Reason for sizing OCPD 125% for continuous load?

[infinity]

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.

I'm not sure how the conductors would degrade even if they continuously carried 100% of the ampere rating of the circuit breaker. Would a 30 amp continuous load on #10 conductors cause the temperature of the conductor to exceed its insulation temperature rating?

 

[wwhitney]

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.

While the mechanism you mention is real, the ampacity is a continuous rating. So it is a current level that has been determined not to overheat the conductor insulation, even when the load is continuous.

Cheers, Wayne

 

[wwhitney]

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.

No OCPD means it's not a branch circuit per the definition, so Article 210 does not apply.

Allowances for circuits without OCPD are rare in the NEC, but IIRC there is such an allowance within Article 690 on PV. Because PV can be a current limited source.

Cheers, Wayne

 

[wwhitney]

Would a 30 amp continuous load on #10 conductors cause the temperature of the conductor to exceed its insulation temperature rating?

Only a significant possibility if the ampacity is less than 30A due to adjustment or correction.

Cheers, Wayne

 

[wwhitney]

Allowances for circuits without OCPD are rare in the NEC, but IIRC there is such an allowance within Article 690 on PV. Because PV can be a current limited source.

From the 2026 NEC First Draft:

690.9(A)(1) Circuits Where Overcurrent Protection Not Required.
Overcurrent protective devices shall not be required where both of the following conditions are met:
(1) The conductors have sufficient ampacity for the maximum circuit current.
(2) The currents from all sources do not exceed the maximum overcurrent protective device rating specified for the PV module or electronic power converter.

690.8(B)(1) Without Adjustment and Correction Factors.
The minimum conductor size with an ampacity not less than the maximum currents calculated in 690.8(A) multiplied by 125 percent.
Exception No. 1: Circuits containing an assembly, together with its overcurrent device(s), if any, that is listed for continuous operation at 100 percent of its rating shall be permitted to be used at 100 percent of its rating.

Note that 690.9(A)(1) only requires the ampacity to be not less than the maximum current; no 125% factor for continuous currents. And 690.8(B)(1) Exception #1 says that if everything connected to the circuit is rated for continuous operation, then the required ampacity is 100% of the maximum current, not 125%.

[I'm using the 2026 NEC First Draft because it added the words "if any" to 690.8(B)(1) Exception #1 per one of my PIs. I figured that was implicit, but should be made explicit, and for once a CMP agreed.]

Cheers, Wayne

 

[jim dungar]

690.8(B)(1) Without Adjustment and Correction Factors.
The minimum conductor size with an ampacity not less than the maximum currents calculated in 690.8(A) multiplied by 125 percent.

Again there is only a 100% exception when an OCPD is involved.

 

[wwhitney]

On this topic of continuous currents and "is it the conductor or is it the OCPD" that would not perform properly if not upsized, I have so far overlooked the most direct evidence on the question: 2023 NEC 210.19(A) Exception and 215.2(A)(1) Exception 2. The latter states:

Exception No. 2: Where a portion of a feeder is connected at both its supply and load ends to separately installed pressure connections as covered in 110.14(C)(2), it shall be permitted to have an ampacity not less than the sum of the continuous load plus the noncontinuous load. No portion of a feeder installed under this exception shall extend into an enclosure containing either the feeder supply or the feeder load terminations, as covered in 110.14(C)(1).

This allows sizing the conductor at 100% of the continuous load for a portion of the conductor that runs from junction box to junction box, without terminating on the OCPD or the equipment. That shows that it is the termination on the OCPD that causes a 125% upsizing on the conductor.

So for example, the following is an allowable installation when there is no ampacity correction or adjustment applicable: A feeder with a continuous load of 100A. 125A OCPD at the source, with #1 Cu (75C ampacity 130A) terminated on the OCPD. The #1 Cu goes to an adjacent separate junction box and is connected to #3 Cu (115A 90C ampacity) with 90C rated connectors. Then the #3 Cu proceeds to a junction box to be connected to #1 Cu with 90C connectors, and the #3 Cu goes on to an adjacent panelboard.

Cheers, Wayne

 

[wwhitney]

Again there is only a 100% exception when an OCPD is involved.

Read the quoted exception to 2026 NEC First Draft 690.8(B)(1) again. It states "an assembly, together with its overcurrent device(s), if any, that is listed for continuous operation at 100 percent of its rating". No overcurrent device required for the exception to apply.

