Marina Calculation

[wesley1]

I have recently been asked to caculate how many 50 amp/ 250 volt shore power receptacles can be added to an existing 600 amp/ 240 volt panel at a local marina.The panel currently has 28 30 amp/ 125 volt shore power receptacles fed from this panel. I found in article 555 that the total load can be derated 70% and if I add the new receptacles a 60% derating can be done.My question is the table 555.12 says ( the sum of the rating of the receptacle). When figuring feeder caculations in article 220 the loads are figured as kva or kw. Do I simply add the amp rating of all receptacles and derate by 70% or do I change the 30amp rec to 125voltsX30 amps=3,750w then multiply by the 28 rec = 105,000w, then derate by 70% =73,500w, on a 240 volt feeder this =306.25 amps.since starting my company I have had very little experience with marinas and definetely want to get it right so breakers are not tripping this 4TH of July when the docks are full.Any help would be appreciated!

 

[charlie b]

You need to do all math in terms of KVA, and only convert to amps at the end. For the purposes of marinas, I should think that there would be very few motor loads (the occasional air conditioner, for example). So KW should be the same as KVA.

Take the 30 amp receptacles times 120 volts (i.e., the nominal voltage level, not the 125 volt rating of the receptacle). Take the 50 amp receptacles time 240 volts. Add it up, and apply the demand factor. Then divide by 240 to see if the existing panel can handle the load.

Please note also that if single slip will have both a 30 amp and a 50 amp receptacle, you only count the 50. It is presumed that no single boat will use both, so your load calculation only has to account for the larger of the two.

 

[don_resqcapt19]

Wesley,
Pay attention to the FPN after Table 555.12.

FPN: These demand factors may be inadequate in areas of extreme hot or cold temperatures with loaded circuits for heating, air-conditioning, or refrigerating equipment.

Don

 

[haskindm]

I second what Don pointed out regarding the FPN. Boat owners have a reputation for adding loads until the breakers trip. I would be very careful about derating at all. The person with a million dollar yacht is not going to be happy to find that he cannot run his air conditioner on July 4th because the service is overloaded.

 

[hardworkingstiff]

The person with a million dollar yacht.......

That's not much of a boat these days. I don't do any boating (too busy working for them there yachters), but when I asked about some of the prices of them,.... WOW!

 

[wesley1]

marina calculation

marina calculation

Thanks for the input. Today I was at the same marina changing lift station pumps so I checked on this , using your input .I found that with the 10 50amp receptacles that will be added the wattage comes to 220,800w. this will be a total of 38 receptacles so I derated by 60% ( table 555.12) this brought me to 132,480w , then divided by 240v = 552 amps. When talking to the marina manager I expressed my concern about the heavy a/c loads because we are in Florida. He walked me down the dock and the slips that have the 30 amp receptacles can only accomodate 30' or shorter boats and of the 22 there ,only 2 had a/c.He said that the bigger boats that will be using the 50 amp receptacles will probably all have a/c but there will only be 10.Therefore, I think that the existing 600 amp panel will handle the extra added load. Thanks again!