You're on the right track with 300.3(B)(3), but let's break down the situation and clarify the requirements

Understanding 300.3(B)(3): Parallel Conductors

This section of the NEC specifically addresses parallel conductors. It's designed for larger conductors where the current carrying capacity requirements are high and a single large conductor would be impractical or unavailable. It is typically used for feeders or service conductors 1/0 AWG and larger (see 310.10(H)(1)).

Key Implications of Using Parallel Conductors



Same Electrical Characteristics

All parallel conductors for each phase, neutral, and grounding electrode conductor must have the same electrical characteristics: Same conductor material (e.g., copper, aluminum) Same gauge size Same insulation type Terminated in the same manner

Ampacity Adequacy

The combined ampacity of the parallel conductors must be sufficient for the load they're serving.

Impedance Balancing

The physical layout and installation of parallel conductors is crucial to ensure equal impedance in each raceway. This is why 300.3(B)(3) emphasizes the same length and conductor characteristics.

Applying This to Your Situation

What you're proposing isn't exactly "parallel conductors" in the way the NEC typically envisions it. You're essentially splitting a single circuit into two conduits, each carrying one leg of the power and the neutral and ground.

Is it Permissible?

The short answer is: Yes, but not under the rules of 300.3(B)(3) for "parallel conductors". You can do it by treating it as two separate circuits that happen to share the same neutral and ground.

Here's why and how:1. Not True Parallel Conductors

True parallel conductors are multiples of the same conductor connected at both ends. You are not running two identical conductors in parallel for each phase.2.

Treating as Two Circuits

Instead, view it as two separate 120V circuits that share a common neutral and equipment grounding conductor. This is permissible, provided you meet all other code requirements.3.

Wiring Configuration

In each of the 1" conduits, you'll need: One hot conductor (L1 in one conduit, L2 in the other) A neutral conductor (Common neutral) An equipment grounding conductor (Common Ground)4.

Conductor Sizing

Size the shared neutral based on the maximum unbalanced load. Since you are splitting a 240V circuit, if you put 12 Amps on L1 and 8 Amps on L2, your neutral will be carrying 4 Amps. The neutral conductor must be large enough for the maximum calculated unbalanced load of both circuits combined. The equipment grounding conductor must be large enough for the overcurrent protection of both circuits.5.

Overcurrent Protection

Each hot conductor (L1 and L2) must have its own overcurrent protection (circuit breaker) rated for the circuit.6.

Grouping/Identification

The neutrals and grounds need to be properly grouped and identified at both the source and the load end. The hots also need to be identified per code requirements.7.

Ampacity Derating

If you're running a significant number of current-carrying conductors in the same conduit, you may need to derate the ampacity of the conductors based on the number of current-carrying conductors in each conduit (NEC Table 310.15(C)(1)). However, with only one current-carrying conductor (the hot) in each conduit, this is unlikely to be a factor unless the shared neutral carries a substantial load imbalance, in which case the neutral would also be considered current carrying.8.

300.3(B)(1) Compliance

Make sure the neutral and grounding conductors are run together with the ungrounded conductor in each raceway. This helps to minimize impedance and inductive reactance, providing a better path for fault current.

Why This Works (and Addressing Inductance Concerns)

You are satisfying the basic requirements of a circuit by having each hot leg, neutral, and ground present. Since the conduits are non-metallic (PVC), there's no issue with inductive heating of metallic conduit. By running the neutral and ground with each hot conductor, you're minimizing the loop area and inductance.

Important Considerations and Best Practices



Documentation

Clearly document this wiring configuration in your electrical panel schedule or wiring diagrams.

Labeling

Label the conduits and junction boxes to clearly indicate that these circuits are interconnected and share a neutral and ground.

Professional Judgment

If the load is highly unbalanced or has significant harmonic content, this arrangement might not be ideal. In such cases, consider running separate circuits with dedicated neutrals or increasing the size of the shared neutral.

In Summary

You can* split the L1 and L2 feeds into separate PVC conduits with a shared neutral and ground. However, this is

not considered "parallel conductors" under NEC 300.3(B)(3). You must treat this as two circuits sharing a common neutral and ground, and ensure you comply with all applicable code requirements for such an installation. Ensure the neutral conductor is sized adequately for the maximum calculated unbalanced load of both circuits. This approach is generally acceptable for residential applications as long as it's done safely and in accordance with the NEC. Flag for review