The National Electrical Code requirements for conductor sizing and overcurrent protection have always been confusing and complex. Factors that must be consider include:

  1. Continuous loads
  2. Terminal temperature ratings
  3. Conductor insulation
  4. Conductor ampacity
  5. Special application
  6. System voltage

NEC Section 240-3 requires the branch circuit, feeder, and service conductors to be protected against overcurrent in accordance with their ampacities as specified in Table 310-16. However, Section 240-3 contains twelve rules that modify the general requirement and permit the conductors not to be protected in accordance with their ampacities, they include:

  1. Power Loss Hazard
  2. Devices Rated 800 Amperes or Less
  3. Tap Conductors
  4. Motor-Operated Appliance Circuit Conductors
  5. Motor and Motor-Control Circuit Conductors
  6. Phase Converter Supply Conductors
  7. Air-Conditioning and Refrigeration Equipment Circuit Conductors
  8. Transformer Secondary Conductors
  9. Capacitor Circuit Conductors
  10. Electric Welder Circuit Conductors
  11. Remote-Control, Signaling, and Power-Limited Circuit Conductors
  12. Fire Alarm System Circuit Conductors

With so many different Code rules that modify the general requirements, it does become overwhelming to a circuits conductor and overcurrent protection device. However the following steps and examples should help you understand the basic rules of conductor sizing and protection.

Step 1 - Size the overcurrent protection device in accordance with Sections 210-20(a), 215-3, and 384-16(d). These three NEC rules required the overcurrent protection device (breaker or fuse) be sized no less than 100% of the noncontinuous load, plus 125% of the continuous load. Section 240-6(a) contains the list of standard size overcurrent protection devices.

Step 2 - Select the conductor to comply with Sections 110-14(c), 210-19(a), 215-2, and 230-42(a). Sections 210-19(a), 215-2 and 230-42(a) required the conductor to be sized no less than 100% of the noncontinuous load, plus 125% of the continuous load. In addition, Section 110-14(c) requires a consideration of the temperature rating of the equipment terminals when sizing conductors. Section 110-14(c) requires the circuit conductors to be sized according to the 60°C column of Table 310-16 for equipment rated 100 amperes and less, unless marked otherwise, and equipment rated over 100 amperes must be sized to the 75°C column of Table 310-16.

Step 3 - The selected conductor must be protected against overcurrent in accordance with Section 240-3. Section 240-3. This requires the branch circuit, feeder, and service conductors be protected against overcurrent in accordance with their ampacities as specified in Table 310-16. However, Section 240-3(b) permits "the next size up device" if the conductors are not part of a multioutlet branch circuit supplying receptacles, and the ampacity of the conductors does not correspond with the standard ampere rating of a overcurrent protection fuse or a circuit breaker as listed in Section 240-6(a), and the next higher standard rating selected does not exceed 800 amperes.

Branch Circuit Continuous Load Example: What size branch-circuit overcurrent protection device and conductor (THHN) is required for a 19 kVA of nonlinear loads (75°C terminals). The branch-circuit is supplied by a 208/120 volt, 4-wire, 3-phase, Wye connected system.

Step 1 - Size the overcurrent protection device in accordance with Sections 210-20(a) and 384-16(d). The first thing that we must do convert the nonlinear load from kVA to amperes:

Amperes = VA/(Volts x 1.732), Amperes = 19,000/(208 volts x 1.732), Amperes = 52.74 amperes, rounded to 53 amperes

Step 2 - Select the conductor to comply with Sections 110-14(c) and 210-19(a). Section 210-19(a) also requires the branch-circuit conductor to be sized no less than 125% of the continuous load, 53 amperes x 125% = 66 amperes. We must select the conductor according to the 75°C terminals temperature rating of the equipment terminals. No. 6 THHN has a rating of 65 amperes at 75°C and can not be used, therefore we must select a No. 4 which has a rating of 85 amperes at 75°C.

Step 3 - The No. 4 THHN conductor must be protected against overcurrent in accordance with Section 240-3. We must verify that the No. 4 THHN is properly protected against overcurrent by the 70 ampere overcurrent protection device. Since we have more than three current-carrying conductors in the same raceway, we must correct the No. 4 THHN conductors ampacity as listed in the 90°C column of Table 310-16. Corrected Ampacity No. 4 THHN = Ampacity x Note 8(a) Adjustment Factor Corrected Ampacity No. 4 THHN = 95 amperes x 80% Corrected Ampacity No. 4 THHN = 76 amperes

Feeder Continuous Load Example: What size feeder overcurrent protection device and conductor (THHN) is required for a 184 ampere continuous load on a panelboard (75°C terminals) that supplies nonlinear loads. The feeder is supplied by a 4-wire, 3-phase, wye connected system.

Step 1 - Size the overcurrent protection device in accordance with Sections 215-3 and 384-16(d). The feeder overcurrent protection device must be sized not less than 125% of 184 amperes, 184 amperes x 125% = 230 amperes. According to Section 240-6(a) we must select a minimum 250 ampere overcurrent protection device.

Step 2 - Select the conductor to comply with Sections 110-14(c) and 215-2. Section 215-2 also requires the feeder conductor to be sized no less than 125% of the continuous load, 184 amperes x 125% = 230 amperes. We must select the conductor according to the 75°C temperature rating of the panelboards terminals. No. 4/0 THHN has a rating of 230 amperes at 75°C.

Step 3 - The No. 4/0 conductor must be protected against overcurrent in accordance with Section 240-3. We must verify that the No. 4/0 THHN conductor is properly protected against overcurrent by the 250 ampere overcurrent protection device. Since we have more than three current-carrying conductors in the same raceway, we must correct the No. 4/0 THHN conductors ampacity as listed in the 90°C column of Table 310-16. Corrected Ampacity No. 4/0 THHN = Ampacity x Note 8(a) Adjustment Factor Corrected Ampacity No. 4/0 THHN = 260 amperes x 80% Corrected Ampacity No. 4/0 THHN = 208 amperes

The No. 4/0 THHN which is rated 208 amperes after ampacity correction is not considered protected by a 250 ampere overcurrent protection device. This is because "the next size up rule" in Section 240-3(b) would only permit a 225 ampere protection device on the 208 ampere conductor [240-6(a)]. Therefor we must increase the conductor size to 250 kcmil in order to comply with the overcurrent protection rules of Section 240-3.


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