Special permission?

[Dale001289]

So you are telling me when the OCPD fails, it will only fail by 25% above its rating?

Not saying that at all. Just sayin’ the additional 25% feeder size provides safety in case the CB fails to open under an overload condition.


Sent from my iPhone using Tapatalk

 

[oldsparky52]

If you have a 20A CB rated 80% on a lighting panel, the cable needs to be min #12 or 20A x 125%, i.e. 25A,

Humm, I've always understood if you have a 20A CB you need a 20-amp rated conductor. If you have an 80% rated breaker then you can only continuous load that breaker to 16-amps, or non-continuous to 20-amps. Either way, the conductor is rated for 20-amps, not 25.

 

[oldsparky52]

Let us keep in mind at a high level, the NEC is about safety. Even though as you say "You cannot put 1200A of continuous load on the 1200A MCC" you and I both know someone out there will do it - load it up to 1200A or more 'till the CB trips. That's why the associated feeder must be sized up 125% of the 1200A.

1st, we don't inspect on possible future loads.

2nd, that's what future permits and inspections are supposed to prevent.

 

[Isaiah]

Humm, I've always understood if you have a 20A CB you need a 20-amp rated conductor. If you have an 80% rated breaker then you can only continuous load that breaker to 16-amps, or non-continuous to 20-amps. Either way, the conductor is rated for 20-amps, not 25.

210.19(A)(1)(a)


Sent from my iPhone using Tapatalk

 

[david luchini]

Thus, the 25% is added to the feeder size to ensure a margin of safety. This is the intent of NEC 215.2(A)(1).

Except NEC 215.2(A)(1) doesn't tell you to add 25% to the feeder size. So that's obviously not it's intent.

 

[david luchini]

Humm, I've always understood if you have a 20A CB you need a 20-amp rated conductor. If you have an 80% rated breaker then you can only continuous load that breaker to 16-amps, or non-continuous to 20-amps. Either way, the conductor is rated for 20-amps, not 25.

You understand it correctly.

 

[Isaiah]

Except NEC 215.2(A)(1) doesn't tell you to add 25% to the feeder size. So that's obviously not it's intent.

That’s right but it does say add 25% to continuous load. The intent here is safety. The installation is safe in my opinion.


Sent from my iPhone using Tapatalk

 

[oldsparky52]

210.19(A)(1)(a)


Sent from my iPhone using Tapatalk

That doesn't say what you are saying it says.

 

[don_resqcapt19]

Not saying that at all. Just sayin’ the additional 25% feeder size provides safety in case the CB fails to open under an overload condition.


Sent from my iPhone using Tapatalk

So then you are telling me that code compliant installations are not "essentially free from hazard" as stated in 90.1(B).
The code cannot cover all of the "what-ifs" and possible modes of failure.

 

[wwhitney]

That’s right but it does say add 25% to continuous load. The intent here is safety.

So are you saying that using the exception with a 100% rated breaker is less safe? Since it doesn't have the 25% margin?

As the multitude of responses here shows, you are mistaken as to the intent.

Cheers, Wayne

 

[Dale001289]

So then you are telling me that code compliant installations are not "essentially free from hazard" as stated in 90.1(B).
The code cannot cover all of the "what-ifs" and possible modes of failure.

So then you are telling me that code compliant installations are not "essentially free from hazard" as stated in 90.1(B).
The code cannot cover all of the "what-ifs" and possible modes of failure.

Why 25% Instead of 15% or 30%? Code making panels come up with this stuff based on decades of risk analysis.


Sent from my iPhone using Tapatalk

 

[wwhitney]

Why 25% Instead of 15% or 30%? Code making panels come up with this stuff based on decades of risk analysis.

More likely a number that sounded reasonable 80 years ago, and there's never been any reason to change it.

As I understand it, the only safety risk that would result in not using a 125% factor for continuous loads in breaker/wire sizing would be the result of any ensuing nuisance tripping. E.g. wearing out the breaker because the user keeps resetting it, or underprotecting the conductor at short time scales because the user is induced to upsize the breaker without upsizing the conductor, etc.

Cheers, Wayne

 

[Dale001289]

More likely a number that sounded reasonable 80 years ago, and there's never been any reason to change it.

As I understand it, the only safety risk that would result in not using a 125% factor for continuous loads in breaker/wire sizing would be the result of any ensuing nuisance tripping. E.g. wearing out the breaker because the user keeps resetting it, or underprotecting the conductor at short time scales because the user is induced to upsize the breaker without upsizing the conductor, etc.

Cheers, Wayne

Let’s view this concept in another way. Consider motor overloads for example. Say you have an induction motor with a 1.15 SF. Why are the OL’s sized at 125% of motor FLA? And why is the motor branch circuit also sized at 125% of motor FLA? It’s all about HEAT, right?
It’s the same principal with the feeder 125% rule. It has nothing to do with nuisance trips. It’s all about protection against overheating. Whether 80 years ago or 100 years ago doesn’t really matter, the physics aspect hasn’t changed.


Sent from my iPhone using Tapatalk

 

[Isaiah]

So are you saying that using the exception with a 100% rated breaker is less safe? Since it doesn't have the 25% margin?

