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Tap rule calculator

Choose the tap scenario and enter the feeder protection, the tap conductor's ampacity, and its length. The calculator checks the length and ampacity tests and lists any that fail.

Built on the 2026 NEC · Free, no sign-up · Updated October 2026

Rating of the OCPD ahead of the tap. For a transformer secondary tap, the OCPD protecting the transformer primary.

Ampacity of the tap conductor after any derating.

From the splice to the device.

Transformer secondary tap only. The single OCPD where the secondary conductors terminate, 240.21(C)(6)(2).

Transformer secondary tap only. 240.21(C)(6)(1) uses the primary-to-secondary voltage ratio.

Transformer secondary tap only.

The result updates as you type.

Result

✓ Tap is compliant

How it was calculated

  1. 240.21(B)(1): tap conductors ≤ 10 ft.
  2. 240.21(B)(1)(4): tap ampacity ≥ feeder OCPD ÷ 10 = 40.00 A.
  3. All checks pass.

Assumptions

  • Evaluating the 10-ft-feeder-tap scenario per 240.21.
  • This tool checks length and the 1/10 rule only. 240.21(B)(1)(4) applies the 1/10 rule to field installations where the tap leaves the enclosure or vault; the tap ampacity must also be at least the calculated load and the rating of the equipment or OCPD it supplies (240.21(B)(1)(1)), and the enclosure/raceway conditions in (B)(1)(2)–(3) must be met.

Sections used

240.21

These are the sections the calculator applies. Look them up in your code book before you rely on the result, and check the code your AHJ has adopted.

The 10 ft tap rule

Under 240.21(B)(1), a tap no longer than 10 ft can be protected by the feeder's overcurrent device if all of these are true:

  • The tap's ampacity is at least the calculated load and at least the rating of the device or equipment it supplies.
  • The tap doesn't extend beyond the switchboard, panelboard, disconnect, or control device it supplies, and it's enclosed in a raceway from the tap point to that equipment.
  • For field installations where the tap leaves the enclosure it's made in, the tap's ampacity is at least one-tenth of the feeder overcurrent device rating.

The 25 ft tap rule

Under 240.21(B)(2), a tap up to 25 ft needs an ampacity of at least one-third of the feeder overcurrent device. The tap ends at one breaker or one set of fuses sized to hold the load to the tap's ampacity, and the tap needs physical protection.

Transformer secondary conductors up to 25 ft

Under 240.21(C)(6), secondary conductors up to 25 ft need an ampacity of at least one-third of the primary overcurrent device times the primary-to-secondary voltage ratio. They have to end in a single overcurrent device that is no larger than their ampacity.

What the calculator checks

The calculator checks the length and ampacity tests, which are the parts with numbers. The enclosure, raceway, and physical-protection conditions are up to you on site.

Worked example

An 8 ft tap with 100 A ampacity from a feeder protected at 400 A:

  • Tap scenario: 10-ft feeder tap
  • Feeder OCPD (amps): 400
  • Tap conductor ampacity (amps): 100
  • Tap length (ft): 8
  • Primary voltage (volts): 480
  • Secondary voltage (volts): 208

Result

✓ Tap is compliant

How it was calculated

  1. 240.21(B)(1): tap conductors ≤ 10 ft.
  2. 240.21(B)(1)(4): tap ampacity ≥ feeder OCPD ÷ 10 = 40.00 A.
  3. All checks pass.

Assumptions

  • Evaluating the 10-ft-feeder-tap scenario per 240.21.
  • This tool checks length and the 1/10 rule only. 240.21(B)(1)(4) applies the 1/10 rule to field installations where the tap leaves the enclosure or vault; the tap ampacity must also be at least the calculated load and the rating of the equipment or OCPD it supplies (240.21(B)(1)(1)), and the enclosure/raceway conditions in (B)(1)(2)–(3) must be met.

Sections used

240.21

These are the sections the calculator applies. Look them up in your code book before you rely on the result, and check the code your AHJ has adopted.

Questions electricians ask

What is the 10 ft tap rule?

A feeder tap up to 10 ft long can skip overcurrent protection at the tap point if it meets the conditions in 240.21(B)(1). Those include an ampacity of at least one-tenth of the feeder device for field taps that leave the enclosure.

What is the 25 ft tap rule?

Under 240.21(B)(2), a tap up to 25 ft needs at least one-third of the feeder device's rating in ampacity, and it has to end in a single overcurrent device that limits the load to that ampacity.

How do I measure tap length?

From the point where the tap connects to the feeder to where it ends at the overcurrent device or equipment.

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How this calculator was built

The calculator runs the same code as the Ask Shorty iPhone app. Automated tests check it against hand-worked answers, and every section it names is checked against the 2026 NEC text. It shows section numbers, not code text. Read the sections in your own code book and confirm which edition your AHJ enforces.

How we test the calculators. See something wrong? Tell us and we will check it against the code.

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