First off, 20.8 amps is approximately the amperage draw on a single element furnace (nominally 5 kw.) So if you have 3 or 4 elements, (15 or 20 kw capacity) the problem is the remaining elements are not being enrgized. Look to see if there is a W1 and W2 and even possibly a W3 terminal on the terminal strip. If so, and its not a heat pump, there should be thermostat wires on all three, indicating three stages of heat. To energize those properly, the thermostat should be a multi-stage heating thermostat providing W1 and W2 at the minimum. If you do not have a multi stage thermostat, and there are no wires on W2 and W3, jumper W1, W2 and W3 together and the furnace will come on at full capacity with the single stage thermostat.
If you DO in fact have wires on the additional W terminals, you more than likely have a heat sequencer that is not energizing the 2nd and possibly the third stage of heat. A three element furnace is typically called a 15kw furnace and the three elements should draw approximately 19 to 20 amps each. So when each stage draws in, the clamp on amprobe on the main incoming wires will correspondingly increase with each stage.
Also, some 15 and 20 kw furnaces require two circuits to supply the elements, and I have been on calls before where the second breaker was not turned on, and in one case, the second circuit wasn't even wired in from the electrical panel box.
DO NOT pull the wire off the fan while the heaters are energized as they will rapidly cycle on an overtemp device. Some actually have a "one time" thermal link that once melted render the respective element inoperable until that thermal link is replaced. They will melt very easily. An energized element MUST have airflow across it !!!
Testimonial: "This is a shop heater, 20a 240v dual element the only terminals available are L1 L2 (power) and T1 T2 (thermostat) quite the answer but not much of it applies to this issue."
The problem with a lot of the questions is that they themselves leave a lot to be desired. We cannot guess what it is you are having a problem with. A model number does nothing to speak of as far as giving information unless you are conferring with brand specific service techs. Your response provided more insight to your problem than the original question you posted.
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