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Figure 01
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For EC&M Magazine
By Mike Holt, NEC® Consultant
Here's the follow-up to yesterday's newsletter.
This includes the answers to the questions sent, so you can see how you did.
Note: The answers to these questions are based on the 2017 NEC.
Underlined text indicates a change in the rule for the 2017 NEC.
Q1. What is the Code requirement for sizing the overcurrent protection of conductors?
A1.[240.4] Except as permitted by (A) through (G), conductors must be protected against overcurrent in accordance with their ampacity after ampacity correction and adjustment as specified in 310.15 [240.4].
(A) Power Loss Hazard. Conductor overload protection isn’t required, but short-circuit overcurrent protection is required where the interruption of the circuit will create a hazard; such as in a material-
handling electromagnet circuit or fire pump circuit.
(B) Overcurrent Protection Devices Rated 800A or Less. The next higher standard rating of overcurrent protection device listed in 240.6 (above the ampacity of the ungrounded conductors being protected) is permitted, provided all of the following conditions are met:
(1) The conductors aren’t part of a branch circuit supplying more than one receptacle for cord-and-plug-connected loads.
(2) The ampacity of a conductor, after the application of ambient temperature correction [310.15(B)(2)(a)], conductor bundling adjustment [310.15(B)(3)(a)], or both, doesn’t correspond with the standard rating of a fuse or circuit breaker in 240.6(A).
(3) The overcurrent protection device rating doesn’t exceed 800A.
Author’s Comment:
•
This “next size up” rule doesn’t apply to feeder tap conductors [240.21(B)] or transformer secondary conductors [240.21(C)].
(C) Overcurrent Protection Devices Rated Over 800A. If the circuit’s overcurrent protection device exceeds 800A, the conductor ampacity, after the application of ambient temperature correction [310.15(B)(2)(a)], conductor bundling adjustment [310.15(B)(3)(a)], or both, must have a rating of not less than the rating of the overcurrent protection device defined in 240.6.
(D) Small Conductors. Unless specifically permitted in 240.4(E) or (G), overcurrent protection must not exceed the following after any ambient temperature and number of conductors have been applied:
Author’s Comment:
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The general rule for common circuit conductor and overcurrent protection sizes, such as 14 AWG with 15A protection, 12 AWG with 20A protection, and 10 AWG with 30A protection [240.4(D)(5), (6), and (7)] are often used but there are many conditions where the overcurrent protection is safely permitted to exceed the ampacity of the conductors as specified in 240.4(E) or (G).
(1) 18 AWG Copper—7A
(2) 16 AWG Copper—10A
(3) 14 AWG Copper—15A
(4) 12 AWG Aluminum/Copper-Clad Aluminum—15A
(5) 12 AWG Copper—20A
(6) 10 AWG Aluminum/Copper-Clad Aluminum—25A
(7) 10 AWG Copper—30A
(E) Tap Conductors. Tap conductors must be protected against overcurrent as follows:
(1) Household Ranges, Cooking Appliances and Other Loads, 210.19(A)(3) and (4)
(2) Fixture Wire, 240.5(B)(2)
(3) Location in Circuit, 240.21
(4) Reduction in Ampacity Size of Busway, 368.17(B)
(5) Feeder or Branch Circuits (busway taps), 368.17(C)
(6) Single Motor Taps, 430.53(D)
(F) Transformer Secondary Conductors. The primary overcurrent protection device sized in accordance with 450.3(B) is considered suitable to protect the secondary conductors of a 2-wire (single voltage) system, provided the primary overcurrent protection device doesn’t exceed the value determined by multiplying the secondary conductor ampacity by the secondary-to-primary transformer voltage ratio.
Example: What’s the minimum secondary conductor size required for a 2-wire, 480V to 120V transformer rated 1.50 kVA with 60ºC terminals. Figure 01
Solution:
Primary Current = VA/E
VA = 1.5 kVA x 1,000 = 1,500 VA
E = 480V
Primary Current = 1,500 VA/480V
Primary Current = 3.13A
Primary overcurrent protection [450.3(B)] = 3.13A × 1.67
Primary overcurrent protection = 5.22A or 5A Fuse
Secondary conductor must have an ampacity no less than five times the primary protection ampere rating.
