Conductor%20Sizing%20and%20the%20National%20Electrical%20Code
  NEC Rules - Conductor Sizing


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.

    Authorís Comment. These important Code rules were added to the 1993 and 1996 NEC and are often overlooked. The purpose of these new rules are to insure that the heat generated on the equipment terminals can be properly dissipated without damaging the conductors. For all practical purposes, most electrical equipment is design to accept conductors 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.

    Authorís Comment. The ampacity of a conductor is itís current rating in amperes that it can carry continuously, after applying conductor ampacity reduction factors for conductor bundling and ambient temperature. In addition, the 1996 clarified in Section 110-14(c), that the ampacity reduction of THHN (90°C) conductor is based on the conductors ampacity as listed in the 90°C column of Table 310-16 and not on the terminal temperature rating.

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

    The branch-circuit overcurrent protection device must be sized not less than 125% of 53 amperes, 53 amperes x 125% = 66 amperes. According to Section 240-6(a) we must select a minimum 70 ampere overcurrent protection device.

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

    The No. 4 THHN which is rated 76 amperes after ampacity correction is properly protected by a 70 ampere overcurrent protection device in compliance with the general requirements of Section 240-3.

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.


Free Commercial/Industrial Wiring and Raceway Chart

If you have any questions or comments regarding this article, please let me know, mike@mikeholt.com.

Call Toll Free 1-888 NEC CODE

Mike Holt Enterprises, Inc. 7310 West McNab Road #201 Tamarac, Florida 33321
Our office hours - 8:30 am (sometimes earlier) to 5:00 PM (sometimes later) Eastern standard time.
  Go to top of page
Newsletter Registration   |   Stay Connected:
 

888.NEC.CODE (632.2633) 3604 PARKWAY BLVD, STE 3, LEESBURG FL 34748  

Tell a Friend About This Site

  NEC® and National Electrical Code® are registered trade marks
of the National Fire Protection Association (NFPA).
  ©Copyright 2011 Mike Holt Enterprises, Inc.