80% vs 100% LV Circuit Breaker

[EP007]

The circuit breaker is 800 A frame and 80% rated, which is 640 A, The adjustable rating plug, can be easily set to say 500 A. The trip unit, which is separate, can be set as a multiple of 500 A. Can the breaker carry 500 A as that is less than 640 A, or does it now become limited to 400 A?

 

[dpc]

From an NEC perspective, for an 800 A frame breaker with an adjustable rating plug, the conductor must be sized per the MAXIMUM setting of the rating plug, unless it is in a supervised location. That's the basic rule. The same logic applies to an adjustable trip unit. Normally rating plugs are not adjustable - that's the whole point of a rating plug.

If you are asking if the breaker is going to actually trip at exactly 500 A, I'd say probably not.

If you are asking if the NEC/UL will let you consider the 500 A a 100% rating, I'd say probably not since the 100% rating depends on the combination of the breaker, the trip unit and the enclosure. In addition, wiring must be done with 90 deg C insulation applied at the 75 deg C rating.

Also, the "80% rating" applies to continuous loads only.

 

[EP007]

They have new plugs that are adjustable, so it now looks like the other settings, with dials. See for example p11 of the following:

http://www.eatoncanada.ca/ecm/groups/public/@pub/@electrical/documents/content/il01209005e.pdf

My location is industrial, well controlled, and that is supervised. I don't know what is inside, but perhaps the sensor is like a CT with a transformer tap. I would think that the 80% rating relates to thermal at a max 640 amp and the main current paths. That would suggest 500 A is less of a thermal effect on a 500 A plug setting.

 

[dpc]

I know adjustable plugs exist. I'm just saying they are not normally used because that defeats most of the original intent of rating plugs. I don't see any difference between an adjustable rating plug and an adjustable trip unit as far as it relates to this issue. The 80% limit is the flip side of the NEC requirement to size continuous loads at 125%. You're right, it is historically based on the sensitivity of the thermal trip element of the breaker. A 100 A molded case breaker can carry 100 A continuously in open air. The use of solid-state trip units may change the physics of the situation, but it doesn't change the requirements of UL or the NEC.

Also, when you buy a 100% rated breaker, you get a specific 100% rated trip unit. There are two different trip units, IIRC. Whether there is any actual difference, I don't know.

You could contact GE or Eaton and ask them. They have specialists in MCCBs who can give you their opinion.

 

[wroggent]

I have often wondered the same thing. IIRC, as an example, GE's spectra breakers (electronic trip units) will trip from 105% to 135% of the continuous current rating; you can see this looking at the TCCs and I think its mentioned in a manual somewhere. Those breakers are insensitive to ambient temperature as well. Seems like it's high time for the NEC to address electronic versus thermal magnetic trip units, particularly adjustable ones.

 

[EP007]

There is the issue of conductor sizing and terminal temperature. If the conductor is at a temperature then any terminal it is connected to is also. This article is helpful:

http://www.eaton.com/ecm/groups/public/@pub/@electrical/documents/content/ap01200008e.pdf

Even if the cb is taking the lower current of the lower plug setting, but the conductor was sized for that lower current, the conductor may be at the same temperature.