Secondary Conductor Ampacity = Primary Protection Ampacity × (Primary Voltage/Secondary Voltage)
Secondary Conductor Ampacity = 5A × 480V/120V
Secondary Conductor Ampacity = 5A × 4 times
Secondary Conductor Ampacity = 20A
Answer: 12 AWG rated 20A at 60°C [Table 310.15(B)(16)].
(G) Overcurrent Protection for Specific Applications. Overcurrent protection for specific equipment and conductors must comply with the requirements referenced in Table 240.4(G).
Air-Conditioning and Refrigeration [Article 440]. Air-conditioning and refrigeration equipment, and their circuit conductors, must be protected against overcurrent in accordance with 440.22.
Author’s Comment:
• Typically, the branch-circuit ampacity and overcurrent protection size is marked on the equipment nameplate [440.4(A)].
Air-Conditioning Example: What size branch-circuit overcurrent protection device is required for an air conditioner when the nameplate indicates the minimum circuit ampacity is 23A, with maximum overcurrent protection of 40A?
Solution: Table 240.4(B) directs us to Article 440 Parts III and VI but the nameplate values for listed air-conditioning and refrigeration equipment can be used to size the branch-circuit conductors and short-circuit protection device size [110.3(B)]. No other calculations are needed.
Answer: 10 AWG, 40A protection, Table 310.15(B)(16), 110.14(C)(1)(a), 10 AWG rated 30A at 60°C
Author’s Comment:
• Air-conditioning and refrigeration nameplate values are calculated by the manufacturer according to the following:
- Branch-Circuit Conductor Size [440.32]
18A × 1.25 = 22.50A, 10 AWG rated 30A at 60ºC [110.14(C)(1)(a), Table 310.15(B)(16)]
- Branch-Circuit Overcurrent Protection Size [440.22(A)]
18A × 2.25 = 40.50A, 40A maximum overcurrent
protection size [240.6(A)]
- Motors [Article 430]. Motor circuit conductors must be protected against short circuits and ground faults in accordance with 430.52 and 430.62 [430.51]
If the nameplate calls for fuses, then fuses must be used to comply with the manufacturer’s instructions [110.3(B)].
Motor Example: What size branch-circuit conductor and overcurrent protection device (circuit breaker) is required for a 7½ hp, 230V, three-phase motor with 60ºC terminals?
Solution: Table 240.4(G) directs us to several parts of Article 430.
Step 1: Determine the branch-circuit conductor size [Table 310.15(B)(16), 430.22, and Table 430.250]:
FLC = 22A [Table 430.250]
22A × 1.25 = 28A [240.6(A)]
Step 2: Determine the branch-circuit overcurrent protection size [240.6(A), 430.52(C)(1) Ex 1, and Table 430.250].
Inverse Time Breaker: 22A × 2.50 = 55A
Answer: 10 AWG rated 30A at 60°C branch-circuit conductor [110.14(C)(1)(a), Table 310.15(B)(16)] with a 60A (next size up) overcurrent protection device [240.6(A)].
Motor Control [Article 430]. Motor control circuit conductors must be sized and protected in accordance with 430.72.
Remote-Control, Signaling, and Power-Limited Circuits [Article 725]. Remote-control, signaling, and power-limited circuit conductors must be protected against overcurrent in accordance with 725.43.
Q2.What is the NEC requirement for overcurrent protection of flexible cords and fixture wires?
A2. Flexible cord must be protected by an overcurrent protection device in accordance with its ampacity as specified in Table 400.5(A)(1) or Table 400.5(A)(2) [240.5(A)]. Fixture wires must be protected against overcurrent in accordance with their ampacity as specified in Table 402.5. Supplementary overcurrent protection, as discussed in 240.10, can provide this protection.
(B) Branch-Circuit Overcurrent Protection.
(1) Cords for Listed Appliances or Luminaires. If flexible cord is used with a specific listed appliance or luminaire, the conductors are considered protected against overcurrent when used within the appliance or luminaire listing requirements.
Author’s Comment:
• The NEC only applies to premises wiring, not to the supply cords of listed appliances and luminaires.
(2) Fixture Wire. Fixture wires can be tapped to the following circuits:
(1) 20A–18 AWG, up to 50 ft of run length
(2) 20A–16 AWG, up to 100 ft of run length
(3) 20A–14 AWG and larger
(3) Extension Cord Sets. Flexible cord used in listed extension cord sets is considered protected against overcurrent when used within the extension cord’s listing requirements.
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