Transformer Secondary Conductor Calculator

Inputs

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The breaker or fuse protecting the transformer primary.
A
Line-to-line.
V
Line-to-line.
V
From the transformer secondary terminals to the first overcurrent device.
m
The computed load the secondary conductors serve.
A
Rating of the single breaker or fuse set the conductors land on. 0 if there is none (this disqualifies the 25-ft and outside paths).
A
Nameplate secondary FLA (kVA ÷ (√3 × V_s) for three-phase). Used by the industrial 25-ft path; 0 to skip.
A

Results

Default result
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Required conductor ampacity(I_min)
200A
Pass

Governing: 240.21(C)(2) — 10 ft rule. Other eligible paths: 240.21(C)(6).

Take this to the ampacity table of the adopted code — no table is embedded here.

Also computed

Primary OCPD ÷ ratio (referred)(I_p·r)253.8A

The primary device seen from the secondary side — the basis of the 1/10 and 1/3 provisions.

10 ft rule minimum200A

max(calculated load, termination device, 1/10 × referred primary OCPD)

25 ft rule minimum200A

max(1/3 × referred primary OCPD, the single termination OCPD)

Method notes 4 notes
  • Rule structure per NEC 240.21(C) (verified against two agreeing walkthroughs); the adopted edition and the AHJ govern. NO ampacity table is embedded — take the required ampacity to the adopted code's table with its correction factors.
  • Each path carries installation conditions beyond ampacity: raceway/physical protection, termination in the named device(s), and for (C)(4) outside runs, the disconnecting-means location rules.
  • (C)(2) conductors must not extend beyond the equipment they supply; (C)(6) requires the single termination OCPD rated no more than the conductor ampacity.
  • The governing path shown is the least-restrictive eligible one — any other listed path is equally code-compliant if its conditions suit the installation better.

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