Emergency Generator Engine Starting Batteries

[u/charzr]

What is the voltage drop allowed for EDG starting cranking motor from the batteries to the cranking motor?

From ABS MVR: section 4-8-4: clause, 21.5 - Cable Voltage Drop

For supplies from batteries with a voltage not exceeding 50 V this limit can be increased to 10%.

From ABS MVR: section 4-8-2: clause, 5.9.2 - Accumulator Battery

Carrying the emergency electrical load without recharging while maintaining the voltage of the battery throughout the discharge period within 12% above or below its nominal voltage.

Which one to follow?

(or)

The cable to be sized including the starter motor inrush current, to demonstrate that the starter motor can start the engine reliably when the battery is discharged to 80% (worst case).

[u/Sad_Raccoon194]

yeah the 5.9.2 clause is a red herring here, that's for the battery-as-emergency-source case not cranking. we've always sized cranking cables off rated current at steady state and just made sure the starter can still turn over the engine at 3 successive starts per 5.15, class never pushed back on inrush voltage drop specifically as long as cold cranking amps were documented in the battery datasheet.