Receptacle rating for marina load calculation

[charlie b]

Re: Receptacle rating for marina load calculation

On the marina study that I mentioned earlier, it was my recommendation that they bring 480 volt, 3-phase onto the pier. My design had three transformers on the pier, located at the one sixth, three sixths, and five sixths distance points along the pier. Each transformer was 480 ? 120/240, single phase, so I staggered the primary feeds (AB, then AC, then BC) to help equalize the primary currents. Each transformer fed slips both up and down the pier. In other words, the transformer at the one sixth point fed all slips along the first third of the pier. My design included use of power pedestals similar to the ones that hardworkingstiff described.

 

[hardworkingstiff]

Re: Receptacle rating for marina load calculation

Charlie,

About how far was it from shore/bulkhead to the three transformers in your study?

When you do this with the transformers, you have to take a ground wire back to shore?

Thanks,
HWS

 

[charlie b]

Re: Receptacle rating for marina load calculation

I don't recall the actual distances (it was over 3 years ago), but I think the piers were close to 1000 feet long. I believe that I did include a ground wire back to the shore, for each transformer.

 

[hardworkingstiff]

Re: Receptacle rating for marina load calculation

That would make sense. I did not due the cost calculations, but my instinct tells me that after about 600-700 feet, the transformer is the way to go.

 

[stew]

Re: Receptacle rating for marina load calculation

Hrard worker. I just got done with one of these tyrpes of installs and learned a lot about how the calculations are done. Your AHJ is absolutley wrong in telling you to use 100% of each receptacle . Youe need to read the 555 article in its entirety. Lets say you have 30 30 amp receptacles. 30 x 30 x 120 v =108000 va Divide by the 240 v supply voltage or 450 amps . in your case you have 208 v 3 phase so you would divide by 208x1.732 or 300 amps per phase.The only time you use 100% is in the intial calculation to get a va rating . Then you apply the demand factors in 555.12 . After you make that calculation to get va at the delivery voltage of 120 then you calculate the current from that based on your input voltage and phase. I had an inspector also try to make me use 100% until I asked the question the same as you have and the very intelligent sources on here showed me the proper way to calculate it. I sat down with him and showed him the math and the table and he finally realized that there were people out there that knew more than he did about it. We bot learned a lot from the experience as well. Hope this helps. I forgot you also had some 50 amp receptacles which are used for the very large boats typically 45 ft and up. Make sure and include them when figuring the total va then do your demand and then calculate the current by whatever the supply voltage and phase is.

[ August 21, 2005, 10:32 PM: Message edited by: stew ]

 

[hardworkingstiff]

Re: Receptacle rating for marina load calculation

Stew,

Thanks for trying to straighten me out, although I'm not sure what I typed to make you think I needed to be straightened out.

I finished working on one last year that had 72 power centers, 30 of them had 50/50 side A and 50/50 side B. That would be a total of 4 receptacles on one power center that according to 555.12 would require 100% load calculation for the feeder feeding that unit (unless you loop feed, but who wants to wrestle with a 400-amp rated type W cable?).

The one I start next month has 53 power centers rated at 100 amps each. I had to show the electrical engineer how to use the table in 555.12 so we could get by with smaller panels (using 2 800-amp and 2 600-amp).

Yea, I appreciate you keeping me straight, but when it comes to basic marina wiring, I think I have a handle on it.... THANKS!

 

[augie47]

Re: Receptacle rating for marina load calculation

hws:
Ever come across a prefab concrete dock, 4" EB conduits cast in the concrete with "quazite" covers at each brach off point ?
Watching one being installed in my area. Only questionable thing I saw was the fitting at each pedetal were plumbing "wye" fittings.

 

[hardworkingstiff]

Re: Receptacle rating for marina load calculation

Originally posted by augie47:
hws:
Ever come across a prefab concrete dock, 4" EB conduits cast in the concrete with "quazite" covers at each brach off point ?
Watching one being installed in my area. Only questionable thing I saw was the fitting at each pedetal were plumbing "wye" fittings.

I've only seen drawings of them. Seems like they are limited in what you can do in the future if I understand the construction of them. If they are not properly engineered for proper wire sizes from the beginning.....

I'm lucky in that I'm working with a dock builder that uses wood construction and the utility chases are open like floor joists in a house. Pretty slick actually.