The wire is a small resistor in series with the heater. Amps = 120 V ÷ (heater ohms + wire ohms). Volts lost = amps × wire ohms.
Current goes out on the black wire and comes back on the white wire. So a run of 200 feet each way has 400 feet of wire carrying current.
Wire resistance comes from Chapter 9, Table 8, for solid uncoated copper at 75 °C. Per 1,000 feet, 14 AWG has 3.07 ohms, 12 AWG has 1.93, and 10 AWG has 1.21. Cooler wire has a little less.
Heater resistance comes from the nameplate. Ohms = volts × volts ÷ watts. A 1,000 W heater at 120 V is 14.4 Ω, and a 500 W heater is 28.8 Ω. The picture keeps the heater’s resistance fixed.
The panel stays at 120.0 volts here. A real panel also drops a little under load. The picture counts only the loss on this branch circuit. The bus bars carry power in from the service, which is not drawn.
The lost volts turn into heat along the whole run. Each foot gets only slightly warm. The glow and wavy marks are drawn bigger so you can see them. Every choice here stays under the wire's rating.
An informational note under 210.19 suggests sizing branch-circuit wire to lose no more than 3 percent. That is 3.6 V at 120 V. Counting the feeder too, the target is 5 percent (215.4(A)(2)). For an ordinary branch circuit this is only advice. Notes are not enforceable, 90.5(C).
14 AWG copper goes on a 15 A breaker, 240.4(D)(4). Bigger wire on the same breaker is fine. If you upsize the hots for voltage drop, you must upsize the ground wire in proportion, 250.122(D). Each cable here has a ground the same size as its hots.
A smaller AWG number means a thicker wire. 10 AWG is thicker than 14 AWG, so it has fewer ohms per foot.
Cable makers color the jacket white for 14, yellow for 12, and orange for 10. The NEC does not require these colors.
The dots show amps. More dots passing each second means more amps. The dots do not show how fast electrons move. Real electrons drift only a fraction of a millimeter per second. Wall power is AC, so the charges really move back and forth. The dots show how much current flows.
Only trained workers should measure live voltage. Use a meter rated for the job and wear the right gloves and gear. Take covers off only to test. Test before you touch (trade practice).
Here the breaker is also the heater's shutoff. If the breaker is more than 50 feet from the heater or out of view, it must lock in the off position, 424.19(B)(1). "In sight" means visible and no more than 50 feet away, 110.29.
The picture shows one building with one heater on its own circuit. Many dwelling rooms also need AFCI protection, 210.12(B).
The bare ground wire carries no current unless something fails. The heating element's wire is the heater's resistance.