the 4800 watts heater does not seem to get even hot

[kajvid]

I just bought one of those 4800 watts dimplex heaters for my garage that is 25 x 30 with non insulated celling. Was hoping to warm things up at least a bit. But this heaters coil is not even getting dull red. I am looking for ideas what might be the problem, yes its made in China but there is hardly an alternative these days in North America.

I wired the (new) outlet to the back of my 240v compressor/ welder plug which is on the outside of garage and this heater plug is on the inside wall.

the cable is solid 10g with bare 12g, twisted. At the fuse box it is wired in this fashion: red and black to the 40A double squareD breaker
The white wire is screwed to buss bar and the bare 12g is not connected to anything.

I suspect that I have made an error when I recently wired the above mentioned compressor/welder plug, as the compressor is hard to get going in this cold weather also.
I am getting 120V horizontaly across the outlet terminals but also 240V if I touch the rounded terminal simultaniously with one of the side o

At any rate, while the compressor is kinda sluggish the heater doesnt get hot at all. Anyone here to set me straight

Thank you for all suggestions

 

[big-all]

do you mean 480w??
if so it wont make much difference

 

[kajvid]

As posted the heater is Dimplex 4800 Watt type. Not 480watt.
Everybody I asked was raving about it so I bought one. While is rather small it is rated the construction grade so I think that the element should get cherry red at least, if not glowing.