Is the breaker box already in the garage? Is an attached or detached garage?

I think the temperature rating has more to do with the installation environment than what the wire is going to heat to under full load. I've put 15 amps through a light gauge extension cord and it only got warm to the touch, maybe not more than 120 degrees. As long as you're not running in a hot attic, or bundling with a bunch of other wire, I think the 8/2 would be more than good.

Seabornman is right about the temperature rating. The only practical use of it is to determine what the wire-size to ampacity calculation shall be (and a small amount for use in wiring high heat fixtures which you are not doing here).

His comment on extension cords is spurious, however. Cordage actually gets rated for higher ampacities than building wiring because it's assumed they'll be sitting in free air, rather than inside walls or conduits.

The thing draws more than 20A however (but less than 30A by a good margin), so in fact, the instructions are well advised (and you're obliged to follow anything that the instructions say are REQUIRED) anyhow. This means that you're going to need at least 10g wiring and a breaker to match (30A). Yes, you can use a larger cable (provided it doesn't exceed the terminal size on the unit), so your 8g UF would be fine, you'd still want to use the 30A breaker if that's what the heater recommend.

You haven't quite given the details of the installation. UF can be used surface mounted in the garage in most places, though depending on how your "heater" is mounted, you may not want to use it to go that last little distance between the wall and the unit.

All useful and helpful so far. Further details, if it matters:

Garage is a 900 sqft attached garage, with sheetrocked, uninsulated, 2X6 walls, double-sheetrocked ceiling with no insulation. and insulated doors. Heater has a steel hanging bracket that you bolt to the ceiling joists. Minimum distance from any wall is 6” and bracket provides automatic spacing from ceiling. The breaker box is on a wall about 6‘ from the location of the heater and I was planning for the cable to go about 18” from the back of the heater through an armored jacket until I got to the wall, and then bring the wire up through the wall cavity to the open and accessible attic, and finally back down through the wall cavity to the breaker box. I already have a 30 amp, double pole breaker, and two free slots. I’m in the Pacific NW, so the climate is mild. Would only be using the heater on winter days when the temps outside are in the mid-40’s F down to possibly the mid-20’s. The inside of the garage on the coldest days never gets worse than the high 30’s. I just want to be comfortable doing woodworking.

If the walls are uninsulated, I'd certainly look into adding insulation. You won't believe how much of an improvement this makes in comfort even if there's no heating or air conditioning.

If the walls are uninsulated, I'd fish the 8-2 from the panel up to the on the wall near the heater. Put a disconnect box there (technically, if you can see the breaker panel, you don't need one, but I like the idea of having disconnects near the equipment). Use a piece of flexible conduit (commonly called an appliance whip) to go from the heater to that box.

Alternately, you install a regular box rather than the disconnect.

Alternately, you could run the 8-2 on the surface of the wall to the box/disconnect in most locations.

Ron-

Thanks...that's all helpful. A couple of questions:

1. What is the safety advantage of a disconnect box for this type of equipment?

2. Would this be an adequate and proper disconnect box from the standpoint of specs?: Disconnect box

3. If I just use a regular box and make the connections from the appliance whip to the cable in that, what would be the preferred connection from an electrical efficiency standpoint: wire nuts; soldered + wire nuts; or insulated splice connectors (the kind with hex screws)?

Ron Natalie- I’m bumping this back up in the hope that you’ll notice it and give me your input on my followup questions. Thanks. :-)

1. The advantage is that the control of the disconnect is easily under the person working on it (especially handy when we're talking about something you have to get on a ladder to work on).

2. Yep, that would work just fine. You don't even need one with overcurrent protection. The kind I tend to use just have a pull out T-handle.

Any listed fastener can be used for the connections. Wire nuts would be just fine.

Yes, you have to size up continuous loads. I already took this into account. The 21A load here was put on a 30A circuit (it could likely be on 25A circuit, but such are uncommon). Besides, if the manufacturer instructions require 30A, that's what you must use.

There's no such thing as BX anymore. Even if you had it it would be ILLEGAL. AC and MC cables have a similar visual appearance but are fundamentally different with regard to grounding than historic BX cable. Romex is a tradename commonly ascribed to Type NM cable. Type UF cable (as previously mentioned) is legal for use where NM is allowed indoors.

Nobody mentioned multiple heaters on this circuit nor a selection between 120 and 240.

The advantage of running a higher voltage (and this is why your transmission lines are at thousands of volts), is that the current is less, which means the wires can be smaller and less loss over distances.

Ron- Is this kind of appliance whip ok to run from the disconnect box to the heater?

Non-metallic whip

I was thinking of a standard armored cable but this non metallic version is the type I keep running into. It has THHN wires, which satisfy the 75 C temperature spec by the heater manufacturer, and the non-metallic jacket is rated for 80 C, so I assume I'm good. Is there anything I'm missing?

Yep, essentially, it's just a premade up piece of conduit with the THHN/THWN already inside. They're pretty much designed for A/C compressors (outside), but they should work fine for your heater in the garage. In fact, it's probably would have been my choice anyhow rather than trying to dig up a few feet of flexible conduit and the end fittings to make my own.

Thanks, Ron. I think I'm about set ;-)