SettingsSettings for this calculationUS
The sum of the wattage of everything the transformer will serve.
Add up the rated wattage of all connected equipment, panels, or buildings the transformer feeds.
How efficiently the load uses apparent power — 1.0 is purely resistive, motors and electronics are typically lower.
0.8 is a common general-purpose assumption for a mixed residential/light-commercial load when the actual value isn't known.
Recommended transformer size
15 kVA
This is a planning estimate — final transformer sizing for a real installation should be confirmed by an electrical engineer or your utility, accounting for load diversity, future growth, and inrush current from large motors.
- Minimum required capacity
- 12.5 kVA
They open the calculator with your figures already in it
Transformer Sizing Calculator: 15 kVA — shown in imperial, US market. The link sets both, so the result they see is the one on your screen.
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How this was calculated
Formula source(s)
- Required kVA = connected watts / 1,000 / power factor; standard distribution transformer sizes commonly include 5, 10, 15, 25, 37.5, 50, 75, 100, 150, and 225 kVA
Inputs used
- Total Connected Load (W)
- 10000
- Power Factor
- 0.8
Intermediate steps
- Minimum required capacity
- 12.5 kVA
Confidence note: This is a planning estimate — final transformer sizing for a real installation should be confirmed by an electrical engineer or your utility, accounting for load diversity, future growth, and inrush current from large motors.
What this calculation does not cover
- This is a watts-to-kVA conversion of the connected load, not a code load calculation. It applies no demand or diversity factors and no continuous-load multiplier, so it does not produce the load figure an inspector or the serving utility will ask for, and it does not size the primary or secondary overcurrent protection, conductors or grounding that the transformer still needs.
- No headroom for growth is included. The result is the smallest listed standard size that covers today's connected load exactly, so if you want the spare capacity the FAQ describes you have to add it to the load figure yourself before entering it.
- Motor starting is not modelled. Inrush and locked-rotor current can dip the voltage unacceptably on a transformer that is entirely adequate for the running load, and sizing for a large motor, welder or compressor start is a separate calculation this does not perform.
- Phase, voltage, impedance and derating are outside the model. It never asks single- or three-phase, and the standard-size list it selects from is a generic distribution series — three-phase dry-type units are commonly built in sizes that are not on it, and nameplate kVA is a rating at a stated ambient and altitude that this makes no adjustment for.
- One power factor is applied to the entire load. A mixed installation has a different power factor on each branch, and if the figure you have is already in VA or kVA rather than real watts, dividing it by power factor again oversizes the answer.
Add the equipment this sizes
This result is a specification — 15 kVA — not a quantity. Put the thing it sizes into your project: how many, what you call it, and your supplier’s price.
Computed in your browser — nothing you enter is uploaded. Presented in US customary units and US trade terminology. Where a formula follows a published standard, that standard and its edition are cited beside it on this page; where none governs, the page says so. Local amendments override model codes — verify against the code in force where you build.
Sources checked 2026-09-02 · in the site-wide review of 2026-09-06 · v1.0.1
Regulatory standards & verification citations1
- Required kVA = connected watts / 1,000 / power factor; standard distribution transformer sizes commonly include 5, 10, 15, 25, 37.5, 50, 75, 100, 150, and 225 kVA
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