As the multitude of responses here shows, you are mistaken as to the intent.

Cheers, Wayne

Not at all. Remember we’re not worried so much about the breaker sizing - It’s the cable we’re concerned about.
Conceptually I maintain the need to upsize the cable based on continuous load. The real question is what exactly is the continuous load, 960A or is it 1200A? If it’s 960A based on the 80 percentile of the MCB then the cable is OK. If it’s 1200A then the cable needs to be increased to 1500A. If the CB is 100% rated then nothing has to change


Sent from my iPhone using Tapatalk

 

[wwhitney]

Conceptually I maintain the need to upsize the cable based on continuous load. The real question is what exactly is the continuous load, 960A or is it 1200A? If it’s 960A based on the 80 percentile of the MCB then the cable is OK. If it’s 1200A then the cable needs to be increased to 1500A. If the CB is 100% rated then nothing has to change

That just shows that it's about the breaker, and not the cable. If you use a 100% rated breaker, then you don't need to upsize the cable, the cable's rating is continuous. The only reason you have to upsize cable is if you have to upsize the breaker because of the limitations of regular breakers.

Cheers, Wayne

 

[wwhitney]

It’s all about HEAT, right?

Yes, for resistive conductors, current limits are all about HEAT.

But in this case, it's about the heat in the breaker, not the heat in the cable. Let me draw your attention to the definition of ampacity:

Ampacity. The maximum current, in amperes, that a conductor can carry continuously under the conditions of use without exceeding its temperature rating.

(italics by me). So the conductor can carry current up to its ampacity continuously without generating excess heat. The conductor doesn't need an extra 25% factor, the table ampacity already reflect the necessary safety margins.

And if you use a 100% rated breaker, then it can carry current up to its rating continuously without risk of nuisance tripping. It's only because of the limitations of "regular" breakers that it's ill advised to do that, and the NEC requires you to upsize the breaker by a 25% factor to avoid that nuisance tripping. Then, because the breaker has been upsized, you must upsize the conductors to be adequately protected.

Cheers, Wayne

 

[Isaiah]

That just shows that it's about the breaker, and not the cable. If you use a 100% rated breaker, then you don't need to upsize the cable, the cable's rating is continuous. The only reason you have to upsize cable is if you have to upsize the breaker because of the limitations of regular breakers.

Cheers, Wayne

There’s never been any argument about the 100% rated breaker. That’s actually the best scenario, then you don’t have worry about the cable uplift.


Sent from my iPhone using Tapatalk

 

[don_resqcapt19]

Why 25% Instead of 15% or 30%? Code making panels come up with this stuff based on decades of risk analysis.


Sent from my iPhone using Tapatalk

This was totally based on heat at the breaker terminals and was put in the code to correspond with the breaker standard of 80% for continuous load. I has nothing to do with safety.

 

[wwhitney]

There’s never been any argument about the 100% rated breaker. That’s actually the best scenario, then you don’t have worry about the cable uplift.

Great. So say you have a 1000A continuous load, and let's consider a few scenarios:

A) 1000A rated conductors, 1000A breaker, 100% rated. We agree that this is allowed. The conductors are rated for 1000A continuous current, as is the breaker. The breaker properly protects the conductors.

B) 1000A rated conductors , 1000A normal breaker. The breaker still protects the conductors, and the conductors are rated for the continuous current. However, the breaker is not rated for the continuous current, so it may trip. If that weren't an issue, this would be fine.

C) 1000A rated conductors, 1250A normal breaker. The breaker is now rated for 1000A continuous current, so there will be no nuisance tripping. But the conductors are no longer properly protected by the breaker, so not allowed.

D) 1250A rated conductors, 1250A normal breaker. Allowed.

As you can see, it's all about the limitations of the breaker, and the conductor upsizing is just a consequence.

Cheers, Wayne

 

[Isaiah]

Great. So say you have a 1000A continuous load, and let's consider a few scenarios:

A) 1000A rated conductors, 1000A breaker, 100% rated. We agree that this is allowed. The conductors are rated for 1000A continuous current, as is the breaker. The breaker properly protects the conductors.

B) 1000A rated conductors , 1000A normal breaker. The breaker still protects the conductors, and the conductors are rated for the continuous current. However, the breaker is not rated for the continuous current, so it may trip. If that weren't an issue, this would be fine.

C) 1000A rated conductors, 1250A normal breaker. The breaker is now rated for 1000A continuous current, so there will be no nuisance tripping. But the conductors are no longer properly protected by the breaker, so not allowed.

D) 1250A rated conductors, 1250A normal breaker. Allowed.

As you can see, it's all about the limitations of the breaker, and the conductor upsizing is just a consequence.

Cheers, Wayne

A) Agreed.
B) 800A continuous load x 125% equals 1000A which requires 1000A conductors and Standard Breaker.
C) Agreed.
D) 1000A continuous load x 125% equals 1250A which requires 1250A conductors and Standard Breaker.

continuous load is the driving element behind both conductor and CB sizing, per NEC


Sent from my iPhone using Tapatalk