The PSX series of circuit breakers and Auto Reversers [PSX-AR] will trip within about 200us by default (CV55=0) of the current exceeding the set trip value. The PSX measures the current on the rising edge of each DCC bit to detect the excess current as quickly as possible. HOWEVER, series inductance in the wiring will slow the rate of rise of the current with respect to the leading edge of the waveform. This can slow the PSX response and if there is enough inductance, the PSX will not trip. If there is enough inductance to prevent the PSX from tripping, there is also enough rounding of the leading edge of the DCC waveform to prevent reliable data transmission. Per NMRA Standard S 9.1 "transitions that cross the region between -4 volts and +4 volts shall occur at 2.5 volts per microsecond or faster." This is equivalent to 3.2 us to cross the region from -4 to +4 volts. This is the standard to which any DCC decoder is designed.
Inductance is essentially electrical inertia. The more the inductance, the more the inertia. It is inertia that keeps your car from doing 0 - 60 in 0.1 seconds. It is electrical inertia (inductance) that can keep the DCC signal from doing -4 volts to +4 volts in 3.2us. OK. What is inductance? As the current in a wire begins to increase, it generates a magnetic field. The magnetic field is changing, and induces a voltage in the wire that OPPOSES the rise in current. The net effect is to slow down the rate of current increase. The key here is the magnetic field around the wire. The less magnetic field, the less inductance. It turns out that the overall inductance in a circuit is proportional to the loop area of the circuit.
OK. Why the Engineering 102 lecture? Because the physics tell us how to wire a layout for DCC. Note that since a DC layout does not have a rapid voltage change, inductance makes not difference. Hence, a DC layout wired with individual wires will work great. Solid, individual wires, however, have a large loop area and generate maximum inductance; the worst case scenario for a DCC layout.
Our recommendation is to use speaker wire to run the main buses of a DCC layout, particularly for a club size layout since inductance is also proportional to wire length. Parts Express SKRL-12-100 is a good example. The wires are physically close making the loop area very small. This means that equal and opposite currents in the two parallel wires produce equal and opposite magnetic fields that cancel each other...very low inductance! The next best thing is to use stranded wire and twist it tightly, about 4-6 twists per foot.