Point at or tap any part to see its name. Tap the heater case or the 20 A breaker to change them.
Amber dots show the heater's normal current. Red diamonds show fault current, and neutral current on ground wires.
The dots show how much current flows, on one scale. Real electrons drift slower than a snail, a fraction of a millimeter per second.
The 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.
The fault and the trip are shown 100 times slower than real life.
The transformer makes a new system, called a separately derived system. Its neutral has no wire back to the service, so it gets its own bond, 250.30(A).
The system bonding jumper ties the neutral point, XO, to the transformer case and the ground wires, 250.30(A)(1). It can go at the transformer, as here, or at the first disconnect. It goes in one place only.
Fault current on the 120 V side has to get back to the 120 V winding. The ground wires and the bonding jumpers are that path, 250.4(A)(5).
The 6 AWG wire from the transformer case to the panel is the supply-side bonding jumper, 250.30(A)(2). The readout shows the current on that wire only. The heater's own ground wire is a separate wire.
The 480 V ground wire runs back to the 480 V source. It does not lead to the 120 V winding, so it carries none of this fault current.
The bare wire to building steel is the grounding electrode conductor, 250.30(A)(5). It lands on the ground bar, which Exception No. 1 allows when the jumper is a wire. The steel ties into the building's grounding electrode system, 250.30(A)(4) and 250.68(C)(2). It carries no current in these setups.
With the jumper out and no fault, nothing holds the new system's voltage to ground steady. That is what floating means, so the readout says Floating. A meter may still read some voltage there, from stray capacitance.
With the jumper out and the fault on, the case stays at ground through the 480 V ground wire and building steel. The neutral is what moves. It sits 118 V from the case, and the other two hots sit 208 V to ground.
Past the bonding jumper, the neutral must not connect to ground wires or metal parts, 250.30(A) and 250.142(B). A second bond puts neutral current on the ground wire during normal use. That is objectionable current, 250.6(A).
With both bonds, the neutral current splits by resistance. 10 ft of 1/0 neutral is 0.00122 Ω. The other way back, through the second bond, the 6 AWG supply-side bonding jumper, and the system bonding jumper, is 0.00511 Ω. So 2.4 of the heater's 12.2 amps, about one fifth, comes back on the jumper.
The EMT between the transformer and the panel is also a ground path, and it carries some of that current too. The model counts only the wires.
This is a teaching model. The transformer is 45 kVA, 480 V delta to 208Y/120 V, with 4 percent impedance, a typical nameplate value. That works out to 0.038 Ω per phase (0.04 × 120 V ÷ 125 A), added to the wires as plain resistance. The 480 V supply is drawn as stiff. Readings are rounded.
The secondary is 10 ft of 1/0 copper to a 150 A main breaker, Table 310.16, 75 °C column. The system bonding jumper, the supply-side bonding jumper, and the wire to steel are 6 AWG copper, Table 250.102(C)(1) and Table 250.66.
The 480 V feeder is 4 AWG copper on a 70 A breaker, with an 8 AWG ground wire, Table 250.122. The transformer draws 54 A at full load on the 480 V side. With the 150 A main as secondary protection, 70 A is well under the 250 percent allowed by Table 450.5(B). 4 AWG copper is good for 85 A, Table 310.16, 75 °C column.
The heater circuit is 50 ft of 12 AWG copper on a 20 A breaker, 240.4(D)(6), with a 12 AWG ground wire, Table 250.122. That wire is 1.98 Ω per 1,000 ft, Chapter 9, Table 8. The EMT to the heater is also a fault path beside that ground wire, so the real fault current would be higher. The model counts only the wire.
A fixed heater counts as a continuous load, 424.5. The 1,500 W heater draws 12.5 A, and 12.5 A × 1.25 is 15.6 A, under the 20 A breaker.
A typical 20 A breaker trips on its magnet in about one cycle, 17 ms, at 10 times its rating or more. That is maker data, not code. The 150 A main trips much slower at this current, so it stays on.
The covers are off only to show the inside. Working inside live equipment is a trained task. Before work, turn off and lock the 480 V feeder breaker, then test before you touch.
Wire colors are trade custom, not code: brown, orange, and yellow on the 480 V side, and black, red, and blue on the 208 V side.