.20 says the OCPD needs to be sized to protect the oversized adjusted conductor.

There is an exception to both the rating of the OCPD and the sizing of the conductor.
There are similar requirements for feeders.
 
jim dungar said:
Using the 2017 edition.
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The fact that 210.19 precedes 210.20 does not imply anything about causality.

Say you have a 200A continuous load and a 200A current limited source that does not require OCPD. You can use a 200A ampacity conductor, as the ampacity is a continuous rating.

Now say your source is no longer current limited so you need OCPD. You can use a 200A 100% rated OCPD, and there is no other change.

Or you can use a non-100% rated OCPD, and you are required to increase the OCPD size to 250A, and also required to upsize the conductor ampacity to 250A. Why did the OCPD size change? Because of the issue of nuisance tripping for an OCPD not in free air. Why did the conductor size change? Because a 200A conductor may not be protected at 250A.

It is the OCPD size change that drives the conductor size change.

Cheers, Wayne
 
electrofelon said:
What I mean is, it is commonly stated that the conductors are oversized on a continuous load to act as a better heat sink and pull more heat out of the breaker.
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In my apprenticeship, our instructor told us that conductor sizing is based upon amps but the loading was sporadic and that allowed for cooling of conductors. But with continuous loading, there was not this cooling time and then the wire would heat up and possible degrade the jacketing. Thicker gauge wiring has less resistance (or some other sparky magic) and therefore less heating. Not about sucking heat from breaker but all that heating up wiring in conduits.
 
wwhitney said:
Say you have a 200A continuous load and a 200A current limited source that does not require OCPD. You can use a 200A ampacity conductor, as the ampacity is a continuous rating.
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Not according to 210.19. Its exception only applies if there is an OCPD protecting the conductor. No OCPD would mean the conductors need to be upsized.
 
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