What is the maximum 12/2 wire length from a 20A breaker to a baseboard heater?
Using the values from Chapter 9 Table 8 of the National Electrical Code, you can calculate the maximum circuit distance as follows.
Find allowable voltage drop
The NEC recommends a maximum 3% voltage drop at the &quot;farthest outlet of power, heating, and lighting loads, or combinations of such loads&quot; (210.19(A) FPN No. 4). So you'll want to start by calculating the maximum voltage drop as follows:
Max Voltage Drop = Voltage * 3%
Max Voltage Drop = 240V * 0.03
Max Voltage Drop = 7.2V
Calculate the maximum length
To calculate the maximum length of the circuit, you'll first have to know how much resistance the conductor being used has. Table 8 from Chapter 9 of the NEC, makes this simple.  If you're using 12 AWG solid uncoated copper, you'll find that it has a resistance of 1.93 ohms per thousand feet.



For the following formula, you'll need to know the resistance per foot. This is easily calculated, by dividing the value above by 1000.
1.93 ohms per 1000 feet / 1000 = 0.00193 ohms per foot
Next you'll need to know the amount of current flowing along the conductor. Since you didn't specify this in the question, I'll use 20 amperes as an example.
The formula
Length = maximum voltage drop / ( 2 * Ohms per foot * current)
Length = 7.2 V / ( 2 * 0.00193 Ohms * 20 Amperes)
Length = 7.2 / ( 0.00386 * 20)
Length = 7.2 / 0.0772
Length = 93.264248704663212435233160621762 ft.

tl;dr
If you have a circuit with 20 amperes of current flowing along a 12 AWG uncoated solid copper conductor, and you want to keep the voltage drop below 7.2 V (3% or 240 V). You'll want to keep the length of the circuit (one way) less than 93' 4&quot;. Unless may calculations are wrong (which they sure could be).
NOTE:
Since you didn't mention the actual current drawn by the heater. The above calculations are for example purposes only.

Using Ohm's law, you can figure out the current based on the wattage you've stated.

I = P / V
I = 2000 watts / 240 volts
I = 8.33333 amperes
You can then plug this into the formula above.
Length = 7.2 Volts / ( 2 * 0.00193 Ohms * 8.33333 Amperes )
Length = 7.2 / ( 0.00386 * 8.333333 )
Length = 7.2 / 0.0321666666666667
Length = 223.8341968911917 ft.