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May 1, 2001 12:00 PM, By James Stallcup, Jr., NEC and OSHA Consultant

Sizing Tap OCPDs - Art. 240

Choose the best answer:

What size OCPD is required for a tap, less than 10 ft in length, from a junction box that is supplied by a feeder circuit with a 500A OCPD?

(a) 30A (c) 50A

(b) 40A (d) 60A
What size OCPD is required for a tap, over 10 ft but not more than 25 ft in length, that originates at a junction box supplied by a feeder circuit with a 500A OCPD?

(a) 100A (c) 150A

(b) 125A (d) 175A
What is the maximum size secondary OCPD required for a tap up to 25 ft in length (including transformer) that is supplied from a feeder circuit with a 300A OCPD that has a 480V primary and 208V secondary?

(a) 200A (c) 300A

(b) 250A (d) 400A
What size OCPD is required for a tap that travels 25 ft vertically and 75 ft horizontally in a high-bay manufacturing building, served from a junction box that is supplied by a feeder circuit with a 300A OCPD?

(a) 100A (b) 125A

(c) 150A (d) 175A

answers and discussion

(c), 50A. Tap conductors less than 10 ft long and terminated at a panel do not require an OCPD at the point of the tap. However, you must protect it with conduit, tubing, or metal gutter, and size it to carry the total load.

Step 1: Calculate the minimum size tap per Sec. 240-21(b)(1)(d).
A=500A OCPDö 10=50A

Step 2: Based on Table 310-16, 50A requires a No. 8 THWN cu. conductor.

Step 3: Select OCPD based on the ampacity of the conductor per Sec. 240-21(b)(1)(d). (50A is equal to 50A.)
(50A× 10=500A)

Solution: Select OCPD per Secs. 240-3(b) and 240-21(b)(1)(d). A 50A conductor requires a 50A OCPD.

Stallcup's Code Loop: Secs. 240-3(b), 240-6(a), and 240-21(b)(1); Table 310-16.
(d), 175A. Tap conductors more than 10 ft in length but less than 25 ft do not require an OCPD at the point of the tap. However, you must install an OCPD at the end of the tap; protect the conductor with conduit, tubing, or metal gutter; and properly size it to carry the total load. The ampacity of the tap conductors must also be at least ⅓ the rating of the OCPD.

Step 1: Calculate the minimum size tap per Sec. 240-21(b)(2)(a).
500A OCPD×⅓ = 166.7A

Step 2: According to Table 310-16, 166.7A requires a No. 2/0 cu. conductor.

Solution: Select OCPD per Secs. 240-3(b) and 240-21(b)(2)(a). A No. 2/0 cu. conductor requires a 175A OCPD.

Stallcup's Code Loop: Secs. 240-3(b), 240-6(a), and 240-21(b)(2); Table 310-16.
(c), 300A. Tap conductors for a transformer (primary plus secondary) longer than 10 ft in length but less than 25 ft do not require an OCPD at the point of the tap. You must meet the following requirements:
- Ampacity of tap conductor must be at least ⅓ that of the feeder's OCPD.
- Ampacity of secondary conductor must be at least ⅓ that of the feeder's OCPD, based on the transformer's primary-to-secondary ratio.
- Total length of conductors, primary plus secondary, must not be more than 25 ft.
- Secondary conductors must terminate at a suitably sized fuse or circuit breaker.
Step 1: Calculate the minimum size secondary tap per Sec. 240-21(b)(3)(a). Determine the transformer ratio and multiply by the load (3 is the number of phases):
480V÷208V÷3×300A=231A

Step 2: Based on Table 310-16, 231A requires a 250kcmil conductor.

Solution: Select OCPD for secondary tap per Secs. 240-3(b), 240-6(a), and 240-21(b)(3)(a). A 250kcmil conductor allows a 300A OCPD.

Stallcup's Code Loop: Secs. 240-3(b), 240-6(a), and 240-21(b)(3); Table 310-16.
(a), 100A. If the tap is in a high-bay manufacturing building and is no longer than 25 ft horizontally or 75 ft vertically, you can place the protection device at the end of the tapped conductors. However, the total run must be 100 ft or less. You must also meet the following requirements:

Ampacity of the tap conductors must be at least ⅓ the rating of the OCPD protecting the feeder conductors.
Tap conductors must terminate at a single circuit breaker or set of fuses. This single overcurrent device can supply any number of additional OCPDs on its load side.
Tap conductors are sized No. 6 cu. or No. 4 aluminum, or larger.

Step 1: Calculate the minimum size tap per Sec. 240-21(b)(4)(c).
300A OCPD×⅓=100A

Step 2: According to Table 310-16, 100A requires a No. 3 cu. conductor.

Solution: Select OCPD for tap per Secs. 240-3(b) and 240-21(b)(4). A No. 3 cu. conductor requires a 100A OCPD.

Stallcup's Code Loop: Secs. 240-3(b), 240-6(a), and 240-21(b)(4); Table 310-16.