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One 8 kw heater at 208 3 phase will operate at approx 22 amps each
Which calculates at 2 x 22 or 44 amps.
Code requires a feeder of 125% of the load when wiring heat, so that then calculates at 55 amps total.
For 55 amps, you need #8 THHN wiring, if only a short distance of 100 ft or less and then to #6 THHN wire over that length.
On 55 amps, a 60 amp breaker is closest so that would be used for the main feeder.
Let me know anything further needed,
Sorry, that is #8 copper wire THHN.