# Transformer Calc NEC — NEC Art.450 kVA & OCPD

> NEC Art.450 kVA & OCPD (iPhone/iPad app by Rizvan Adzhoian.)

- Source: https://appshunter.io/ios/app/transformer-calc-nec/id6763311943 (this page in markdown: same URL + `.md`)
- Developer: [Rizvan Adzhoian](https://appshunter.io/developer/1891746079)
- Category: Utilities, Business
- Price: $0.99 with in-app purchases
- Age rating: 17+
- Requires: iOS 13.0 · 32 MB
- Languages: American English
- Released: 2026-04-24
- Data updated: 2026-08-21
- User reviews in markdown: https://appshunter.io/ios/app/transformer-calc-nec/id6763311943/reviews.md

## What is Transformer Calc NEC?

NEC transformer overcurrent protection calculator for electricians, apprentices, and electrical engineers. Size primary and secondary OCPD per NEC Table 450.3(B), calculate full-load amps (FLA), and select conductors from Table 310.16 - all per NEC 2023 (NFPA 70). Works 100% offline on the job site.

Whether you're sizing overcurrent protection for a 75 kVA, 480-to-208V three-phase transformer, or checking if primary-only protection requires 125% or 300%, this calculator gives you accurate NEC-compliant results in seconds. Use it as your go-to transformer protection reference - from kVA rating to OCPD and conductor selection.

TRANSFORMER CALCULATOR FEATURES:
- Primary overcurrent protection per NEC 450.3(B) - 125%/167%/250%/300% rules
- Secondary overcurrent protection per NEC 450.3(B) - 125%/167%/250% rules
- Primary-only and primary+secondary protection methods
- Full-load ampere (FLA) calculation: single-phase and three-phase
- Next-standard-size-up applied only where Note 1 permits (125% cells); 167%/250%/300% treated as ceilings
- All standard OCPD ratings per NEC 240.6(A), including fuse-only ratings 1, 3, 6, 10, 601 for small transformers
- Conductor sizing from NEC Table 310.16 (75°C column, printed code values)
- Copper and aluminum conductor support
- 40 standard kVA ratings: 0.05–500 kVA single-phase, 3–2500 kVA three-phase
- 7 voltage presets: 120V, 208V, 240V, 277V, 460V, 480V, 600V
- Single-phase and three-phase support
- NEC article references for every calculation result

NEC REFERENCES INCLUDED:
- NEC Table 450.3(B) - Transformer overcurrent protection (600V or less)
- NEC 450.3(B) Note 1 - Next-size-up rule for primary-only protection
- NEC 240.6(A) - Standard overcurrent device ratings
- NEC Table 310.16 - Conductor ampacity at 75°C

PROTECTION METHOD RULES (printed Table 450.3(B)):
- Primary Only, 9A or more: 125% maximum — next standard size up permitted (Note 1)
- Primary Only, 2–9A: 167% maximum — ceiling, no round up
- Primary Only, under 2A: 300% maximum — ceiling, no round up
- Primary + Secondary: primary 250% maximum (ceiling); secondary 125% (Note 1) for 9A or more, 167% ceiling under 9A

CALCULATION RESULTS INCLUDE:
- Primary full-load amps (FLA) with formula
- Secondary full-load amps (FLA) with formula
- Primary OCPD size (standard rating per NEC 240.6(A))
- Secondary OCPD size when applicable
- Primary and secondary conductor sizes with ampacity
- Applicable NEC percentage rules and references
- Protection method notes and explanations

PRO FEATURES (one-time purchase):
- No ads - clean interface for professionals
- Save transformer calculation history with labels
- Export calculations as PDF reports for permits and records
- Dark theme support

HOW THE TRANSFORMER CALCULATOR WORKS:
1. Select phase type (single-phase or three-phase)
2. Choose kVA rating from standard sizes
3. Select primary and secondary voltage
4. Choose protection method (primary only or primary + secondary)
5. Select conductor material (copper or aluminum)
6. Get OCPD sizes, conductor recommendations, and NEC references instantly

BUILT FOR ELECTRICIANS ON THE JOB SITE:
This transformer calculator is designed by electricians, for electricians. Enter your transformer parameters, get the correct OCPD size and conductor recommendation instantly - no complicated menus or unnecessary features. One-hand operation, instant results.

PERFECT FOR:
- Licensed electricians sizing transformer protection daily
- Electrical apprentices studying for journeyman exams
- Electrical engineers and designers
- Electrical contractors bidding jobs
- NEC code exam preparation
- Journeyman and master electrician exam study

OFFLINE & PRIVACY:
- Works 100% offline - no internet needed on the job site
- No account required
- No data collection - your calculations stay on your device

Simplify your transformer protection sizing on the job site.

## Pricing and in-app purchases

Base price: $0.99.

**One-time purchases**

| Purchase | Price | Type |
| --- | --- | --- |
| Pro Unlock | $2.99 | one-time |



## Version history (last 5 releases)

### 1.0.10 — 2026-07-10

What's new:
• Secondary conductor sizing now follows NEC 240.21(C): the conductor is sized so the secondary breaker never exceeds its ampacity — no next-size-up on the secondary (e.g. a 75 kVA 208V secondary is 350 kcmil, not 300)
• Large transformers that exceed a single conductor now show parallel sets per NEC 310.10(H), e.g. "2 × 600 kcmil", instead of a blank result
• More precise three-phase current (exact √3)

Thanks for using ElecTools! Send feedback to support@tradetools.dev.

### 1.0.9 — 2026-07-09

What's new:
• Conductor sizing now coordinates with the OCPD per NEC 240.4(D): a 25A breaker can no longer land on #12 copper — small conductors are up-sized to match the breaker (e.g. 15 kVA primary → #10)
• Primary and secondary conductors sized to 125% of FLA per NEC 215.2 (continuous)
• Aluminum small-conductor limits applied correctly

Thanks for using ElecTools! Send feedback to support@tradetools.dev.

### 1.0.8 — 2026-06-10

What's new:
• 167%/300% rules now follow the printed Table 450.3(B): ceilings without round-up (Note 1 applies to 125% cells only)
• Fuse-only ratings 1, 3, 6, 10, 601 per NEC 240.6(A) — correct OCPD for small transformers
• Conductor table corrected to the printed NEC 310.16 75°C values
• 600V preset added; 40 standard kVA ratings listed

Thanks for using ElecTools! Send feedback to support@tradetools.dev.

### 1.0.7 — 2026-05-18

What's new:
• Moved to tradetools.dev — new umbrella site for all our trade calculators
• Updated support email: support@tradetools.dev
• Refreshed privacy & support pages with unified design
• Bug fixes and stability improvements

Thanks for using ElecTools! Send feedback to support@tradetools.dev.

### 1.0.6 — 2026-04-29

What's new:
• Moved to tradetools.dev — new umbrella site for all our trade calculators
• Updated support email: support@tradetools.dev
• Refreshed privacy & support pages with unified design
• Bug fixes and stability improvements

Thanks for using ElecTools! Send feedback to support@tradetools.dev.

## More apps by Rizvan Adzhoian

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- [Breaker Size Calc NEC](https://appshunter.io/ios/app/breaker-size-calc-nec/id6763312865)

All apps by Rizvan Adzhoian: https://appshunter.io/developer/1891746079

## Related topics

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*Data collected daily from the US App Store and indexed by [AppsHunter](https://appshunter.io/). User reviews are verbatim App Store reviews. Ratings, prices and chart positions refresh continuously; this snapshot is from 2026-08-21.*