No, you cannot generally decrease the quantity of parallel sets and decrease the feeder size at a point 500 feet away (or any distance) from the origin of the parallel runs, even if the smaller feeder is protected by the upstream OCPD. This is because it creates a significant safety hazard and violates several key electrical code principles. Here's why: Overcurrent Protection Violations

While the downstream, smaller feeder might be protected by the upstream OCPD against short circuits and ground faults, it is NOT adequately protected against overloads. An overload is a current above the ampacity of the smaller conductor but not high enough to trip the large upstream breaker instantly. The upstream breaker is sized to protect the original, larger, parallel conductors, not the reduced size conductors further down the line. This could lead to overheating and potentially a fire.

Violation of Selective Coordination

The principle of selective coordination requires that, under fault conditions, only the overcurrent protective device closest to the fault should operate (trip). Reducing the conductor size and maintaining the larger upstream OCPD creates a situation where the downstream conductors are the weakest link. An overload or fault condition downstream could damage the smaller conductors and cause a fire before the upstream breaker even trips.

Unpredictable Current Distribution

Parallel conductors share current based on their impedance. Reducing the number of parallel conductors and reducing the conductor size downstream changes the impedance and current distribution within the system. This can lead to unpredictable and potentially hazardous current flows.

NEC (National Electrical Code) Violations

Multiple NEC sections prohibit this practice. For example:

NEC 240.21(C) Tap Rules

These rules allow for certain tap conductors under very specific conditions – primarily for a defined load (e.g., a single motor) and over a limited distance. Your scenario of reducing conductor size and number of parallel sets after 500 feet does NOT meet tap rules.

NEC 310.15 (Ampacity of Conductors)

Conductors must be sized based on the load they serve and are required to be protected at their ampacity.

NEC 300.3(B) Conductors of the Same Circuit

All conductors of the same circuit, including neutral and equipment grounding conductors, must be run together in the same raceway or cable. The change in conductor size and quantity violates this because a portion of the circuit is using the original parallel set and another portion isn't.

Voltage Drop Considerations

While your initial motivation might be voltage drop, reducing conductor size exacerbates voltage drop issues further down the line.

Safe and Code-Compliant Alternatives

If you're facing voltage drop issues and need to adjust the conductor size, here are some acceptable approaches:1.

Increase the OCPD and conductor size upstream

If voltage drop is the issue, use larger conductors and a larger OCPD from the beginning of the run to compensate.2.

Use a Step-Down Transformer

Place a transformer near the load to step down the voltage to a lower level, which will reduce the current and thus reduce voltage drop.3.

Change the Supply Voltage

If possible, increase the supply voltage. Higher voltage means lower current for the same power, reducing voltage drop.4.

Install a Subpanel with a Separate Feeder

Run a new feeder from a different source (e.g., a subpanel) closer to the load.5.

Derate Downstream

IF you are running parallel sets and have calculated that you do not need the full ampacity that is offered by the multiple parallel feeders, it is permissible to derate the system after the load calculation. However, this means that the OCPD must be decreased to match the adjusted ampacity. This is not what you described.

In summary: You cannot arbitrarily reduce the number of parallel sets and conductor size downstream while maintaining the original upstream OCPD. This creates serious safety hazards and violates the NEC. You must size conductors according to the load and protect them at their ampacity or follow specific rules for tap conductors, which your described situation does not meet. Always consult the NEC and a qualified electrical engineer for compliant and safe electrical installations. Flag for review