Andy, as typical electrical (and legal) questions draw a lot of responses. Not being there I cant know if the box you're talking about is a main or a sub (I suspect) panel so I will describe the proper NEC and trade practice and let you figure it all out.
1) ONLY the Main distribution panel should have a Neutral Ground Bond, that's Single Point Grounding. Neutral is bonded to Ground there and the box's metal frame is bonded to the Ground, which is of course bonded to the Neutral Buss. Also at or near the main service entrance panel (Up at weatherhead riser,,,,,,,,Or in the Meter Base,,,,,,,,,,,Or in the Panel) the Neutral is bonded to all readily available "Grounding Electrodes" (via the Grounding Electrode Conductor like #4 bare copper wire) such as metal conductive utility buried pipes or structural steel or ground grids or "made electrodes" such as one or more copper driven into earth Ground Rods.......
2) At a SUB PANEL fed from the main panel NEUTRAL AND GROUND ARE ELECTRICALY INSULATED AND ISOLATED FROM EACH OTHER. There is a separate Neutral Buss and a separate Ground Buss. The metal frame is of course bonded to the Ground Buss (but NOT the Neutral Buss)
3) If you're running 12/2 w Ground wire using a single pole breaker out to your heater, that's good for 120 VAC likely 30 amps subject to enclosure and insulation etc. It has a White "Grounded Conductor" (Neutral), a Bare/Green Equipment Ground, and a Black UNgrounded Conductor (Hot). If you used a 240 ONLY volt heater you could run 12/2 w Ground but the Black and White would be 240 VAC and the Bare/Green the Equipment Grounding Conductor, and of course a two pole breaker.
4) Even if your wire has an ampacity of 30 amps, there is a voltage drop subject to wire size and length and current which may affect your project IE if voltage drop is too much you may need bigger wire.
5) As I recall NO WARRANTY things may have changed, per the NEC any buildings electrical service requires grounding which means if you wire a sub panel at the barn it needs proper grounding.
SOOOOOOOOOO if that box is a sub it needs separate and isolated and Ground and Neutral Busses and if you run yet another sub off it the same.
Hope this helps
John T Longggggggggg retired Electrical Power Distribution Design Engineer and the NEC may have changed and I'm rusty so NO WARRANTY
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