Transformer Sizing Calculator
Size a dry-type transformer from the load: kVA, primary and secondary full-load current, and the NEC 450.3(B) overcurrent protection each side needs. Built for Phoenix commercial work and the APS and SRP service application.
How it’s calculated. Full-load amps: single-phase FLA = (kVA × 1000) ÷ V; three-phase
FLA = (kVA × 1000) ÷ (√3 × V). The tool picks the next standard transformer kVA at or above your load, then sizes
overcurrent protection from NEC Table 450.3(B) (transformers 1000 V or less): primary-only protection is 125%
of primary FLA (≥9 A), 167% (2–9 A), or 300% (<2 A); with secondary protection the primary may go to 250%
and the secondary to 125% (≥9 A) or 167% (<9 A). The 125% values round up to the next standard device (240.6); the 167/250/300% values
are ceilings that round down. Suggested conductors are the smallest 75°C copper carrying 125% of FLA.
Buck-boost. A buck-boost is a small insulating transformer field-connected as an autotransformer, so it only transforms the voltage
change — buck-boost kVA = (load amps × ΔV) ÷ 1000. That’s why a ~1.3 kVA unit can serve a 10 kVA
load when boosting 208 V to 240 V. Three-phase uses a bank of single-phase units — two in open-delta on a 3-wire source, three in wye on a
4-wire source. Always confirm the selected unit’s winding-current rating on the manufacturer’s selection chart.
Low-voltage / control (12–24 V). Control transformers are sized to the sealed (holding) VA of everything energized at once, and must also supply the inrush VA when coils pick up without the secondary voltage sagging below pickup — the tool combines them as √(sealed² + inrush²) and flags the manufacturer regulation check. Lighting / resistive 12–24 V loads are sized at the connected VA plus 25% spare; watch secondary voltage drop on long low-voltage runs.
This is a planning aid, not a stamped design. For commercial transformer, feeder, and service sizing — with the studies and the install — our engineering team handles it as part of design-build. Verify every result against the current NEC and your local amendments.
Need it engineered and stamped?
Specifying a transformer or service for a project? Our Phoenix commercial electricians size the gear, run the studies, and self-perform the install.
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Read the field guide
The how-to behind this calculator, written by our field team and referenced to the NEC.
How to Wire a Three-Phase Transformer →
480V to 208Y/120V: primary and secondary connections, grounding, and the overcurrent protection the sizing above feeds.
How to Do a Commercial Load Calculation →
Where the kVA you entered here comes from: NEC Article 220 demand factors, step by step.
All Field Guides →
Thirty-plus NEC-referenced how-to guides from our Phoenix crews.
Transformer sizing and the utility application in the Valley
In metro Phoenix the transformer question and the utility question arrive together. To open a commercial new-service file, APS wants the official street address and legal description, the structure type and square footage, the estimated load, the voltage and phase you want, and site plans showing the desired meter location. The utility then engineers its system up to your meter from that information — which means the kVA and voltage you settle on here are what the application is built from.
That application should open the same week the design does. Neither APS nor SRP publishes a committed lead time for commercial new service, and the two utilities are genuinely different: APS runs on an Arizona Corporation Commission tariff with a codified feasibility test and a five-year refund window; SRP is self-governed and its contribution in aid of construction is generally non-refundable. Your city does not tell you which one serves you, and Mesa runs its own municipal utility in the middle of it.
Service voltage is a lead-time decision, not just an electrical one
The secondary voltage you choose here sets the gear, and the gear sets the schedule. Confirm the available service voltage with the serving utility before you commit the transformer size, because a design built around one voltage and a service that arrives at another is redesign, not adjustment. Service and distribution covers how we run that coordination.
Which code edition applies
Phoenix, Mesa, Chandler, Glendale and Buckeye are on the 2023 NEC; Scottsdale is on the 2020; unincorporated Maricopa County is still on the 2017. For Gilbert, Peoria and Tempe we confirm the adopted edition with the jurisdiction before design starts. Article 450 protection rules are consistent across those editions.
Need the load study, one-line and utility application done as a set? The engineering page covers what goes into it, and the form at the bottom gets it priced.
Frequently asked questions
What does APS need from me to start a commercial service application?
The official street address and legal description, the structure type and square footage, the estimated load, the voltage and phase you want, and site plans showing the desired meter location. The kVA you size here feeds the estimated load. APS then engineers its system up to your meter from that information.
Which utility serves my project, APS or SRP?
Your city will not tell you. SRP's own service list describes Phoenix, Scottsdale, Glendale, Peoria and Avondale as served in whole or in part, while APS serves parts of each; the boundary runs through neighbourhoods and, in places, individual streets. Mesa also runs its own municipal electric utility over part of downtown. Confirm with the serving utility for the specific parcel.
When should I open the utility application?
The same week design starts. Neither APS nor SRP publishes a committed lead time for commercial new service, and every week the application sits unopened is a week added to the end of the job.
Ready to price your Phoenix project?
Send us the plans, the scope, or just the problem. We will tell you what it takes. Call (602) 396-5216 or request a bid online.