Article 555 - Shore Power

[Npstewart]

The NEC is clear that if (2) receptacles with different voltages are provided in a pedestal, the receptacle with the higher kW demand should only be considered for load calculations. I just want to make sure though that if two receptacles with the same voltage (ie. 240v) are provided that both have to be used for load calculations.

Also, in the picture attached which is from Mike Holt's website, why do they use 30 receptacles instead of 40 because there are actually 40 receptacles total.

 

[charlie b]

My cubicle neighbor and his wife live aboard their boat. Their slip has two 120 volt, 30 amp receptacles that they could use. These are separate from the receptacles that serve the boat across the pier from theirs. They only use one of the 30 amp receptacles at the moment, but they are contemplating upgrading their boat to be able to connect to the second 30 amp receptacle. So yes, I would say that you have to count both, when performing the load calculation.

You would do Mike a favor in pointing out the apparent contradiction in his example calculation. I believe his web site has a contact person for that type of report.

 

[Npstewart]

As long as it is actually a mistake, I will be sure to let them know.

One more question. In addition to the "Shore Power" receptacles, we have 120V maintenance receptacles which will ALSO be located in pedestals but they are strictly for maintenance and they are in difference pedestals. Am I required to use the entire ampacity of the receptacle (ie. 20A) for these as with the shore power, or am I permitted to take these at 180 va each.

The problem is that this is a long run and I want to loop all the 20A receptacles on one circuit. If I use 20A * 4 pedestals that is 80A, so I would need a 80A feeder to feed all the receptacles.

 

[GoldDigger]

The problem is that this is a long run and I want to loop all the 20A receptacles on one circuit. If I use 20A * 4 pedestals that is 80A, so I would need a 80A feeder to feed all the receptacles.

Diversity aside, it is not a good idea to tap a 20A receptacle onto a feeder with OCPD at 80A. If you mean that you will put in a subpanel to feed the four receptacles as separate branches, that is fine.

 

[Npstewart]

This is a really long run, somewhere around 604 feet so one feeder would probably be ideal. I also have to provide voltage drop calcs for this. If I have a #10 wire loaded with only 6 Amps, I could probably get away with a #10 wire without a issue. If I have it loaded with 20A, I would probably need a #6 wire.

My question still stands. If these are located on the floating docks, do I have to use the entire 20A for a load even though these have nothing to do with shore power? Or can I use 180 VA, or maybe 300VA just to be on the safe side.

 

[Npstewart]

You would do Mike a favor in pointing out the apparent contradiction in his example calculation. I believe his web site has a contact person for that type of report.

Charlie, do you think that maybe the diversity comes when you add the "Receptacles per line"?There are 40 total receptacles, but maybe they are only adding the number of receptacles "per line". That would equal 30.

 

[augie47]

Definitely just an opinion, but I would say that you are correct in that the number they use is "per line" and you do only have 30 per line.
As far as the "maintenance receptacles", I would say that 180va would be the number for calculations, however, based on what I see at our marinas, unless those pedestals are equipped with locks, they will become boat power also.

 

[charlie b]

I haven't seen the example on Mike's web site, and I don't have time to look right now. But the image that is shown in the first post of this thread is confusing. It names 40 receptacles, and then counts 30. I don't understand that, nor do I know what is meant by "line 1" and "line 2." I will say, however, that this example appears (I emphasize "appears") to be adding amps from a 120 volt load to amps from a 240 volt load, and coming up with a total amps. That would not be valid. You need to do the math in units of KVA, and only convert to amps at the end.

But here is a quick guess at Mike's intent: 20 receptacles that have 120 volts applied and that are rated at 20 amps would have the same amount of KVA as 10 receptacles that have 240 volts applied and that are rated at 20 amps. That might be why the graphic only counts 10 of the 20 amp receptacles. That would allow the 10 x 20 to be added to the 20 x 30 to get a total of 800 amps, since the receptacles would have the same rating of 240 volts. This would not be my approach, but it might be what that example was trying to do.

 

[Npstewart]

Here is the link to the article/PDF:

https://www.google.com/url?sa=t&rct...=pXYJqMkIGnvb2n_CWMkHtw&bvm=bv.49478099,d.eWU

 

[GoldDigger]

But here is a quick guess at Mike's intent: 20 receptacles that have 120 volts applied and that are rated at 20 amps would have the same amount of KVA as 10 receptacles that have 240 volts applied and that are rated at 20 amps. That might be why the graphic only counts 10 of the 20 amp receptacles. That would allow the 10 x 20 to be added to the 20 x 30 to get a total of 800 amps, since the receptacles would have the same rating of 240 volts. This would not be my approach, but it might be what that example was trying to do.

+1
The rest of the article (just posted) makes it clear that the amperage is being calculated based on a 120/240 supply and so they are assigning/balancing 10 of the 20 120 volt receptacles to each of the two 'lines', L1 and L2. Without that extra information, the illustration alone is certainly confusing.
With the extra information it is exactly what you said, only using an acceptable method of finding the load current directly instead of going via power calculations.

 

[Npstewart]

+1
the illustration alone is certainly confusing.

Sorry

Adding the amps together was confusing me as well even with the article, I ended up changing it to VA.