RV Site Load Calcs

[WA_Sparky]

Questions on RV type load calcs.
We have 100 RV sites planned to be located across a site with 208 3Ø service available.
As im reading through the code there is some confusion as to when and how demand factors may apply.
The thought is that we will be running 3Ø conductors out to a string of RV pedestals each pedestal will be altering conductors so AB,BC,AC,....

551.73 551.73 Calculated Load.
(A) (A) Basis of Calculations.
Electrical services and feeders shall be calculated on the basis of not less than 12,000 volt-amperes per site equipped with 50-ampere, 208Y/120 or 120/240-volt supply facilities; 3600 volt-amperes per site equipped with both 20-ampere and 30-ampere supply facilities; 2400 volt-amperes per site equipped with only 20-ampere supply facilities; and 600 volt-amperes per site equipped with only 20-ampere supply facilities that are dedicated to tent sites. The demand factors set forth in Table 551.73(A) shall be the minimum allowable demand factors that shall be permitted in calculating load for service and feeders. Where the electrical supply for a recreational vehicle site has more than one receptacle, the calculated load shall be calculated only for the highest rated receptacle.
Where the electrical supply is in a location that serves two recreational vehicles, the equipment for both sites shall comply with 551.77, and the calculated load shall only be calculated for the two receptacles with the highest rating.

(B) (B) Demand Factors.
The demand factor for a given number of sites shall apply to all sites indicated. For example, 20 sites calculated at 45 percent of 3600 volt-amperes results in a permissible demand of 1620 volt-amperes per site or a total of 32,400 volt-amperes for 20 sites.
Correct me if im wrong.
Hypothetically I have a 200/3 breaker feeding a string of RV Pedestals, each 50A/2.
1. AB
2. BC
3. AC
4. AB
5. BC
6. AC
7. AB
8. BC
9. AC
At minimum there are 6 RV sites connected to a single phase conductor at a time.

(A)(A) Says feeders shall be calculated on the basis of not less than 12,000 volt-amperes per site equipped with 50-ampere
(B)(B) Says The demand factor for a given number of sites shall apply to all sites indicated
Table 551.73(A) 6 sites = 60% demand factor.

So this is saying the demand load for a string of 9 sites connected in this configuration would be 6(sites connected)*12000VA (defined by (A)(A))*60%((B)(B) demand factor)= 43200VA demand for the given string.
Breaker at MDP could be sized for: 43200VA/208V/1.732=119.91A @208V 3Ø, so say 125A/3
Conductors would be sized based off 125A (not factoring in voltage drop at the moment).

Thanks in advance for your responses.

 

[tortuga]

I would do all the calcs in VA then convert to amps at the end
So if I had 9 sites * 12000 VA per site = 108000 VA
The take the demand of 55% 108000 * .55 = 59400 VA
Then convert to amps 59400 / 360.27V = 165 Amps

 

[mopowr steve]

6 sites @ 12,000va = 72,000va
72,000va x 60% = 43,200va
43,200va / 240volts* = 180Amp minimum
So a 2 pole 200 amp breaker derived from a 3phase service (as you describe connected AB, BC, AC ) could be used to feed a string of 6 RV sites with 50amp receptacle.
***(IIRC the feeder breakers are figured as a single phase load because there is no line to line loads in an RV, they are all 120 volts line to neutral loads. I think you can use the 3 phase calculation at the service for service sizing though just not the feeders)

 

[mopowr steve]

You don’t run 3 phases out thru the pedestals just 2 phases and neutral and EGC. The pedestals don’t have 3 phase feed-thru lugs just phase1, neutral , phase 2. And yes, it is good practice to alternate the phases every pedestal in case you have a larger portion of RV’s that still utilize the 30amp receptacle, which is factory wired to only one of the phases. This way you create a more balanced load. This does not much matter as far as the 50 a receptacles go as those loads are already considered balanced any way among the two phases.

 

[tortuga]

Good point if he's running single phase feeders, but then you would need to use 208V not 240.
for the service calc you could still use 3 phase