CAN OF WORMS ALERT LOL here it comes, look out, your question should draw everyone and their brother in law (INCLUDING ME) out of the woodwork lol. The more the merrier I always say, great bunch of folks here.
NOTE from your post: "I have replaced an old fuse panel to my barn, 250 ft from the house." I take it you're replacing the remote barn sub panel NOT the main panel in home that feeds it right?????????????????????
NOTE I'm taking it as you're installing a new 120/240 panel in the barn right???? IFFFFFFFFFF you're only wiring a single receptacle for an RV in the barn THERE ARE RULES EXCEPTIONS making it far less complicated
NOTE 250 feet is a longggggggggg run, you need to figure VOLTAGE DROP into the calculations as far as the size of wire from home to barn
NOTE you may be grandfathered in if NEC matters????? in your area as far as the old three wire versus new four wire out to remote sub panels for 120/240 single phase. If you have three or four wires out to barn MAKES A DIFFERENCE, see 1 and 2 and 3 below:
HERES THE REAL DEAL UNLESS the NEC has changed OR your local jurisdiction hasn't adopted a particular version of the NEC OR you don't care LOL and IFFFFFFFFFFFF you're talking about a new 120/240 panel in the barn that's fed from your homes main panel, the modern way to do it correctly and comply with the latest NEC (again if that matters or you care) is as follows:
1) AT THE BARNS SUB PANEL IFFFFFFFFFF YOU HAVE FOUR WIRES (Two Hots, Ground, Neutral) OUT FROM HOME
a) It needs two separate isolated and insulated from each other Busses, an Equipment Ground Buss PLUS a Neutral Buss
b) The panels case/frame bonds to the Equipment Ground Buss butttttttttt the Neutral Buss does NOTTTTTTTTTTT
c) The OUTGOING white Neutrals all wire to the Neutral Buss (Well DUH)
d) The OUTGOING Green/Bare Equipment GroundING Conductors wire to the Ground Buss (Well DUH)
e) The INCOMING Neutral wires to the Neutral Buss
f) The INCOMING Ground wires to the Ground Buss
g) The Ground Buss is bonded to a Grounding Electrode Conductor (No 4 bare copper) which wires to a Grounding Electrode such as a "made electrode" like a copper rod driven into mother earth
2) AT THE BARNS SUB PANEL IFFFFFFFFFF YOU ONLY HAVE THREE WIRES (Two Hots, Neutral) OUT FROM HOME
IFFFFFFFF the jurisdiction permits it?????????????????????????????? If you only have three wires out from your home and don't want to upgrade to four, you "could" (IM NOT saying yes or no) wire it under the old code and old three wire method to remote sub panels as done for years which is as follows:
a) The INCOMING Neutral wires to the panels Neutral Buss (or a single common Neutral AND Ground Buss if so equipped).
b) The new panel has EITHER 1) A single common Buss where Grounds AND Neutrals ALL attach ORRRRRRRR 2) Two busses which are bonded together with a cross tie bar or wire.
c) OUTGOING Neutrals wire to the panels Neutral Buss (or common Neutral and Ground Buss if so equipped)
d) OUTGOING Grounds wire to panels Ground Buss (or common Neutral and Ground Buss if so equipped)
REMEMBER IF IT HAS TWO BUSSES THEY ARE BONDED TOGETHER IN THIS THREE WIRE OUT SITUATION
e) The panels Neutral (or combination Neutral and Ground) buss wires to a Grounding Electrode Conductor (NO 4 bare copper) which wires to a "Grounding Electrode"
3) IFFFFFFFFFFFFF you're replacing the homes main distribution panel NOTTTTTTTTT the remote barn sub panel:
a) Incoming Utility Neutral wires to panels Neutral Buss ORRRRRRRRRRRR single common Neutral and Ground Buss if so equipped.
b) If new panel has separate Ground and Neutral Busses, they bond together with a cross tie bar or wire
c) Grounds to Ground Buss, Neutrals to Neutral Buss or a single common Buss if so equipped
d) Neutral at weatherhead riser orrrrrrrrrrrrr meter base orrrrrrrrrrrrr panel wires to a grounding electrode.
GOT IT CLEAR AS MUD I'M SURE
DISCLAIMER I'm rusty on the latest NEC so no freaking warranty, you can do as I say,,,,,,,,,,,,ORRRRRRRRR as other trained professional electricians say,,,,,,,,,,, ORRRRRRRRRR as non electricians say,,,,,,,,,,,,,,,,,ORRRRRRRRRRRR what your brother in law says,,,,,,,,,,,,,BUT I SUGGEST YOU DO AS LOCAL AUTHORITY AND THE NEC SAYS
Sorry I'm in a bit of a hurry, so I know I missed something grrrrrrrrrrrrrr if so I hope the other fine gents here can add to this and/or correct my likely mistakes.
Best wishes, God Bless all and God Bless the USA
John T Too long retired electrical engineer and rusty so no warranty hear me, this may be right or wrong as rain, don't have a calf, I warned you lol