STANDARDS & FORMULAS

Generator Sizing

The generator for the emergency loads you tick, sized step by step; the biggest step decides.

Standards and regulations

ISO 8528-1
Generating sets - application, ratings and performance
The kinds of rating (ESP, PRP, LTP, COP), the 0.8 power factor and the 70 % average load.
IEEE 446
Emergency and standby power systems (Orange Book)
The voltage dip when the largest motor starts.
NFPA 20
Stationary fire pumps
A fire pump’s controller dips 15 % at most as it starts.

Formulas

1 · Running load

P = Σ P(demand) × diversity · S = P / pf · R1 = max(P / 0.8, S) × (1 + spare)

2 · Non-linear loads (UPS, drives)

R2 = (max(P / 0.8, S) + (k − 1) × S(non-linear)) × (1 + spare)

k = how many times the non-linear kVA is counted

3 · Largest motor start (voltage dip)

R3 = S(start) × X′d × (1 − ΔV) / ΔV

S(start) = the motor’s running kVA × its starting multiple (DOL, star-delta, soft starter, VFD)
X′d = the alternator’s transient reactance (0.15 – 0.25 as a rule)
ΔV = the dip allowed (15 – 20 % as a rule)

4 · Engine while the motor starts

R4 = (P − P(motor) + S(start) × pf(start)) / 0.8

5 · Average load (ESP / PRP)

R5 = P × load factor / (0.70 × 0.8)

Site derating and the set

Required kVA = max(R1 … R5) / (Ft × Fa) → the next standard rating

Ft = heat derating, Fa = altitude derating (the maker’s figures)

Design rules it applies

Its current, breaker, cable, ATS and fuel worked out
Put on the board it feeds and drawn in the single line through an ATS
The calculation as a PDF report

For reference. MO.tool works these out to help your design; the engineer remains responsible for it. Always check the edition of each standard your project uses and the requirements of the local authority (DEWA, ADDC, Civil Defence…).

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