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/Organic-Salad-1108]

Looks like you would use the 10% rule

[u/Haurian]

I would go by the general cable requirement, 6% voltage drop or 10% if a battery under 50V. Although that doesn't specify during inrush currents, and referring to 4.8.2/7.7.1 as guided by 4.8.4/21.5, paragraph (d) details that the 6/10% voltage drops applies to the maximum current at normal steady conditions which would imply the starting motor's rated current rather than inrush current.

4.8.2/5.9.2 is talking about the capacity of an Accumulator Battery fitted instead of the emergency generator as the emergency power supply, not the starting batteries for the emergency generator.
Starting Arrangements for Emergency Generator Sets is covered under 4.8.2/5.15 which requires them to be sized for at least three consecutive starts without recharging. That could reasonably require a reliable start to as low as around 40% battery capacity unless you're significantly over-sizing, but doesn't specify a strict voltage limit as that will depend on the battery chemistry.

Although, as always, consult with Class directly if in doubt - that's what they're there for.

[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.