Ok I have searched and still think I am correct and the electrician at the store that informed my dad of the lack of a required separate ground conductor is wrong... But also maybe my dad misinformed him of the exact situation or something because I should think between me and a electrician I'd be the incorrect one. We are starting an airbnb property out in the woods in Washington State. They are also putting in an old 70s single wide for their housing the few months a year they're here. Power comes to the pole and the same box houses the meter socket and 200A main disconnect, which I'm pretty sure have the neutral and ground bonded inside and there are 2 ground rods one at pole maybe 10' away to the other. Now about 15' from the pole were building a "shed" to house equipment AND a 200A panel. This panel will feed the mobile now and the campsites later on as well as the well, septic pump, etc...

So here's the deal I found 4/0-4/0-2/0-4 mhf I planned to use but the trailer is now getting here sooner than planned and I really need to cover this ditch today... Local town had 4/0-4/0-2/0 (is this the stuff referred to as sweetbriar or something?) So I sent him in to get 125' of the stuff plus a 4th direct burial conductor to act as the grounding conductor and run them all in the same ditch (2.5" conduit just from each panel into dirt then buried). While there somebody said "hey I'm an electrician and up here in Washington you don't run a 4th conductor to mobile homes... I assume this would be true if the meter socket/main panel was AT the mobile but it's not... Before I let him fill this ditch do I need to go back and get another wire? So pole 200A disco bonded N/G and ground rods. Shed 200A separate N/G and install ground rod/s AND mobile panel separate N/G AND more ground rod/s.

 

Everything after this point needs a four wire feed and the ground and neutral are kept separate - not bonded in the panels.
As for the cables I am not up on the codes for those. I will leave that part for another person.

 

Was this electrician an employee at the store? I don't know if the state of Washington licenses electricians and if the state is the local jurisdiction or not, it's different in every state. I would recommend you check with your local AHJ (Authority Having Jurisdiction) and first ask what version of the National Electric Code they have adopted and then ask your question of them. The 4th ground wire has been required by the NEC for quite a while and I seriously doubt your local AHJ would still be on a version of the NEC so old that just a 3 wire feeder would be required. This just might explain why the alleged electrician works in a store and not as an electrician.