80% rule

[billydrowne]

would anyone mind explaining the 80% rule for sizing breakers/wire. someone asked me recently and i couldnt exactly explain it well

 

[wwhitney]

As I understand it, the issue is that a thermal magnetic circuit breaker will not reliably hold 100% of it rated current continuously. It has a good chance of tripping under those conditions.

As a result, various NEC rules limit the continuous current allowed on the breaker to 80% of its rating. This is in the form of a 125% multiplier on continuous current for the purposes of breaker sizing.

A consequence, however, is that the wire gets upsized by 125% as well. Because while wire is rated to carry 100% of its ampacity continuously, upsizing only the breaker would cause the wire to be no longer be protected at its ampacity. So the wire size has to increase as well.

Cheers, Wayne

 

[rnatalie]

Not just the breaker, the assumption is that if you have a CONTINUOUS load, the listed rating of all the components might not be able to handle it. So you have to compute things at 125% of the CONTINUOUS load. If everything in the assembly is capable of continuous loads at the rated ampacity, you don't need to do the size up.

 

[gadfly56]

As I understand it, the issue is that a thermal magnetic circuit breaker will not reliably hold 100% of it rated current continuously. It has a good chance of tripping under those conditions.

As a result, various NEC rules limit the continuous current allowed on the breaker to 80% of its rating. This is in the form of a 125% multiplier on continuous current for the purposes of breaker sizing.

A consequence, however, is that the wire gets upsized by 125% as well. Because while wire is rated to carry 100% of its ampacity continuously, upsizing only the breaker would cause the wire to be no longer be protected at its ampacity. So the wire size has to increase as well.

Cheers, Wayne

IIRC, 100% rated breakers are available. At higher cost, of course.

 

[wwhitney]

Not just the breaker, the assumption is that if you have a CONTINUOUS load, the listed rating of all the components might not be able to handle it. So you have to compute things at 125% of the CONTINUOUS load. If everything in the assembly is capable of continuous loads at the rated ampacity, you don't need to do the size up.

But wire amapacities are continuous ratings. So as far as the premises wiring goes, it's only the breaker that isn't capable of continuous loads at the rated current.

Cheers, Wayne

 

[billydrowne]

thank you guys. brought up some stuff i completely forgot about

 

[Jraef]

There is no such thing as an "80% rule" for circuit breakers, that's just a shorthand term we use because the real story is too difficult to explain quickly. ALL circuit breakers are tested and listed to carry 100% of their rated load, but in "free air", meaning all sides of the breaker can dissipate heat (this becomes important later).

Here is where the 80% rule comes from:

• You are required to size conductors at 125% of the continuous load, right?
• Breakers protect the conductors, right?
• So if I have a 200A continuous load, I am required to size the conductors for 250A (125% of 200A).
• I size the breaker to protect the 250A conductors, so a 250A breaker.
• But BECAUSE the conductors were over sized per the rule, the continuous LOAD on that 250A breaker should never exceed 200A.
• 200A / 250A = .8, ergo "80% rule".

Now, BECAUSE of this fact, the breaker MANUFACTURERS are going to expect that a breaker is never continuously loaded to more than 80% of it's rating. So when designing a PANELBOARD or SWITCHBOARD to house the breakers, they base everything in terms of heat rise on that 80% factor, because, remember, the 100% rating was based on FREE AIR, and in a panelboard where there are other breakers on either side, you do NOT have free air to dissipate heat in the breaker. So the breakers are DESIGNED to work in a panelboard or switchboard assuming no more than 80% continuous loading, based not on the breaker's maximum capacity, but on how they are being USED.

When you want a "100% rated breaker", it has to be used in a SEPARATE enclosure all by itself, NOT in a panelboard or switchboard arrangement (there are a few exceptions though, like Sq. D I-Line panels)., and there has to be sufficient air space around the breaker to dissipate heat. So the Main Breaker in a piece of gear, including MCCs, can be 100% rated, because they will have their OWN SEPARATE compartments, sized by the manufacturer to have sufficient air volume around it under the design operating conditions and maximum heat rise of the breaker.

There are other issues regarding conductor and termination ratings based on the NEC rules for using 100% rated conductors that can also get in the way, but those have nothing to do with the so-called "80% rule" on breakers.

 

[wwhitney]

You are required to size conductors at 125% of the continuous load, right?

The NEC is written that way, but my understanding is that is due to the limitations of the OCPD. You are suggesting that the manufacturers are designing to what the NEC requires, whereas I am suggesting that the NEC was written based on the technological limitations of the products (at some point in time, I don't know if the technology has improved since then).

Is it now technically feasible to design inexpensive circuit breakers that can protect a conductor at its ampacity for all load profiles, including at its full rating continuously, when 40-80 are packed next to each other in a panelboard? I infer from the non-availability of such a product that for that thermal environment, an inexpensive breaker that would hold for the fully-rated continuous load would necessarily have a trip curve that was too slow at various levels of overload.

Cheers, Wayne

 

[billydrowne]

There is no such thing as an "80% rule" for circuit breakers, that's just a shorthand term we use because the real story is too difficult to explain quickly. ALL circuit breakers are tested and listed to carry 100% of their rated load, but in "free air", meaning all sides of the breaker can dissipate heat (this becomes important later).

Here is where the 80% rule comes from:

• You are required to size conductors at 125% of the continuous load, right?
• Breakers protect the conductors, right?
• So if I have a 200A continuous load, I am required to size the conductors for 250A (125% of 200A).
• I size the breaker to protect the 250A conductors, so a 250A breaker.
• But BECAUSE the conductors were over sized per the rule, the continuous LOAD on that 250A breaker should never exceed 200A.
• 200A / 250A = .8, ergo "80% rule".

Now, BECAUSE of this fact, the breaker MANUFACTURERS are going to expect that a breaker is never continuously loaded to more than 80% of it's rating. So when designing a PANELBOARD or SWITCHBOARD to house the breakers, they base everything in terms of heat rise on that 80% factor, because, remember, the 100% rating was based on FREE AIR, and in a panelboard where there are other breakers on either side, you do NOT have free air to dissipate heat in the breaker. So the breakers are DESIGNED to work in a panelboard or switchboard assuming no more than 80% continuous loading, based not on the breaker's maximum capacity, but on how they are being USED.

When you want a "100% rated breaker", it has to be used in a SEPARATE enclosure all by itself, NOT in a panelboard or switchboard arrangement (there are a few exceptions though, like Sq. D I-Line panels)., and there has to be sufficient air space around the breaker to dissipate heat. So the Main Breaker in a piece of gear, including MCCs, can be 100% rated, because they will have their OWN SEPARATE compartments, sized by the manufacturer to have sufficient air volume around it under the design operating conditions and maximum heat rise of the breaker.

There are other issues regarding conductor and termination ratings based on the NEC rules for using 100% rated conductors that can also get in the way, but those have nothing to do with the so-called "80% rule" on breakers.

great thank you!

 

[Carultch]

IIRC, 100% rated breakers are available. At higher cost, of course.

And with strings attached / probably harder to get. You'd be looking at an application where all your loads in a given enclosure containing the breakers are continuous, where you'd intend to use continuous rated breakers. And an application where there is an advantage over using the counterpart product with ordinary duty breakers.

It isn't enough that the breaker itself is 100% rated, but it needs to be 100% rated in the enclosure within which it is used within. It is usually more practical to just apply the 1.25 safety factor and use a breaker rated for ordinary duty.