How to use this calculator
- Refer the primary device. Primary OCPD × (Vp/Vs) — the basis of the 1/10 and 1/3 provisions.
- Find the eligible paths. Length, location and secondary configuration decide which of the five 240.21(C) routes apply.
- Meet the governing floor. The calculator names the least-restrictive eligible rule and its required ampacity.
- Pick the conductor from the adopted table. No ampacity table here — apply your code edition’s table with its correction and adjustment factors.
How it works
Overcurrent protection does not reach through a transformer — the turns ratio re-scales the amps — so secondary conductors live under their own rule set, 240.21(C), with five routes out:
(C)(1) 2-wire/Δ-Δ · (C)(2) ≤10 ft: ≥ max(load, device, I_p·r/10) · (C)(6) ≤25 ft: ≥ I_p·r/3 · (C)(3) industrial · (C)(4) outside
This screen evaluates every eligible path and names the governing one. The primary device itself comes from the transformer kVA calculator's sizing, the load from the Article-220 style math in the service load calculator, and the conductor's real-world check downstream is the voltage drop calculator.
Worked example
Verified against the live calculator
A 75 kVA 480:208 transformer with a 110 A
primary breaker feeds a panel through 8 ft of
conductor landing on a single 200 A main:
referred primary = 110 × 2.31 = 254 A · (C)(2) floor = max(180, 200, 254/10) = 200 A
The 10-ft rule governs at 200 A — the termination breaker, not the 180 A load or the 25.4 A one-tenth provision, sets the floor, which is the common pattern for panel feeds. Stretch the run to 15 ft and the 25-ft rule takes over at the same 200 A (its one-third floor is 84.6 A); stretch it past 25 ft inside, and no rule applies — the design changes, not the wire size.
Frequently asked questions
Why can’t the primary breaker protect transformer secondary conductors?
Because amps transform: a 110 A primary device at 480:208 looks like 254 A from the secondary side, and for anything but a 2-wire (or delta-delta 3-wire) secondary, an unbalanced fault can draw damaging secondary current the primary device never sees as excessive. That is why 240.21(C) treats secondary conductors as tap conductors with their own rules.
What is the 10-foot rule for transformer secondaries?
Secondary conductors up to 10 ft may run unprotected at their supply end if their ampacity is at least the largest of: the calculated load, the rating of the device they terminate in, and 1/10 of the primary OCPD referred through the voltage ratio — in a raceway, terminating at the supplied equipment.
What is the 25-foot rule for transformer secondaries?
Up to 25 ft, conductor ampacity must be at least 1/3 of the primary OCPD times the primary-to-secondary voltage ratio, and the run must land on a single breaker or fuse set rated no more than the conductor ampacity. For the 110 A / 480:208 example that floor is 84.6 A — though the termination device usually governs.
What if the secondary run is longer than 25 feet?
Inside a building, no tap rule covers it (outside conductors get unlimited length under (C)(4), and supervised industrial sites get their own 25-ft variant): protect the conductors with an overcurrent device at the transformer secondary, or re-plan the run.
Method & assumptions
- NEC 240.21(C) rule structure (verified 2026-06-11 against two agreeing published walkthroughs); the adopted edition and the AHJ govern. No ampacity table is embedded — outputs are required minimums to take to the adopted code's tables with their correction factors.
- Each path carries installation conditions beyond ampacity (raceway/physical protection, termination devices, outside-run disconnect rules) — summarized in the result notes, satisfied in the field.
- The governing path shown is the least-restrictive eligible route; any other listed path is equally compliant where its conditions fit better.
- (C)(5) outside feeder taps of separately derived systems and engineering-supervision variants beyond (C)(3) are outside this screen.