Transformer sizing and loading
Preview Version 0.1.0How many kVA a load needs, and how heavily a transformer of the rating you are considering would carry it.
Apparent power needed
498.8kVA
The load needs 498.8 kVA, which is 99.8% of a 500 kVA transformer.
NIST SP 811 · IEEE C57.91Self-checked · Hand-computed cases for single-phase, three-phase, and amps from kVA match to 0.1 kVA, and 498.8 kVA converts back to 600 A within 0.1 A. No freely published worked sizing example has been found to register. What this means
Screening estimate: the utility or an engineer sizes the transformer for real. · Self-checked only: no independent check has been run yet · Example values · Review pending Report a problem
How we got this
Method
Apparent power is √3 × V line-to-line × A ÷ 1,000 for three-phase service and V × A ÷ 1,000 for single-phase, or kW ÷ power factor. Loading is kVA ÷ rating, and headroom is rating − kVA.
Steps
- Load: √3 × 480 V × 600 A ÷ 1,000 = 498.8 kVA (three-phase).
- Current at 480 V three-phase: 600 A.
- Loading on a 500 kVA transformer: 498.8 ÷ 500 = 99.8%, with 1.2 kVA of headroom.
- This tool names no standard size: the standard distribution transformer ratings are in IEEE C57.12.00, a paid standard we neither buy nor quote, so enter the rating you are considering. It also does not compute overload time, hot-spot or top-oil temperature, or loss of life; those come from the loading guide and the owner's practice.
Assumptions
- Three-phase voltage is line-to-line, and the load is balanced across phases.
- A load given in kW uses the power factor entered.
Sources
- NIST Special Publication 811, 2008, Section 8: apparent power (volt ampere) and its relation to voltage and current
- IEEE C57.91, Guide for Loading Mineral-Oil-Immersed Transformers, and IEEE C57.96 for dry-type transformers, named only; not quoted or applied, Overload duration, hot-spot temperature, and loss of life
Known limits
- Names no standard rating; the standard list is in a paid IEEE standard.
- Does not compute permissible overload time, hot-spot or top-oil temperature, or loss of life.
- Assumes a balanced three-phase load; unbalanced loads need each phase checked.