Section 5: Feeders

5.1 Minimum Rating and Size
5.2 Ground-Fault Protection of Equipment

5.1 Minimum Rating and Size
Feeder ampacity is required to equal or exceed non-continuation loads plus 125% continuous loads.

Feeder conductors must be large enough to carry 100% of the load.  The load is calculated according to NEC Article 220.

If the calculated load is continuous the conductor must be sized to carry 125% of the calculated load.  If the conductors are in an ambient temperature greater than 86°F, or if there are more than 3 current-carrying conductors in conduit, the size of the conductors must be increased.

If a load is continuous and there are more than 3 current-carrying conductors in conduit or the ambient temperature is above 86°F, take the larger of either (1) the continuous load (125%) or (2) the combined adjustments for a hot ambient and more than 3 current-carrying conductors.

Example for continuous and non-continuous loads:

The calculated load on a feeder includes 50 amps of non-continuous loan and 50 amps of continuous load.  Solution:  Add the non-continuous loat at 100% and the continuous load at 125%.  40 amps + 62.5 amps (50 amps x 1.25) = 112.5 amps.

Example for continuous loads and other ampacity adjustment conditions:

What is the minimum size 75 degree, cu conductor to feed a 110 amp continuous load with 4 current-carrying conductors in a conduit?  Solution.  First calculate the load with 4 current-carrying conductors.  110 amps à · .8 = 137.5 amps.  Since the result of both calculations is the same, select a conductor that can carry 137.5 amps.  1/0 cu.

There is an exception that permits feeder conductors supplying continuous loads that are connected to overcurrent devices which are listed for operation at 100% of their rating to be calculated at 100%, not 125%.  Another exception permits grounded conductors to be sized at 100%, if they are not connected to overcurrent devices.

Review Question Section 5.1

35. What is the minimum ampacity of a feeder conductor that supplies a non-continuous load of 75 amps and a continuous load of 100amps?

5.2 Ground-Fault Protection of Equipment

Ground-fault protection for equipment is required for feeder disconnects rated 1000 amps or more for systems that are a Wye-connected, solidly grounded, and that exceed 150 volts to ground but do not exceed 600 volts phase-to-phase.  For example, a 277/480 volt, system solidly connected to a grounding electrode is not required to have ground-fault protection for equipment unless it is rated 1000 amps or more.

Ground-fault Protection of Equipment on large feeders prevents ground faults from becoming phase-to-phase faults and causing massive damage to electrical equipment.

NEC Article 290.35 has the following requirements for ground-fault protection for service equipment:

  • Setting: maximum permitted setting of the ground-fault protection is 1200-A; and the maximum time delay is 1-second for ground-faults of 3000-A or greater.
  • Fuses:  switch and fuse combinations are required to use fuses that have faster response time than the switch which may take up to one second or longer to open.
  • Performance Testing:  a written record of a performance test conducted when the equipment was initially installed is required to be available to the AHJ.  Manufacturer’s instructions generally require periodic performance testing throughout the life of the equipment.
Review Question Section 5.2

36. Which of the following Wye-connected feeders, rated 277/480 volts, is not required to have ground fault protection.

Ground-fault protection for equipment is required for solidly grounded 480/277 volt feeder disconnects rated 1000 amps or more.