Services, feeders and transformers
Master candidates are tested on the equipment that feeds a building: service conductors, feeders, taps, transformers and the fault current that equipment must withstand. Most questions combine one sizing rule with a table lookup.
A timed 100-question exam in the ICC R16 format on the 2023 NEC, or the two-part Texas exam. Every answer has a worked explanation and you get a score by topic.
Or start with the free services, feeders and transformers drillSizing feeder and service conductors
- Continuous loads count at 125% and noncontinuous loads at 100% when you size the conductors and the overcurrent device (215.2(A)(1), 215.3, 230.42(A)).
- For a dwelling service or main power feeder of 100 to 400 A that carries the whole dwelling load, the conductors may have an ampacity of 83% of the service rating (310.12).
- Check the termination temperature: equipment rated 100 A or less uses the 60°C column unless marked otherwise, and larger equipment the 75°C column (110.14(C)).
Feeder taps
- Ten-foot tap: the tap conductors must carry the load they serve and be at least as large as the device they end in. Field-installed taps leaving the enclosure need at least one-tenth of the feeder overcurrent device rating (240.21(B)(1)).
- Twenty-five-foot tap: the tap conductors need at least one-third of the feeder overcurrent device rating and must end in a single device that limits the load to their ampacity (240.21(B)(2)).
Transformer overcurrent protection: Table 450.3(B)
| Protection method | Current | Maximum rating |
|---|---|---|
| Primary only | 9 A or more | 125% (next standard size up permitted) |
| Primary only | 2 A to less than 9 A | 167% |
| Primary only | Less than 2 A | 300% |
| Primary and secondary | Primary | 250% |
| Primary and secondary | Secondary 9 A or more | 125% (next standard size up permitted) |
| Primary and secondary | Secondary less than 9 A | 167% |
Practice service and transformer questions
A timed 100-question exam in the ICC R16 format on the 2023 NEC, or the two-part Texas exam. Every answer has a worked explanation and you get a score by topic.
Or start with the free services, feeders and transformers drillWorked example: 112.5 kVA transformer
112.5 kVA, 480 V delta primary, 208Y/120 V secondary
- Primary current = 112,500 ÷ (1.732 × 480) = 135.3 A.
- Primary-only protection: 135.3 × 125% = 169.1 A, so the next standard size, 175 A, is permitted.
- Secondary current = 112,500 ÷ (1.732 × 208) = 312.3 A.
- With primary and secondary protection, the secondary device may be 312.3 × 125% = 390.4 A, so 400 A.
Available fault current
Equipment must have an interrupting rating at least equal to the current available at its line terminals (110.9), and non-dwelling service equipment must be field marked with the available fault current and the date it was calculated (110.24).
For a transformer with unlimited primary supply, the maximum fault current at the secondary is the full-load current divided by the per-unit impedance. A 500 kVA, 208 V transformer with 2% impedance has a full-load current of 1,387.9 A, so about 69,400 A of fault current.
Common questions
What is the 83% rule for dwelling services?
For a dwelling service or main power feeder rated 100 to 400 A that carries the entire dwelling load, the conductors may be sized at 83% of the service rating (310.12). A 200 A service needs 166 A of ampacity, which is 2/0 copper in the 75°C column.
Can I round a transformer primary breaker up to the next size?
Yes for primary-only protection when the primary current is 9 A or more: 125% is the limit and Note 1 to Table 450.3(B) permits the next higher standard rating.
Who must mark the available fault current?
The installer marks it on service equipment in other than dwelling units, with the date of the calculation (110.24).
Practice service and transformer questions
A timed 100-question exam in the ICC R16 format on the 2023 NEC, or the two-part Texas exam. Every answer has a worked explanation and you get a score by topic.
Or start with the free services, feeders and transformers drillMore master exam guides
- Master electrician load calculations
- Grounding and bonding for the master exam
- Motor calculations (Article 430)
- Special occupancies and emergency systems
- Hazardous (classified) locations
- Master electrician practice test
- ICC Master Electrician Practice Test (R16)
- Texas Master Electrician Practice Test (TDLR)
Section numbers follow the 2023 National Electrical Code (NFPA 70); rules are paraphrased, not quoted. Independent preparation, not affiliated with NFPA, ICC, PSI, Pearson VUE or any state board.