80% vs. 100% rated breakers

[JDBrown]

This is one of those things that I feel like I ought to know, but I don't. I've heard of 80% rated breakers and 100% rated breakers in passing, but I'm not really clear on what those terms mean, or on how common one or the other is in the wild. So, can somebody explain to me what an 80% rated breaker is vs. a 100% rated breaker?

I'm guessing that maybe a 100% rated 30 amp breaker could supply a continuous load of up to 30 amps, while an 80% rated 30 amp breaker could only supply a continuous load of 24 amps (80% of 30 amps) -- but that's only a guess. Can someone please confirm, or explain to me what this really means?

Also, how common are 100% and 80% rated breakers? One of the guys I work with claims that they stopped making 80% rated breakers years ago, and all new breakers are 100% rated. Another guy I work with claims that most breakers are 80% rated, and 100% rated breakers are special-order items that you have to specifically request if you need them. But the reality of the situation is that none of us are actually out in the field, working with and handling breakers on a daily basis--so I seek knowledge from those of you who do.

 

[infinity]

I'm guessing that maybe a 100% rated 30 amp breaker could supply a continuous load of up to 30 amps, while an 80% rated 30 amp breaker could only supply a continuous load of 24 amps (80% of 30 amps) -- but that's only a guess. Can someone please confirm, or explain to me what this really means?

That's it.

 

[JDBrown]

Thanks, Rob. It's nice when terms make sense like that.

As for the second part of my question, is there a "standard" for thermal-magnetic breakers (80% or 100%)? Or is it just whatever the EE decides to specify (or, if the drawings don't call for one or the other, whatever the EC decides to install)?

 

[Jraef]

Sort of...

All breakers are "capable" of handling 100% load, but you cannot necessarily use them that way. The rules really are based on the conductor sizing rules, which have a lot of ifs, ands or buts to allow them to be used at 100%, rather than 80% (the inverse of being sized at 125% of load). So since most conductor installations fall into that 125% of load category, there is no reason to take the extra steps of rating every breaker at 100%, so the "80% rated" breakers are really just regular breakers that have NOT been specifically tested and UL listed at 100% load.

Read this, it is one of the better explanations I have found.
http://w3.usa.siemens.com/us/SiteCollectionDocuments/LVTB-RCIRB-0711(lo-res).pdf

 

[JDBrown]

Read this, it is one of the better explanations I have found.
http://w3.usa.siemens.com/us/SiteCollectionDocuments/LVTB-RCIRB-0711(lo-res).pdf

You're right; that explanation is excellent. Thanks for the link. :thumbsup:

 

[templdl]

Also, you won't find 100% rated breakers in smaller ampacities. They will only be available in much larger frame sized.
Basically 100% breakers are used to save on costs. It could be that you may be able to step down a frame size by going from an 80% to a 100% rated breaker. You mat also even save on cable cost also. But, you must run the numbers in order to see if would be advantageous to apply a 100% rated breaker.
To apply a 100% rated break you must pay attention to where the breaker must be install. Then it is imperative the you use 90degC rated wire. The wire ampacity is based upon 100% of the continuous load plus 100% of the non-continuous load as opposed to 125% plus 100% as would normally be done. Because you are allowed 100% of the continuous load and not 125% it may be possible to use a smaller wire for a cost savings. But, the 90degC rated wire still must be applied at 75degC.
As such you must run the numbers to verify the breaker frame size. The 100% rated breakers cost more.
Then, will there be any cost savings if you are allowed you use smaller wire,
It is important to note that 100% rated beakers are no better than 80% rated breakers.

 

[zxfabb]

Generally, most regular MCCB are 80% rated while all LVPCB are 100%. Current rating of CB is verified by temperature rise test.

Per UL 489, the temperature rise test of a regular MCCB is performed in free air, when it is installed in an enclosure (switchboard, or the like) supplying continuous load, a derating factor of 80% applies due to worse heat dissipation. Optionally, a 100% rated MCCB shall be tested specially in an enclosure.

Per UL 1066, the termperature rise test of LVPCB shall be carried out in an enclosure. Therefore derating is unnecessary when used in switchboard with similar conditions.

Other derating factors shall be considered in special service conditions such as high altitude, high ambient terperature, small enclosure type.

Hope this helps.

 

[JDBrown]

Thanks for the replies, guys. I think I've got it. I appreciate all the help.