Appropriate sizing for 75kw Genset Output Breaker/Cable

A

ahoyhoy

Member
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?
 
jim dungar said:
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.
Click to expand...
Vessel is subject to either ABYC or CFL, but they are pretty darn similar to NEC for cable sizing/ derating.

I would like to design the system for the full load since there are some heavy motor loads for refrigeration components and lots of heaters on board. Coast guard limits us to 115% OCPD on generators.
 
Birken Vogt said:
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?
Click to expand...
Nope, no output breaker, and not adjustable.
Actual load could get as high as 200A intermittently I believe (lots of heaters, a cooking stove, and compressor and circ pumps for refrigeration.)
Cable will be 600v 105c stranded copper cable.
 
Standard breakers, listed to UL489, are going to have terminations for 75°C and Class B stranding.
Decades ago I sold 'standard' breakers that were also Listed as marine duty. However, I cannot remember what made them different.
 
So when you said cable, that confused the issue.

Cable has a specific definition, something with a jacket and multiple wires inside most often.

MTW/THHW/TEW/AWM are wire insulation and not a type of cable. You probably have them in conduit.

In that case I would just use the table found in 310.
 
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