Cause if you only have two wires going to your wall plug, you're not grounded. Keep water away. To extend them, use the same size wire on each of them and screw them together with wire nuts inside a junction box. Don't use any tape.
My HPS 250 is the same, it has 3 wires going to the wall, but only two from the bulb socket.

I already have the wire nut junctions, but can't I tape around them?

If you have to, cut a little piece of wire from each to match the size at the hardware store.
I was thinking about that but if this means like having to flower under 250HPS for several days because of all the hassle, maybe I don't dare to...

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