80% rating of breakers

[jgarner]

Just registar as I like this website with all of the useful information. Been doing electrical for a long time and always been told you couldn"t load a breaker more than 80% of its rating. Is this right and if so do you have NEC reference. Thanks

 

[iwire]

A standard breaker may be loaded to 100% non-continuously (3 hours or less).

See Article 100 for the definition of continuous load.

210.19(A)(1) for branch circuits

215.2(A)(1) for feeders

 

[Jraef]

Where the 80% comes from is in the conductor ratings. If you have a 100A load, you pick a cable that has a rating which is 125% of that load, or 125A in this case. So if you have cable rated for 125A, you protect it with a breaker rated 125A, but you can only load that cable to 1/125%, or 80% (do the math). So knowing that, breaker manufacturers know that a standard breaker will only be loaded continuously to 80%, and they build them accordingly. As mentioned, you can load them to 100% temporarily.

If you need 100% out of a circuit, you can buy what are called "100% rated" circuit breakers in larger sizes, but the conductor issue gets trickier.

 

[iwire]

Where the 80% comes from is in the conductor ratings.

I have to disagree there, where the 80% comes from is the breakers listing.

And you can load both conductors and overcurrent devices 100% up to 3 hours at a shot.

Now good design and the minimum NEC requirements are often not the same.

 

[brian john]

You can load a CB to over 100% for short periods, such as inrush currents from transformers and motor loads. Just to add more to the mix.

 

[Mr. Bill]

A standard breaker may be loaded to 100% non-continuously (3 hours or less).

See Article 100 for the definition of continuous load.

210.19(A)(1) for branch circuits

215.2(A)(1) for feeders

What iwire said. That 80% rule has been an old rule of thumb to allow quick (over conservative) calculations for all situations. Then the idea got entrenched into everyone's minds. When I first started I was told by several engineers in my office that circuit breakers have to be derated to 80% and fuses were good at 100%. This place help straighten me out.

I've also seen 80% used as a generic power factor conversion. Some thing along the lines of 80 kW is equal to 100 kVA. It just became another quick calculation to adjust for power factor. They didn't care if the PF was unity or 0.5. Assuming 0.8 PF was a means to allow them to save time and avoid annoying math with the NEC's rules and exceptions complicating things. Most new office and school buildings I've seen have had the PF around 0.92 so assuming 0.8 is very conservative. Well, conservative for the building, not necessarily every load.

 

[suemarkp]

There was clarification of this in 2008 as seen by 210.19(A)(1) Exception 2: You can size the grounded conductors which don't connect to an overcurrent device at 100% of the continuous and non-continuous loads. It is the breaker which is causing the 80% load problem.

 

[tom baker]

A standard CB has two tripping means, thermal and magnetic. The breaker is calibrated to trip properly with a conductor sized at 125% of the continuous load. The wire acts as a heat sink to keep the breaker from tripping on the thermal portion.
This is a UL listing requirement, I did not see this in the UL white book though.

 

[iwire]

You can load a CB to over 100% for short periods, such as inrush currents from transformers and motor loads. Just to add more to the mix.

That is a good point, breakers would trip all the time if not for that ability.

 

[brian john]

And to add to Tom's, this over current (in the range of 6 to 10 times) is the magnetic portion of the OCP device.

And 80% is no guarantee of a no trip, ambient can play into this, loading of adjacent CBs.

 

[templdl]

If one simply calcultates the load based upon 100% of the non-continuous load + 125% of the continuos load, the cable selcted must be sized to carry that load, then select a breaker to protect the cable. If you do this the breaker will be applied correctly.
If all of the load is continuous then the cable is sized 125% of that load. The cable must be sized to carry that load, the breaker sized to protect the cable. The breaker will then be applied automatically at no more than 80% of the load.
The problem is if you undersize the breaker which would result in nuisance tripping which if I'm not mistaken would be a code violation refledctin a poorly designed an coordinated system. I believe there is a code reference as such but I can't locate it.