Point at or tap any part to see its name. Tap the heater to turn it on or off.
Amber dots show current on the hots and the neutral. Red diamonds show current on the ground wire and the metal can. That current belongs on the neutral.
The dots show how much current flows, on one scale. They do not show how fast electrons move. Real electrons drift slower than a snail, a fraction of a millimeter per second.
Wall power is AC. It runs 60 cycles a second, so it reverses 120 times a second. The dots show where the current flows and how much.
At the service, the main bonding jumper ties the neutral to the ground bar and the can, 250.24(C). In this building, that is the only place they join.
Past the service disconnect, the neutral must not connect to ground wires or metal parts, 250.24(B) and 250.142(B).
A panel's ground bar is bonded to its metal can. Ground wires go on the neutral bar only where neutral and ground are allowed to join, 408.40.
A main bonding jumper that is only a screw must be green, 250.28(B). Many panels come with one so they can serve as service equipment. In a subpanel it stays out.
Current on ground wires and metal parts during normal use is called objectionable current. Grounding must be set up to prevent it, 250.6(A).
A transformer and a separate building each follow their own rules, 250.30 and 250.32. This picture is one building fed by one service.
With the screw in, the current splits by resistance. 60 ft of 8 AWG neutral is 0.047 Ω, and 60 ft of 10 AWG ground wire is 0.074 Ω. So about 39 percent comes back on the ground wire.
Those come from Chapter 9, Table 8: stranded copper, 0.778 Ω per 1,000 ft for 8 AWG and 1.24 Ω for 10 AWG.
Any other metal path back to the service, like a metal pipe or conduit, would carry some of this current too. The model leaves those out.
With this one heater, the can rises less than 1 volt above the service ground. More load raises it. The trouble is current on metal parts all the time, not the voltage.
With the neutral loose, the ground wire is the only way back. If it comes loose too, the can and the heater case can sit at 120 volts.
The feeder is 8 AWG copper cable on a 40 A breaker. NM cable uses the 60 °C column, 334.80(A), and 8 AWG copper is good for 40 A there, Table 310.16. Its 10 AWG ground wire follows Table 250.122.
The heater circuit is 25 ft of 12 AWG solid copper at 1.93 Ω per 1,000 ft, on a 20 A breaker, 240.4(D)(6). The 1,500 W, 120 V heater acts as 9.6 Ω.
This is a teaching model. The service holds 120 volts from the hot bus to its neutral bar. The utility side is left out. Readings are rounded.
A basement receptacle in a home needs GFCI protection, 210.8(A)(6). The same current goes out and comes back through the GFCI, so it stays on in every setup.
The covers are off only to show the inside. Working in a live panel is a trained task. Before work in the subpanel, turn off and lock the 40 A feeder breaker, then test before you touch.
In the service panel, the wires from the meter stay live even with the main breaker off.
Cable jacket colors show wire size by maker custom, not by code. Black is 8 AWG and yellow is 12 AWG.