I was working on a street light .The con-ed feed was a two wire feed #6 wire. No load it read 120 v,with a 150 w load the voltage went to 70 v, why is this ? Could it be a faulty neutral and if so why does it read only a partial voltage ?

There is most likely a poor connection ahead of the light.

I was working on a street light .The con-ed feed was a two wire feed #6 wire. No load it read 120 v,with a 150 w load the voltage went to 70 v, why is this ? could it be a faulty neutral and if so why does it read only a partial voltage ?
TKRussell made a good point. A poor connection means resistance. Just as any load means resistance. All that resistance shares the same wiring, so they share the same voltage.
The result is that over each resistance voltage is build up, resulting in a lower voltage over the light bulb.
Besides that the light burns lower, the problem is dangerous, as it heats up the poor connection, and can even result in fire and burning out.
So check the wiring and find and sort out that poor connection.
;)