marina service sizing

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need some help with a load calc in a marina. have a system with 48 ea 30 amp outlets and 21 20 amp outlets resulting in a total of 1830 calculated amps. Using the table provided the demand factor multiplier is .40 resulting in a figure of 732 amps. As this is a 120/ 240 single phase system would i then divide by 2 to get 366 per phase and use a 400 amp service or am I all confused and do I really need an 800 amp service?
 
Re: marina service sizing

You are right (except for a minor math error: It?s 1860, not 1830), but I think you need to know why. Demand factors should be applied to loads that are expressed in terms of power, not current. Each of the circuits you name is a 120 volt load, but the service is 120/240 volts. So here?s the math:

48 circuits times 30 amps times 120 volts = 172,800 VA
21 circuits times 20 amps times 120 volts = 50,400 VA
Total = 223,200 VA

Apply demand factor of 40 % times 223,200 = 89,280 VA

Current = 89,280 VA / 240 volts = 372 amps.

Conclusion: A 400 amp service is sufficient for the marina that you have described.
 
Re: marina service sizing

as usaual you have cleared the fog again. thanks much. I often try to make things too hard by thinking too much.
 
Re: marina service sizing

the 21 circuits thatare currently in use may eventually want to be upgraded to 30 so a 600 amp service would then do this job well with expansion for future it seems. should be able to split this load up nicely on 3 200 amp panels eh?
 
Re: marina service sizing

Joe,
Please look at the FPN after Table 555.12. If the marinia in question is expected to be busy, you may need to use a higher demad factor to prevent the main from tripping. A busy July 4th weekend may find all of the slips in use and all of the AC going full blast.
Don
 
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