Hi
Im opening an RV park in mainland mexico and have been giving a few different stories on my power needs for my park. We have 35 spaces for our RVers to hook up. The sites are going to be 30amp and 50 amp. My question is what size transformer and what gauge wire should I run to each site? The furthest site being some 250 meters from where I plan on placing my transformer.

Good questions.

Using the National Electric Code as a guideline as if the park would be built in the USA, which a link to review can be found in the Read First Sticky Note of this forum, per Section 551.73 states if a site is supplying a 50 amp feed, each feed shall be based on a load of 9600 VA ( AC Watts), assuming that each site can is intended for either a 50 amp OR a 30 amp unit.

Then use Table 551.73 (A) to calculate the demand factor based on the quantity.

35 sites falls into the 25-35 site range, so each site of 9600 VA is then derated to 42 %, or 4032 VA for each.

9600 VA * .42 * 35= 141,120 VA. Assuming 120/240 volt service, the total load to size the transformer is a minimum of 141.12 KVA or 588 amps.

There is a disclamer,called a Fine Print Note under Section 551.73 (C) for sites in extreme hot or cold temepratures that the derating may be too low due to heating, or in this case of Mexico, air conditioning. Special attention will be needed to allow additional capacity for each site having an air conditioner running constantly. This may be best calculated by adding the derated amount of 4032 VA plus the load of the AC typically in an RV. This is open to discussion for now.

As far as sizing the feeder conductors, this is pretty intense and detailed calculations to perform. I would need to know each distance for each feeder, and if there is more than one site on a feeder to be accurate. Would also need to know what type of wire is planned to be used, copper or aluminum, and if the cables will be buried, and what the ambient temperature of the earth may be.

Hope this gets you going in the right direction. Take some time to review the NEC at the link I mentioned to get a flavor of where I am getting the detail. Be aware, take your time reading this stuff, it is pretty dry material, ( I happen to like reading it), it can make you run screaming into the night.

This is a project indeed.

I believe Mexico has a code under this title: NOM-00-SEDE--which conforms to NEC and Canadian code.