Calculating voltage drop

I don’t know if your RV is somehow different, but generally, 50A RV’s are two 120 volt 50 amp circuits.

I'm not following. What do you mean 'which wire', and what does voltage drop have to do with it?

Use 240 volts and 50 amps in the calculator. 25 amps would be incorrect.

https://www.myrv.us/electric/Pg/50amp_Service.htm

https://www.customcableusa.com/pages/voltage-drop-calculator

Don't get all confused because there is an RV plugged into the standard common NEMA 14-50 outlet. No different calculation should be used to accommodate what is plugged in.

50A is 240/120V 4 wire which is used as 2x 50A circuits. 240V appliances are not the norm for a RV. The 2 hot wires and neutral are each rated for 50A which is the maximum. ie You have a total of 100A @ 120V for the 2 circuits or 12KW.

Use 50A load for the wires which are normally 6ga unless long distances are involved.

2oldman wrote:
I'm not following. What do you mean 'which wire', and what does voltage drop have to do with it?

OP, explained her question perfectly clearly.
OP, you would use the V and A for the service (IE the main breaker in the sun panel which you state as 50A, 240V). Those are the values to plug into a voltage drop calculator.

Theoretically you could use “all” the power that a 50A 240 main breaker would handle, with a 50A RV setup which is 50A 120v pulling separately off of each leg of the 240v service.

Enter this and then scroll down:

https://www.southwire.com/ca/en-ca/calculator-vdrop

for 50 feet and 50 amps:

55
Amps Rated ampacity of selected conductor
0.4662
Ohms Resistance (Ohms per 1000 feet)
0.0510
Ohms Reactance (Ohms per 1000 feet)
0.9
Power Factor
3.60
Maximum allowable voltage drop at 3%
2.209
Actual voltage drop loss at 1.84% for the circuit

Only the power is halved, not the amperage. One 250 V 50 amp circuit is the same as two 120 V 50 amp circuits.

And 5% voltage drop is fine. No need to go 3% for just a normal residential type load.

stevenal wrote:
Only the power is halved, not the amperage. One 250 V 50 amp circuit is the same as two 120 V 50 amp circuits.

The split phase has the advantage of sharing the neutral and reducing voltage drop.

Here is a good explanation of 50 amp service.

https://www.myrv.us/electric/

It really is 240 vac 50 amp separated into two 120 vac 50 amp circuits. And most all RVs only use the 120 vac circuits. But some high end RVs do actually have some 240 vac appliances. But since it is protected by a 50 amp two pole linked circuit breaker you must size the wire for 50 amps.