Continuous Load

[gburch@ameritech.net]

when applying the nec rules for sizing feeder conductors and overcurrent protection, 125% of continuous plus 100% of noncontinuous load, is it up to the designer to determine how to categorize the loads? i am aware of the definition of a load that runs for three hours or more is considered continuous, but when calculating the service for a building based on nec 220, there are cases such as apartment buildings where none of the individual loads would run for three hours. is there any generally accepted practice for categorizing building load types as continuous/noncontinuous?

as an example:
assume the calucated load for the apartment building above is 723 amperes, is a 800 ampere circuit breaker rated at 80% acceptable?
assume the calucated load for an individual apartment above is 105 amperes, is a 125 ampere circuit breaker rated at 80% acceptable?

would the application of the circuit breakers in the examples violate UL rules for molded case circuit breakers?

 

[cpal]

Apartment buildings as a commercial application are a mixed load
the individual dwellings are most likely not presenting loads that are traditionally considered continuious in the NEC, but the common areas (lighting for example) may.

The entire feeder demand would not be increased by 25% just those loads that may operate for 3 hours or more

 

[haskindm]

You will need to look at each load and determine if it is continuous or non-continuous. In general dwelling loads are not considered to be continuous. For example, a table lamp may be expected to be left on for 3-hours or more, but that is a very small percentage of the ampacity of the circuit so we would not consider the circuit to be continuously loaded. On the other hand, parking lot lights or public hallway lighting would be a continuous load because the circuit would be loaded for 3-hours or more. Figure your non-continuous loads at 100% and the continuous loads at 125% and then add the figures together. As long as this value does not exceed 100% of the fuse or breaker rating, it is OK.
Hope that helps,