Section 4: Branch Circuits

4.1 Rating
4.2 Ground-Fault Circuit-Interrupter Protection for Personnel
4.3 Conductors – Minimum Ampacity and Size
4.4 Outlet Devices
4.5 Heating, Air-Conditioning, and Refrigeration Equipment Outlet

4.1 Rating
The rating of the overcurrent protective device determines the branch circuit rating.

The rating of a branch circuit is based on the maximum permitted rating of the fuse or circuit breaker.  Branch circuits with multiple outlets are rated 15, 20, 30, 40 and 50 amps.

However, when branch circuits are very long, conductor size is often increased to prevent voltage drop.  In these cases, the rating of the branch circuit is based on the actual OCPD (Overcurrent Protection Devices) used rather than on the maximum OCPD permitted for the larger conductor.

For example, if a 120 volt, single-phase, 30 amp circuit was needed to supply a load located 70-feet from a panelboard, a #10 AWG copper conductor could be used and the branch circuit would be rated at 30 amps.

But if the same load were 200-feet from the panelboard a #6 AWG copper conductor would be required to maintain a 3% voltage drop.  If a 30 amp PCPD was used to protect the #6 AWG conductor, the circuit’s rating is still 30 amps.

Review Question Section 4.1

30. What determines the rating of a branch circuit?

4.2 Ground-Fault Circuit-Interrupter Protection for Personnel

The GFCI (Ground Fault Circuit Interrupter) protection for personnel required for single-phase 15 & 20 amp, 125 volt receptacles is provided by Class A GFCIs designed to trip when the current to ground is 6 mA or higher.  They are available as circuit breakers and receptacles.  GFCI protection shall be installed in a readily accessible location.

The requirements and exceptions for non-dwelling locations are listed in NEC article 210.8(B) which requires GFCI protection for 125 volt, single phase, 15- and 20-ampere receptacles as follows:

  • Bathrooms
  • Kitchens (all kitchen receptacles, not just those serving countertops)
  • Rooftops (Receptacles must be readily accessible from the rooftop only)
  • Outdoors
  • Sinks (where receptacles are installed within 6-feet of the outside edge of the sink), and
  • Indoor wet locations
  • Locker rooms with associated showering facilities
  • Garages, service bays, and similar areas other than vehicle exhibition halls and showrooms

Review Question Section 4.2

31. Receptacles rated 125 volts, 15 and 20 amps, and located __________ shall be GFCI protected.

GFCI protection is required for 125 volt, 15 and 20 amp receptacles to service HVAC equipment on rooftops of non-dwelling occupancies.
4.3 Conductors – Minimum Ampacity and Size

The ampacity of branch circuit conductors is required to equal or exceed the load supplied.

This section has been added as general information for inspectors and thermographers.

The ampacity of branch circuit conductors is required to equal or exceed the maximum load supplied.  When supplying a combination of non-continuous and continuous loads the minimum ampacity is required to be 125% of the continuous load plus 100% of the non-continuous load.

Continuous loads are loads that are on for 3 or more hours.  Current flowing into a termination for 3 hours or more will cause increased heating of the Over-Current Protective Device (OCPD) terminals which may overheat the OCPD thermal trip mechanism, causing it to trip.  By increasing the ampacity of the wires to 125% for continuous loads, the larger conductors act as “heat-sinks” drawing heat away from the terminals and minimizing problems caused by continuous loading.

For example, to calculate a 50 amp continuous load:  50 amps x 125% = 62.5 amps.  The electrician would install a conductor that is rated at least 62.5 amps.

Grounded conductors are generally not connected to OCPDs, so overheating caused by continuous loads is not a problem; consequently, the ampacity of grounded conductors is permitted to be rated for 100% of continuous loads.

When a load is continuous, and there are more than 3 current-carrying conductors in the conduit, or the ambient temperature is hotter than 86 degrees F., compare the size of the conductor needed for a continuous load (125%) with the size of the conductor needed to account for more than 3 current-carrying conductors or a hot ambient temperature and select the larger of the two.  In other words, don’t triple derate.

For example; What is the minimum size 75 degree conductor required to supply a 50-amp continuous load with 7 current-carrying conductors in conduit (75ðC terminals)?

  • Step 1:  50 amp continuous load = 50 x 1.25 = 62.5 amps.  NEC Table 310.15(B)16 indicates a No.6 THWN CU conductor has an ampacity of 65 amps which is more than adequate to carry 62.5 amps.
  • Step 2:  NEC Table 310.15(B)(3)(a) indicates that for 7 current-carrying conductors the ampacity in NEC Table 310.15(B)(16) is reduced to 70% of the table value; adjusted for 7 conductors the ampacity of the No. 6 is only 45.5 amps which is not adequate to carry the load.
  • Step 3:  Apply adjustment factor to the next larger conductor to see if it will carry the load.  NEC Table 310.15(B)16 indicates a No. 4 THWN CU conductor has an ampacity of 85 amps.  Adjusted for 7 conductors the ampacity of the No. 4 is 59.5 amps which is adequate to carry the 50 amp load: 85 amps x .70 = 59.5 amps.
  • Step 4:  Select the No. 4 CU THWN which is the larger of the two conductors.
Review Question Section 4.3

32. What is the minimum ampacity for a 75 amp continuous load?

4.4 Outlet Devices

Outlet devices shall have an ampere rating that is not less than the load to be served.

Lampholders on circuits rated more than 20-A are required to be the heavy-duty type.  This means that Medium-base screw-shell lampholders and fluorescent lampholders are prohibited on circuits rated at more than 20-A.

Receptacles:  Single receptacles installed on individual branch circuits are required to have a rating equal to or greater than the circuit’s rating.  Single receptacles are defined in NEC Article 100 as being “a single contact device” on a yoke.  Duplex receptacles are not single contact devices.

Cord-and-Plug-Connected Loads supplied by a single receptacle connected to a circuit that supplies two or more receptacles or outlets are not permitted to exceed the loads specified in NEC Table 210.21(B)(2) below:

Table 210.21(B)(2)

Table 210.21(B)(2)

Review Question Section 4.4

33. When connected to a circuit supplying two or more receptacles or outlets, what is the maximum load and maximum overcurrent protection for a single receptacle rated at 30 amperes?

Table 210.21(B)(2) describes the minimum load for cord and plug connected receptacles.

4.5 Heating, Air-Conditioning, and Refrigeration Equipment Outlet
Install receptacle for service within 25-feet of HVAC equipment.

The single-phase, 15 or 20-A, 125-V receptacle required by NEC 210.63 is intended to be used to service HVAC & refrigeration equipment.  It is required to be accessible, within 25-feet of the equipment, and located on the same level as the equipment.  If the equipment is at grade level, the receptacle must be installed at that level; however, if it is a rooftop unit, the receptacle is required to be located on the rooftop within 25-feet of the equipment.

Because the equipment may need to be disconnected from power when being serviced, the receptacle is not permitted to be connected from power when being serviced, the receptacle is not permitted to be connected to the load side of the disconnect for the equipment.  The receptacle for servicing the HVAC equipment must be GFCI protected, per NEC 210.8.

A GFCI protected outlet located within 25-feet of the HVAC equipment will make it unnecessary for the servicing technician to use an extension cord that could be plugged into an outlet without GFCI protection.

Review Question Section 5.5

34. Which of the following installations of a 15- or 20- amp, 125 volt HVAC equipment outlet is improper?