# Boat Generator Selection

The boat generator comprises the diesel engine and the attached AC alternator. AC alternator rating selection must consider a number of factors. A total expected load analysis must also be undertaken to calculate the peak loads that might be encountered.  Starting current values may be as high as 5–9 times that of the actual normal running current. The in-rush current at starting causes these high currents, and is the energy required to overcome bearing and load inertia in electric motors, winding magnetization and so on. The actual duration of the peaks is typically less than one second, and most alternators are designed to withstand 250% overloads for nominal periods.

## Boat Generator Alternators

Boat Generator Rating Calculation. From a load analysis, the following calculations can be performed to estimate required minimum alternator size:

a. Generator to run at 80% maximum load with 20% reserve.

Largest Single Load Value = 2400 watts

Max. Start Current (Ir) = Power (watts) = 2400 = 12.50 amps

Volts x Pf = 240 x 0.8

Max.=  Start Current (Is) =    Ir x 4 = 50 Amps

b.  If alternator can withstand overloads of 250% for 10 sec., the starting current (Is) must be divided by 2.5 = 20 Amps.

c.   Rating is therefore 20 x 240 = 4.8 kVA or 0.8 x 22.8 x 240 = 3.8kW. Adding a 20% minimum margin to operate at 80% = 4.5kW.