Appropriate sizing for 75kw Genset Output Breaker/Cable

A

ahoyhoy

New User
Location
Olympic Peninsula, WA
Occupation
Marine Electrician
Hello all!

I am a marine electrician based in Washington State, working on a massive genset compared to my typical installations. I'm usually working with 12kw 120/240v gensets, so to be working with a 75kw 120/208y 3 phase gen is a little intimidating, and I want to make sure I'm sizing everything appropriately.

I used the following formula to calculate the output breaker: FLA = (kW × 1000) / (Voltage × Power Factor × √3)
75000 / (208 x .8 x 1.73) = 260.53

For this type of vessel, we can only use 115% of the Full Load Rating for the genset overcurrent protection, leaving me with a 300A breaker.

The cable is run in open air in the engine room of the boat, and is rated to 105c. The genset is only about 10' from the breaker, and most calculators agree I'm looking at 3/0 cables for the output. Never having worked with AC loads this large, this seems freakishly huge to me, but 300A is a lot after all... What is really tripping me up though is that for the old genset that I am replacing, it looks like the installer only ran 1AWG to the 300A breaker, which is making me second-guess myself.
 
At first in the calculating formula of rated current the efficiency factor is missing. I appreciate efficiency as 0.92-0.93. So, in my opinion, the rated current may be 283 A.If the breaker rating has to be 1.15 times the rating the breaker rating may be 325 A.
According to NEC Art.110.14B b if the terminal of the breaker it is not 90oC rated then 75oC may be the limited temperature. If we are in NEC world then the appropriate conductor according to 310.16 is 400 kcmils copper or 600 kcmils aluminum.
 
I didn't think the NEC applied to the internal wiring of a vessel subject to the ABS rules?

Do you need to have the full rating of the generator? I have seen load calcs that fell between two source sizes. It used to be common to use 200A OCPD on the output of 75kVA 208V transformers. Giving up a few amps was deemed a more acceptable design than upsizing equipment.
 
Last edited:
ahoyhoy said:
Hello all!

I am a marine electrician based in Washington State, working on a massive genset compared to my typical installations. I'm usually working with 12kw 120/240v gensets, so to be working with a 75kw 120/208y 3 phase gen is a little intimidating, and I want to make sure I'm sizing everything appropriately.

I used the following formula to calculate the output breaker: FLA = (kW × 1000) / (Voltage × Power Factor × √3)
75000 / (208 x .8 x 1.73) = 260.53

For this type of vessel, we can only use 115% of the Full Load Rating for the genset overcurrent protection, leaving me with a 300A breaker.

The cable is run in open air in the engine room of the boat, and is rated to 105c. The genset is only about 10' from the breaker, and most calculators agree I'm looking at 3/0 cables for the output. Never having worked with AC loads this large, this seems freakishly huge to me, but 300A is a lot after all... What is really tripping me up though is that for the old genset that I am replacing, it looks like the installer only ran 1AWG to the 300A breaker, which is making me second-guess myself.
Click to expand...

Does the generator already have an output breaker?

Is it adjustable?

What is the actual amount of load?

What type of cable will you be using?
 
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