Continuous Load Correction of Ampacity

Ampacity Correction
Temperature Correction
Continuous Load Correction
Conduit Fill Correction
Minimum Number of Circuits
Bus Bar Ampacity
Conductor Resistance
Resistance Change
Motor Disconnect Sizing
Full Load Current Interpolation
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Where a conductor is selected to provide power to a load which is continuous, expected to continue for three hours or more, then the conductor table ampacity must be corrected (reduced) to determine the correct ampacity for that specific use.

The table value ampacity is multiplied by 80% to find corrected ampacity for a continuous load.

There are many loads which are considered continuous. Here is a partial list:

    all motors except those which can not operate for three hours by design
    parking lot lighting
    store lighting
    office lighting

Other loads specifically mentioned in the NEC which are considered continuous for conductor selection are:

    electric heating
    refrigeration equipment
    air conditioning compressor motor
    water heater

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Note: The NEC presents a small twist to ampacity correction. When a continuous load is supplied by a conductor installed in a raceway which at any point in its travel is included with conductors which are subject to ampacity correction for conduit fill, then only one correction is needed.

So here's the deal. If the correction factor for conduit fill is more restrictive, producing a lower corrected ampacity than would result from a continuous load correction, then the correction for fill is made and the correction for continuous load is not made.

The thought here is that this pair of correction factors are both aimed at reduction of heat around conductors so only the most severe is needed to correct for heat.



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