Marina receptacle load calculation

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Jmeyers

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I have a 71 slip marina. The table specifies a demand factor of 30% (table 555.12) for 71 or more 30 amp marine receptacles. So the required transformer rating will be 0.3 x71x15amps or 319.5amps for a three phase 240 volt transformer.



But a three phase 240volt, 200amp service panel can only support 15 30amp receptacles(15 receptacles at 70% demand factor using 80% of 200amps service panel for continuous load per article 100). So my 71 slip marina needs 5 x200A service panels. That can supply a total of 745amps (0.7 x71 x15amps). And all this power is provided by a 319.5 transformer. Is that the correct interpretation of table 555.12
 
Based on the 71 receptacles with a 30% demand, I get 77 kva.
What is your transformer primary voltage ?
(Note, if this is a single 3 phase transformer due to limits on neutral loading, a 208/120 would be more practical IMO)
 
Your 77kva is the same. 77kva/240volt = 320A. What I’m trying to understand is the the number of service Panels I need for 71 30amp receptacles using 200A three phase service panels.
 
In your calculations your are using a 15 amp load for a 30 amp device(per your OP). Where are you getting the 15 amps from? You can use the 30% diversity in each of the panels to determine your load just like you can for lets say Kitchen Equipment loads.
But if you use the 80% of the 30 amp device, you will need either (2) two section 54 circuit, 300 amp panels or you could use a 4 section, 54 circuits each, 600 amp panels.
 
I was taking the 240volt DP supply and dividing by 2 for the 120 volt 30 amp receptacles. Which converts to 15amps at 240 volts. The 89% comes from the continuous load specified in article 100. Should I drop the 80% and use 100% instead?
 
Jmeyers said:
I was taking the 240volt DP supply and dividing by 2 for the 120 volt 30 amp receptacles. Which converts to 15amps at 240 volts. The 80% comes from the continuous load specified in article 100. Should I drop the 80% and use 100% instead?
Click to expand...
 
Jmeyers said:
I was taking the 240volt DP supply and dividing by 2 for the 120 volt 30 amp receptacles. Which converts to 15amps at 240 volts. The 89% comes from the continuous load specified in article 100. Should I drop the 80% and use 100% instead?
Click to expand...
This is what I would do.

71 x 30 = 2,130
2,130 x 30% = 639 amps @ 120V or 320 amps @ 240V, or 77kva.
 
Jmeyers said:
Can you tell if table 555.12 should be used at each service panel or just at the transformer?
Click to expand...
I always understood it's for each panel as loaded (if you had 2 panels for your loads you could only use a 60% diversity factor instead of 30% for all the loads).

ETA:
In your example, you have a 208/120 service. If you have 2 panels, 36 * 30 = 1,080. 1,080 * 60% = 648A spread across 3 phases is 216A. So a 225A 42ckt 3-phase panel would be able to serve 50% of your receptacles. This does not leave much in the way of spare capacity. If there is ever a chance a boat might need a 50-amp receptacle, you should abandon the 3-phase service and go to single-phase.
 
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I think by Code you could get by with 3 panels if you apply the demand factor to each panel (allows you roughly 26 receptacles per panel).
I'm always nervous applying the demand on marinas and RVs. That's areas I don't think the Code has kept up with the amount folks plug in.

oldsparkey took it a step further than I did :)

side question: Where is your transformer located ?
 
So taking the 555.12 Adjustment for the entire marina to the individual 30a receptacles is correct. That translates to 30A x.3% = 9A for 71 receptacles. That was also my understanding.
 
So your thinking one 555.12 Calculation for transformer/switch panel and a separate 555.12 calculation for the service panel?
 
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