Short answer: Conductors are normally protected by an overcurrent device at the point where they receive their supply (NEC 240.21). Tap rules are the listed exceptions. A feeder tap of not over 10 feet needs an ampacity at least equal to the calculated load and to the device it feeds, and at least one-tenth of the feeder breaker if it is a field tap leaving the enclosure. A tap of not over 25 feet needs at least one-third of the feeder breaker rating and must end in a single breaker or set of fuses. Each rule has more conditions, so read the whole rule before you size a tap.
PSI lists feeder overcurrent protection in the Feeders subject area of the Texas Journeyman exam: 3 scored NEC Knowledge questions and 3 scored Calculations questions. PSI does not say how many cover tap rules alone. The Texas Journeyman Practice Pack has 1,000 original questions and 10 timed mock exams in the real two-part format, with step-by-step solutions for every calculation. Try free practice questions without signing up. Get the full pack for US$39 with one payment.
Tsai Electro is an independent learning site. Not affiliated with TDLR, PSI or NFPA. We explain the ideas in our own words and do not reproduce NEC text. Check every rule and table value in your own 2026 NEC, and follow your supervisor and the local inspector.
Why tap rules exist
A tap is a smaller conductor connected to a larger feeder, protected only by the feeder’s overcurrent device. On its own, that breaks the basic rule that every conductor is protected at its ampacity. The tap rules allow it in limited cases: short lengths, a protective device at the end, and physical protection. The shorter the tap, the smaller it may be compared with the feeder breaker.
The main feeder tap rules
| Rule | Maximum length | Minimum tap ampacity | Key conditions |
|---|---|---|---|
| 10-foot tap (NEC 240.21(B)(1)) | 10 ft | At least the calculated load and the rating of the device supplied by the tap; for a field-installed tap leaving the enclosure, also at least 1/10 of the feeder device rating | Ends at a device; raceway enclosure where the rule requires it |
| 25-foot tap (NEC 240.21(B)(2)) | 25 ft | At least 1/3 of the feeder device rating | Ends in a single breaker or one set of fuses that limits the load to the tap ampacity; protected from physical damage |
| High-bay manufacturing tap (NEC 240.21(B)(4)) | 25 ft horizontal, 100 ft total | At least 1/3 of the feeder device rating, with a minimum conductor size | Only in high-bay manufacturing buildings with qualified maintenance |
| Outside tap (NEC 240.21(B)(5)) | No length limit stated | Ends at one overcurrent device that limits the load | Outdoors except at the termination; protected from damage |
Worked example 1: a 25-foot tap from a 300 A feeder
A 300 A feeder breaker. A 20-foot tap runs to a single breaker feeding a panelboard.
- Minimum tap ampacity: 300 ÷ 3 = 100 A.
- The tap must end in a single breaker or set of fuses rated not more than the tap ampacity.
- A conductor with at least 100 A ampacity, ending in a 100 A breaker, meets the one-third test. The tap must also carry the calculated load and meet the rule’s other conditions. Check the conductor ampacity in Table 310.16 with the correct temperature column and any adjustment factors.
Worked example 2: a 10-foot field tap
The same 300 A feeder. An 8-foot field-installed tap leaves the enclosure and feeds a 60 A fused disconnect.
- Ampacity at least the device supplied: 60 A (the calculated load must also be covered).
- Ampacity at least 1/10 of the feeder device: 300 ÷ 10 = 30 A.
- The larger of the two is 60 A. That is the minimum only if the calculated load is not over 60 A and the other conditions are met.
Transformer secondary taps
Conductors on the secondary of a transformer have their own rules (NEC 240.21(C)).
- Two-wire single-phase secondary. In some cases the primary overcurrent device can protect the secondary conductors, if its rating does not exceed the secondary ampacity times the secondary-to-primary voltage ratio (NEC 240.21(C)(1)). This does not apply to three-phase delta-wye or multiwire secondaries.
- Outside secondary conductors. Outside conductors that end at one overcurrent device are allowed without a stated length limit (NEC 240.21(C)(4)).
- 25-foot secondary tap. The secondary conductor ampacity must be at least the primary-to-secondary voltage ratio times one-third of the primary overcurrent device rating (NEC 240.21(C)(6)). It must end in a single breaker or set of fuses.
Worked example 3. A 480 V to 208 V transformer has a 90 A primary breaker. The secondary conductors run 15 feet to a breaker.
- Voltage ratio: 480 ÷ 208 = 2.31.
- One-third of the primary breaker: 90 ÷ 3 = 30 A.
- Minimum secondary ampacity under the one-third test: 2.31 × 30 = 69.2 A. The conductors must also end in a single breaker or set of fuses that limits the load to their ampacity, and meet the rule’s other conditions.
The next size up, and where it stops
When a conductor’s ampacity does not match a standard breaker size (NEC 240.6(A)), the next higher standard size is allowed if the device is 800 A or less and the circuit is not a multioutlet branch circuit for cord-and-plug portable loads (NEC 240.4(B)). Tap rules use their own limits, so do not apply the next size up to a tap unless the rule allows it. Small conductors such as 14, 12 and 10 AWG copper have their own maximum overcurrent ratings (NEC 240.4(D)).
Check yourself
- What is the minimum ampacity of a 25-foot tap from a 400 A feeder breaker? 400 ÷ 3 = 133.3 A.
- Where is a conductor normally protected? At the point where it receives its supply.
- A 10-foot field-installed tap leaves the enclosure of a 200 A feeder device and ends at a 15 A device, with a calculated load under 15 A. Which governs, 15 A or 20 A? 20 A, one-tenth of 200 A.
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Find 240.21 faster on exam day
NFPA 70, National Electrical Code, 2026 Edition (softcover). Highlight the length and fraction in each feeder and secondary tap rule in 240.21, so each length and fraction is easy to find. The Texas exam is open book. PSI allows a soft-bound NEC with highlighting, underlining and notes made in advance. Loose-leaf, spiral-bound and ring-bound copies are not allowed, and only permanent index tabs made or supplied by the NEC publisher are permitted; homemade and third-party tabs are not.
FAQ
What is the 10-foot tap rule?
A feeder tap of not over 10 feet can be protected by the feeder device if its ampacity is at least the calculated load and the rating of the device it feeds. A field-installed tap leaving the enclosure also needs at least one-tenth of the feeder device rating.
What is the 25-foot tap rule?
A feeder tap of not over 25 feet needs an ampacity of at least one-third of the feeder device rating, must end in a single breaker or set of fuses that limits the load to its ampacity, and must be protected from physical damage.
Can I use the next size up breaker on a tap?
The next size up rule covers conductors protected at their ampacity. Tap rules set their own limits, so follow the specific tap rule.
Are tap rules on the Texas Journeyman exam?
PSI lists feeder overcurrent protection in the Feeders subject area: 3 scored NEC Knowledge questions and 3 scored Calculations questions.
Sources checked October 2026
- PSI Candidate Information Bulletin, Texas electrician examinations (updated September 15, 2026): https://test-takers.psiexams.com/api/content/bulletin/1682
- NFPA 70, National Electrical Code, 2026 Edition, 240.4, 240.6(A) and 240.21
