Continuous loads are another topic that many “experts” seem to dance around in their articles that deal with them. They give you the same information you can get from the NEC itself. What good does that do?
Maybe they believe if they copy and paste code articles in a different font you’ll better understand them. We prefer to offer our understanding and not recite the articles.
Continuous Loads | Why the 125%?
What you’ll notice when reading the sections on continuous loads is that they are directly related to the overcurrent protection devices and/or assemblies. That’s because the equipment’s rating is the exact reason we’re doing it. They’re “derated” or listed to operate at 80% of their rating continuously. This is even clearer when taking a look at the exceptions.
For overcurrent devices and conductors you’re permitted to calculate continuous loads at 100% when the assemblies and overcurrent devices are listed to operate at 100% of their rating.
For conductors the more obvious exception is for grounded conductors (neutrals). When they don’t terminate on an overcurrent device they aren’t required to be calculated at 125%.
In case you didn’t know, taking the load at 125% is the same as 80% of the device rating. 1.25 and 0.8 are reciprocals.
For example: A 120 volt 20 amp circuit’s rating is 2,400 VA (120 x 20). 80% of 2,400 VA is 1,920 VA (2400 x 0.8). Now if you take the 1,920 VA at 125% or times 1.25 you’ll get 2,400 VA.
What else to look out for
By now you should understand that we are simply over sizing the equipment and conductors. Again, it’s because of the overcurrent protection and/or assembly ratings. If we increase the breaker size then the conductor size has to increase too. It has to be protected at its ampacity.
Be careful not to think this changes load values when making calculations. The load is the load as required to be calculated. We’re only taking continuous loads at 125% for the purposes of sizing conductors and overcurrent protection.
There’s one other thing you should take note of. Many specific loads are considered continuous or are to be calculated at 125%. In most cases this is for the branch circuits only. Pay attention to the article section you are in. It’s almost always talking about the branch circuit conductors and overcurrent protection.
It’s common for load’s branch circuit conductors and overcurrent protection to be sized at 125%, but its demand on the service/feeder is 100% or less. Unless of course it’s actually a continuous load as defined in Article 100.