Potentiometer Power Handling
If you put V volts across a potentiometer of R ohms, and draw no current from the wiper, then you will be dissipating \$\frac{V^2}{R}\$ watts. So with 150V and a 5k pot, you need a 5 watt pot.

If you do draw a current from the wiper, this will increase the current in the top part of the pot, which will increase the track dissipation, ideally needing a higher power pot. A 5 watt 5k pot would have a maximum permitted track current of 31.6mA. When driven from 150v, the end to end current is 150/5k = 30mA, leaving very little in reserve for the wiper output. A 10 watt 5k pot would be able to tolerate a current of 44.7mA.