These days THHN is usually rated THWN-2 AND MTW...
THWN-2 is very important since it permits reasonable wire size for underslab conductors.
Trick anything might be possible.
In the commercial trades 80% is a hard limit.
Everything about this circuit smells like the EE thought that 277 VAC would cut it.
Yet the EM ballast world is not oriented towards 277 VAC; namely the battery inverter scheme.
I've seen this addressed time and again only at the last minute as a change order -- in the field.
