Specialized calculations such as marinas and boatyards in Article 555 ...
Specialized calculations such as marinas and boatyards in Article 555 ...
Please pardon this lengthy post. This is an excerpt of information I published earlier this year so that (hopefully) anyone with varying amounts of experience level can understand the use of specialized application loads, such as found within Article 555 for marinas and boatyards (... at least as I understand it 
...)....
Within it's Part I, Article 220 recognizes that specialized loads (such as shore power at marinas and boatyards) may have calculation requirements which are inherent only to the specialized equipment. Section 220.3 informs that the calculation of special application loads are in addition to, or a modification of, those calculation procedures within Article 220 and such specialized loads can be found within Table 220.3. Your review of Table 220.3 will reveal that the boat shore power calculations, from Section 555.12, have such inherent specialized calculation requirements, which can ultimately impact service and feeder calculations as described within Section 220.40 (? for determining that minimum calculation?).
From a service or feeder perspective, this means, at least a portion of marina and boatyard feeder and service conductors have specialized calculation requirements which are governed under Section 555.12.
The demand factors of Table 555.12 will only apply to the portion of feeder and service conductors which supply shore power receptacles for boats. The demand factors of Table 555.12 are also permitted to be subjected to additional modifications based on the table notes. The demand factors of Table 555.12 are not required, but they are permitted to be used. When applied, they are the only demands recognized to be applied by the NEC. This means the permissible demand factors for neutral load reductions from Section 220.61(B) are not permitted to be applied. Portions of marina and boatyard services and feeders which supply general lighting and any other loads are not permitted to be subjected to the demand factors of Table 555.12.
No actual instruction is provided for using Table 555.12, and on the surface its use appears self evident. Where multiple shore power receptacles of differing voltages exist on the same service or feeder, the table application can become less than self-evident. In reality, the service and feeder load for each individual boat slip shore power receptacle must be calculated at 100% of its rating. This means a 50-amp boat slip receptacle has a load on the service or feeder of 50-amps. However, Section 555.12 instructs that boat slip shore power receptacles are permitted to be subjected to the demand factors of Table 555.12 based on the total number of boat slip shore power receptacles which appear on the service or feeder.
For example:
An individual 50-amp, 125/250-volt rated shore power receptacle will have a load on the service or feeder of 50-amps (at the nominal voltage of the supplied system). If the service or feeder is supplying a total of 25 shore power receptacles (regardless of individual receptacle rating or receptacle voltage), each shore power receptacle will only have a service or feeder load of 70 percent of its rating.
. - So -
Twenty-five 50-amp, 125/250-volt shore power receptacles will have a load on the service or feeder of only 875-amps, instead of 1,250-amps:
[50
A X 25
Receptacles = 1,250
A X 70% Demand = 875
A]
Normally, a service or feeder neutral load in excess of 200-amps is permitted to be reduced [see Section 220.61(B)]. However, Section 555.12 informs that where the demand factors of Table 555.12 are applied, the permissible neutral load reductions of Section 220.61(B) cannot be applied. In this example, a full size neutral conductor would be required for this service or feeder.
Where multiple shore power receptacles of differing voltages appear on the same service or feeder it is necessary to either balance the feeder load across the phases or calculate the receptacle load in volt-amperes to determine service or feeder size. When calculating in volt-amperes, the instructions from Section 220.5(A) (nominal system voltages) should be used instead of the voltage rating of the receptacle. Typical receptacle ratings are 125-volts and 250-volts, where nominal system voltages are typically 120-volts and 240-volts.
For example:
A boatyard feeder (120/240-volt, 1-?) is supplying only five 30-amp, 125-volt boat slip receptacles and three 50-amp, 125/250-volt receptacles from eight boat slips. In this example, with a total of eight shore power receptacles, each receptacle will have a demand of 90 percent. In turn, the total feeder load will also have a demand of 90 percent. With an odd number of receptacles, or with receptacles of differing voltages on the same feeder, the feeder load will ultimately have to be determined by some method (... No instruction is provided ...).
The demand can be applied to each receptacle, or the demand can be applied to the total feeder load. The feeder load can be determined by volt-amperes, which will produce the unbalanced load on the feeder. The feeder load can also be produced by laying out a matrix to distribute the load across the feeder conductors, which will produce the balanced load on the feeder ?
I do hope this is helpful ...
mweaver
