# How to size proper Transformer (3phase quesiton)

I have a question regarding the sizing of a proper transformer. I have a source voltage and load. See the following:

Primary Source: 480VAC 100A (3phase) Secondary: 415 VAC (3phase) Load: 2 parallel AC-DC units 10kW, Power Factor = 0.8 (ea).

How do you go about sizing a proper transformer to interface between the source and the load?

You haven't said if your loads are motors or whether your voltages are line or phase. I'll assume voltages are line.

$P_T = 20kW$

$$P_T = \sqrt {3}\ V_{Line}\ I_{Line}\ cos \ \theta$$

$$I_{Line} = \frac {P_T}{\sqrt {3}\ V_{Line}\ cos \ \theta}= \frac {20kW}{\sqrt {3} \times 415V \times 0.8} = 34.78A$$

$$S = kVA = \sqrt {3}\ V_{Line}\ I_{Line} = \sqrt {3}\times 415V \times 34.78A = 25kVA$$

So you need a 25kVA three-phase transformer or slightly larger. If your 10kW loads are motors, add 20% for starting.

Reference sites say If motors are started more than once per hour, increase minimum transformer KVA by 20%.

Transformer Basics

Sizing Transformers

• Hi @Stainless SteelRat, thank you for your input. My load is AC-DC battery chargers. In reference to derating factor would it be necessary to apply a temperature derating factor? – cylus May 12 '17 at 2:49
• Not the Transformers. But the wire would have to be sized for transformer losses, safety, temperature, etc. – StainlessSteelRat May 12 '17 at 3:11

So from what I remember $$P = \sqrt3 V\ I\ PF\$$ and this gives just under 18A for each of your loads. This means there is sufficient supply for only one running at any time.

Choosing the transformer will also need considerations as to starting load - usually different to running load and other things.