Cheers, Wayne

 

[jim dungar]

This allows sizing the conductor at 100% of the continuous load for a portion of the conductor that runs from junction box to junction box, without terminating on the OCPD or the equipment. That shows that it is the termination on the OCPD that causes a 125% upsizing on the conductor.

No it shows the assembly, incuding its enclosure ventilation, is the issue, not specifically the OCPD.

 

[LarryFine]

It seems to me that avoiding creating the heat is better than counting on wire to conduct it away.

 

[wwhitney]

No it shows the assembly, incuding its enclosure ventilation, is the issue, not specifically the OCPD.

Sure, I'll mostly agree with that clarification. I would say it's the OCPD performance in the given thermal environment of the enclosure, the other components in the enclosure, and the ventilation of the enclosure. Which in the simple "the OCPD or the wire" dichotomy, I'll ascribe to the OCPD.

It seems to me that as a practical matter, any single breaker installed in its own enclosure would very likely perform adequately as a 100% rated breaker, even if the combination hasn't been tested and listed as such. Just because the typical such single breaker enclosure wouldn't be that different from free air in terms of the thermal environment. Particularly if installed indoors where the ambient temperature is 25C rather than the 40C testing conditions.

Cheers, Wayne

 

[electrofelon]

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.

I believe that theory is a myth. If that were true, then why can we use 100% rated breaker serving "100% rated wire"? The 100% OCPD rule seems to prove that conductors do not need to be upsized for continuous loads.

 

[electrofelon]

On the topic: are any fused disconnects listed for continuous use? It seems like they should be able to pass the tests no problem.

 

[tortuga]

Why not just require 125% for continuous loads in article 240 and let the conductor size follow automatically?

Thats a interesting idea, until 1999 the 125% requirements was in the calculations in article 220.
In 1998 there was a big technical correlating committee reorganization (Log #1332) where they intended to move anything that was not purely load calculations out of 220 and thats when it got put where it is now 210.19 etc.
As often happens with big changes like that there is little to no public comment as they have already discussed it in committee.
It would be nice if 220 was purely a calculations article as they also forgot to remove the continuous factor from table 220.42(A), and it erroneously remains in there today.

 

[wwhitney]

they also forgot to remove the continuous factor from table 220.42(A), and it erroneously remains in there today.

Erroneously and stubbornly. I submitted a PC on the 2026 First Draft about (renumbered) 120.42(A) adjusting all the table values, because of the new (?) section 120.5(E) which states "Load calculations shall not require continuous loads to be calculated at 125 percent." Pretty sure it was rejected; at one point TerraView was showing all the CMP responses to the PCs, even though the Second Draft release date isn't until March 21, but that is no longer true.

Cheers, Wayne

 

[tortuga]

I was not aware you submitted that, yeah its an issue with service lateral conductors which have no such continuous load requirement.

 

[wwhitney]

I was not aware you submitted that, yeah its an issue with service lateral conductors which have no such continuous load requirement.

Not really an issue in 2026 NEC First Draft, since 120.42(A) footnote tells you that the tabular entries include a 125% continuous factor, and 120.5(E) tells you that the 125% factor is not required, you may back it out and just use 80% of the table values in any computations. Just an unnecessary step that should be eliminated.

Cheers, Wayne

 

[jim dungar]

On the topic: are any fused disconnects listed for continuous use? It seems like they should be able to pass the tests no problem.

The only ones I am aware of use Class L fuses. again ventilation is the issue.

 

[jim dungar]

It seems to me that as a practical matter, any single breaker installed in its own enclosure would very likely perform adequately as a 100% rated breaker, even if the combination hasn't been tested and listed as such. Just because the typical such single breaker enclosure wouldn't be that different from free air in terms of the thermal environment. Particularly if installed indoors where the ambient temperature is 25C rather than the 40C testing conditions.

Most manufacturers off er special, extremely ventilated, enclosures for their 'stand alone' OCPDs.

At one time the Square D-I-Line panelboard/switchboard offered plug-on 100% rated group mounted breaker, partly because the enclosures did not have doors restricting the air movement.

 

[tortuga]

In all the years I have been doing this I don't recall having actually seen or installed a 100% rated breaker or disconnect in the wild.