Continuous Rating on Fused Disconnect 1

[zimmerDN]

Are fusible disconnects 100% rated for continous duty? Circuit breakers are often 80% rated, 15A breaker should only have 12A max of continous load.

What about fuses, are there continous duty ratings on them?

 

[X49]

The same rule applies for fusible switches as for breakers. The continuous current must be limited to 80% unless the fusible switch is listed for operation at 100%. I am not sure how common 100% rated fusible switches are. Non-fusible switches are generally rated for 100% continuous duty.

 

[jraef]

All switches and breakers are "rated", or I should say TESTED, for carrying 100% of their stated capacity. So a 100A switch can carry 100A, a 200A switch can carry 200A, etc. etc.

But if the switch also includes the Over Current Protective Device (OCPD), then you just cannot USE them that way. That's because because the conductors must be sized for 125% of the circuit ampacity anyway, and the purpose of the OCPD is to protect the conductors. So the "80% rating" comes about as the inverse of 125%. Example: I have a load that takes 100A. I must use conductors that are rated 125A minimum. I can use a 125A circuit breaker for those conductors, but I cannot load them beyond 100A. 125A CB, maximum 100A load, so 100/125= 80%

It's a common misconception, fostered by the used of the term "100% rated breaker", which is an entirely confusing issue for most people and cannot be used as often as some people would like, because the conditions of their use are usually well beyond what they understand.

"Will work for (the memory of) salami"

 

[wroggent]

jraef, why not use a 100A breaker in your example?

"That's because because the conductors must be sized for 125% of the circuit ampacity anyway, and the purpose of the OCPD is to protect the conductors."

A 100A breaker would protect 125A conductors...

 

[X49]

wroggent,

You could use a 100A breaker in jraef's example, but only if it is listed for operation at 100%. In this case, the conductors would only need an ampacity of 100A. (NEC 215-2, 215-3)

As a rule, I would recommend keeping things simple and using regular breakers. However, 100%-rated breakers could be useful in increasing the capacity of existing feeders (for continuous loads), since loading the conductors to 100% is possible when using 100%-rated breakers.

 

[dpc]

However, 100%-rated breakers could be useful in increasing the capacity of existing feeders (for continuous loads), since loading the conductors to 100% is possible when using 100%-rated breakers.

I have not checked this for a few years, but it used to be a requirement that if a 100% rated breaker is used, the conductor must have 90 deg C insulation applied at the 75 deg C ampacity.

100% rated breakers, in general, don't help as much as might be suspected at first. The 100% rating is based as much on the enclosure as it is the breaker.

 

[wroggent]

X49,

I know. I just didn't think jraef explain it quite right.

These are the conductor stipulations:
1.) Be rated for at least 125% of the load current.
2.) Be rated greater than or equal to the rating of the breaker.

These are the 80% rated breaker stipulations:
1.) Be rated for at least 125% of the load current.


The conductors depend on the breaker...not the other way around. The stipulation on the breaker is because of the environment the breaker is in when it is tested at 100%. The breaker will trip before 100% due to a higher ambient temperature when used in an enclosure (due to trapped heat), versus being in air when in UL test conditions. The breaker is expected to trip at the appropriate current level when it is sized to 125% of the load current.

 

[shamsdebout]

Jraef would be correct if meant all circuit breakers are fully rated for operation in free air at their listed maximum ambient temperature. Not all circuit breakers will be fully rated if applied in enclosures, which Wroggent alluded to.

Of course all LVPCB's are fully rated. Some MCCB and ICCB are fully rated but not all.

dpc bring up a good point about 90 °C insulation rating. Based on what I read, UL 489 requires that if the terminal temperature rise during 100% rating test exceed 50 °C (90 °F rise), then the circuit breaker should be marked "For use with 90 °C (194 °F) wire and the wire size.

The IEEE blue books says: When the total terminal temperature in contact with insulation is 90 °C (194 °F), then a 90°C (194 °F) rated conductor should be used.

Square touches on the temperature rating here:

http://static.schneider-electric.us...nsulated Case Circuit Breakers/0613DB9902.pdf

Glad you mention this as I didn't know about this until you mentioned it.