Marina Calculations and Diversity

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DockAndMarine

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I have been reading the forum and the net to answer a question on making calculations for a marina. The typical design of a Marina these days is Shore power (480 VAC 3 Phase) Coming out to a Substation (225 KVA) for this example and then the substation having a 220/120 Circuit Panel Supplying Pedestals. A Pedestal typically has a 50A and a 30A Receptacle on Each side. A circuit breaker may power more than one pedestal so this side of the substation may have several circuit breakers. Now here is the calculation question based on Article 555.12 of the NEC. It says that the demand factors as set forth in Table 555.12 shall be permitted for each service and or feeder circuit supplying shore power for Boats with the modifications of notes 1 and 2. For this example, lets say there are 30 Receptacles that are powered from this substation and each circuit breaker on the substation feeds 8 receptacles. Based on this and the word feeder circuit in the code, I would assume that the 70% factor would be used on the diversity of the Substation. Because it uses the words feeder circuit, that seems misleading on the lower voltage side because each of these are branch circuits. So do you use the same table for Demand Factors for these individual branch circuits? If so each branch circuit in the example would use 90%. Any Thoughts??
 
What size breaker feeds each "branch circuit" exiting your substation? I have a feeling the circuits are actually feeders, and not branch circuits. If the pedestals have breakers to protect the individual 50 and 30 amp receptacles, then these are feeders. If the breaker(s) aren't present in the pedestals, I'd like to hear more about the configuration.

Jim T
 
jtester said:
If the pedestals have breakers to protect the individual 50 and 30 amp receptacles, then these (i.e., the conductors from the panel to the pedestal) are feeders.
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I agree with this statement. I don't have my book at home, so I can't confirm your math.
 
Remember that derating is ALLOWED, not REQUIRED. There is an FPN that warns that use of the derating factor may result in an installation that is inadequate where it gets either hot or cold! Where is there a marina where it is never hot or cold? I believe that this FPN was added when the wording for the derating factor was changed from "conductor" to "circuit" which effectively allowed a greater derating; so they give a derating factor to use, and then warn us that if we use it, the installation will probably be inadeguate! I guess that is why the NEC is NOT a design document.
 
the substation having a 220/120 Circuit Panel Supplying Pedestals
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I would ask you to pay close attention to 555.13(A)(3).

This seems to be an odd voltage. I've seen 120/240 single-phase, 208/120 3-phase, and 220/127 3-phase.

When you feed these 50-amp receptacles with 2 legs of a 208/120 service, I believe you are in violation of 555.13(A)(3). From what I've been told by boat manufacturers, you should be OK until you get to 60' and larger boats. Below 60' in length, typically the boat has 2 panels rated at 120-volts each and one phase and a neutral feeds one panel and the other phase and a neutral feeds the 2nd panel. There are no 240-volt loads. Above 60' in length, the boats start to have 240-volt loads.
 
hardworkingstiff said:
When you feed these 50-amp receptacles with 2 legs of a 208/120 service, I believe you are in violation of 555.13(A)(3).
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I agree, except for the typo (replace ?13? with ?19?). But I?ll concede that this is a subject of some controversy.

I once did a survey of boat manufacturers around the country and in a few other countries. Out of all the companies I surveyed who manufacture boats with provisions for 240 volt equipment, not one said that they would accept a power source of 208 volts.
 
A trick I have used in the past for marinas that do not have 240 Volt service, but 208, is to specify a multitap transformer with taps at 2.5%,5%, 7.5% and 10% over and under. Use it as a boost transformer 10% and get within the 5% drop allowed at the service receptacle, 228V. It isn't pretty or perfect, but works. Most marina tenants are thrilled with anything that close. Too low and it damages their AC units and ice makers. Don't want that!!!!
 
charlie b said:

I agree, except for the typo (replace ?13? with ?19?). But I?ll concede that this is a subject of some controversy.

I once did a survey of boat manufacturers around the country and in a few other countries. Out of all the companies I surveyed who manufacture boats with provisions for 240 volt equipment, not one said that they would accept a power source of 208 volts.
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RV's are the same way. 120/208 single phase is prohibited for RV parks.

Jim T
 
boater bill said:
A trick I have used in the past for marinas that do not have 240 Volt service, but 208, is to specify a multitap transformer with taps at 2.5%,5%, 7.5% and 10% over and under. Use it as a boost transformer 10% and get within the 5% drop allowed at the service receptacle, 228V.
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Bill, does that give you a 228/132 single-phase circuit? 132 would be about (or just beyond) the upper limit of the lower voltage, right?
 
Yes it does create the higher voltage for the 120 volt circuit. But since most boaters are used to seeing 115,112 volts they are thrilled with a little extra. Equipment is more tolerant to a higher voltage to a lower one in terms of current draw.
 
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