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Transformer Full Load Current Rating

Industrial and commercial electrical engineering design in Alberta

A 300 kVA, pad-mounted transformer outside of a new commercial building in Fort McMurray, Alberta. This powers up a restaurant and 3 small stores in a strip mall.

To determine the size of transformer for your business, you need to discover the total current required to run your heating, cooling and specific user equipment. From there we can determine the transformer full load current rating to determine the size of electrical service your business needs.

One of the first steps we determine is if we have 1-phase or 3-phase supply in the neighbourhood and apply the appropriate formula:

Single Phase:

transformer kva formula for 1-phase

Three Phase:

transformer kva formula for 3-phase

Both formulas follow the basics of electrical engineering for power calculations:

Power voltage current formula

The kVA of the transformer should be equal to or greater than the kVA of the load.

How Much Load Can A Transformer Support?

During the design process of a commercial building, the architect will ask your electrical consultant how much space is needed for the electrical room, long before the owner even knows how much equipment he or she is going to need for their business.

And that is when we use our electrical engineering judgement to estimate a transformer that will more than adequate for your equipment but not oversized to free up funds for other construction costs.

We re-arrange the above formulas to determine the full load current rating of the secondary side of the transformer to get:

Single Phase

Transformer full rated current load formula for 1 phase.

Three Phase

Transformer full rated current load formula for 3 phase.

As an example, if we estimate a 300 kVA, 120/208V, 3-Phase transformer for a commercial building, it can provide:

Transformer full rated current load example for 3 phase.

To start our preliminary electrical design, we would estimate a 300 kVA transformer, enough to run two restaurants with fully loaded kitchens. With this information, the architect can design around the estimated electrical requirements and the mechanical consultant can then provision for the required HVAC equipment.

A final electrical load calculation can then be finished when the owner provides a complete equipment list. We can downsize the transformer at that time to save even more costs while still providing a powerful electrical source that's compliant to code.

Some general rules of thumb:

We do not size the transformer for the maximum connected load but for realistic peak power levels. Doing so, we 'right-size' the electrical equipment and save on equipment, installation and conductor costs.


For industrial buildings, transformer selection is a bit more complicated because industrial buildings have more motors.

Electric motors are one of the largest consumers of electricity and the connection and starting sequence are very important to ensure safety and longevity of your expensive equipment.

For commercial applications, we use these current calculations every day. So much so, we save time and use our instant transformer full load current rating cheat sheet for quick reference. This is electrical engineering.