Where: T lighting = center tapped transformer servicing the single-phase load. This transformer is commonly referred to as the Lighting transformer. T power = two remaining transformers serving transformers. 1 𝜙𝜙 𝑘𝑘𝑘𝑘 𝑘𝑘 = the total estimated single-phase load. 3 𝜙𝜙 𝑘𝑘𝑘𝑘 𝑘𝑘 =the total estimated three-phase load. Notes: 1) In the above equations, many utilities will three-phase load. These transformers are commonly referred to as the Power size the Lighting transformer for 2/3 the single-phase load plus 1/3 the three-phase load. Sizing the Lighting transformer for full single-phase load plus 1/3 three-phase load allows margin for unbalanced single-phase loading. 2) When single-phase loads are larger, the Lighting transformer may be larger than the power transformers. However, due to circulating currents, the Lighting transformer kVA rating should not be more than twice (2x) the Power transformer kVA rating.
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