Rules for Sizing branch-circuit conductors and overcurrent protection
by Gerald Newton
October 29, 1999
There are many rules in the 1999 National Electrical Code for calculating loads, determining the correct size conductors, and determining the correct overcurrent protection. Simply put, conductors shall be sized to carry the load without overheating, and the conductors and equipment shall be protected from overcurrent to prevent overheating. Additionally, although not addressed here, this overcurrent protection usually provides ground-fault and short circuit protection.
The general rules for branch circuits as found in Section 210 of the NEC can be divided into these parts:
- Sizing conductors
- Selecting the overcurrent protection
- Selecting conductors and overcurrent protection based on temperature limitations.
Sizing the Conductors
Selecting the overcurrent protection
- The basic rule: The ampacity of branch-circuit conductors must not be less than the load to be served. (Section 210-19(a).
- The continuous load rule: The allowable ampacity of branch circuit conductors before the application of any derating factors (for higher ambient temperature and for more than three current carrying conductors in a raceway or cable) shall be equal to or greater than the noncontinuous load plus 125 per cent of the continuous load.
- The continuous allowable ampacities of conductors in cables and raceways are given in Table 310-16.
- These ampacities may have to be reduced for over three current carrying conductors in a cable or raceway or for ambient temperatures higher than 30 degrees centigrade (86 degrees F.) using Table 310-15(b)(2)(a) and the ambient derating factors located at the bottom of Table 310-16.
- Rating of Branch Circuits: Branch Circuits are rated by the maximum permitted ampere rating of the overcurrent device protecting the circuit. The rating for other than individual branch circuits shall be 15, 20, 30, 40, or 50 amperes. (210-3)
Selecting conductors and overcurrent protection based on temperature limitations.
- Where a branch circuit supplies a continuous load or a combination of a noncontinuous load and a continuous load the RATING of the overcurrent device shall not be less than the noncontinuous load plus 125 per cent of the continuous load.
- Apply the rules in Section 240-3 when selecting the overcurrent protection for conductors. This section includes rules for selecting the next higher overcurrent devices for devices rated 800 amperes or less.
Some important Definitions
- The temperature rating of a conductor for the selected ampacity shall be selected and coordinated so as not to exceed the lowest temperature rating of any connected termination, conductor, or device. Conductors with temperature ratings higher than specified for terminations and equipment shall be permitted to be used for derating purposes.
- Termination provisions of equipment for circuits rated 100 amperes or less, or marked for Nos. 14 through 1 conductors shall be used with conductors rated 60 degrees C or with conductors with higher temperature ratings provided that after derating the conductors are not used a higher than the 60 degree C. ampacity.
- Termination provisions of equipment for circuits rated over 100 amperes, or marked for conductors larger than No. 1 shall be used with 75 degree C. conductors or with conductors with higher temperature ratings provided that after derating the conductors are not used a higher than the 75 degree C. ampacity.
- Separately installed pressure connectors shall be used with conductors at the ampacities not exceeding the ampacity of the conductor at its listed insulation temperature rating.
Exceptions and Tricky Rules
- Branch Circuit: The circuit conductors between the final overcurrent device protecting the circuit and the outlet(s).
- Individual Branch Circuit: A branch circuit that supplies only utilization equipment.
- Continuous Load: A load where the maximum current is expected to continue for 3 hours or more.
- Outlet: A point on the wiring system at which current is taken to supply utilization equipment.
- Rule for using higher temperature ratings: Conductors with higher temperature ratings are permitted to be used at the higher ampacity ratings if the equipment is LISTED and IDENTIFIED for use with such conductors.
- Exception for continuous loads: Where the overcurrent devices protecting the branch circuit's conductors and the assembly are LISTED for operation at 100 percent of its rating, the ampere rating of the overcurrent device(s) shall be permitted to be not less than the sum of the continuous load plus the noncontinuous load.
- Small Conductors: Unless specifically permitted in Section 240-3 the overcurrent protection for No. 14 copper conductors shall not exceed 15 amperes, for No. 12 copper shall not exceed 20 amperes, and for No. 10 copper shall not exceed 30 amperes. For aluminum and copper-clad aluminum the overcurrent protection shall not exceed 15 amperes for No. 12 and 25 amperes for No. 10. These limitations apply after any correction factors for ambient temperature and number of conductors have been applied.
- Flexible Cords shall be protected by an overcurrent device in accordance with the ampacities as given in Table 400-5(A) and (B).
- Fixture Wires shall be protected by an overcurrent device in accordance with the ampacities as given in Table 402-5.
- Tap Conductors shall be permitted to be protected in accordance with Sections 210-19(d), 240-21, 364-11, 364-12, and 430-53(d).
- Table 210-24 is a summary of additional branch-circuit requirements that especially apply to receptacles, lamp holders, and permissible loads.
- Also, Section 210-2 lists 33 other Articles and Sections in the Code that address specific purpose branch circuits that supply various equipment.
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