Limiting a Multi-Wire Branch Circuit (MWBC) to 3 circuits (meaning 3 hot wires plus a shared neutral) within a single conduit is generally not required by the National Electrical Code (NEC) based solely on the number of circuits. The NEC focuses on other factors that might lead to a need for derating, such as the number of current-carrying conductors in the conduit and the conduit fill percentage.However, there are several scenarios where such a limit might make sense, either due to practical considerations or due to local jurisdiction requirements beyond the NEC

Reasons a Limit Might Make Sense:
Derating

The NEC requires conductors to be derated (their ampacity reduced) if there are more than three current-carrying conductors in a conduit. While the NEC doesn't directly limit the number of circuits, it effectively limits the number of current-carrying conductors due to the derating requirement. If you have multiple MWBCs sharing a neutral, each hot wire is a current-carrying conductor. If you have 6 hot wires (3 MWBCs), then you are very likely needing to derate the conductor's ampacity.

Neutral Capacity

Although a neutral in an MWBC is generally only carrying the imbalance between the hot legs, there are situations where the neutral can carry a significant amount of current (especially if nonlinear loads are present, creating harmonics). If this scenario exists, you might be putting a lot of strain on a single neutral wire. Some jurisdictions may prefer spreading out the neutral load rather than concentrating it.

Conduit Fill

The NEC has strict rules about how much space conductors can take up inside a conduit (conduit fill). Packing too many wires in can make it difficult to pull the wires, increase heat buildup, and potentially damage the insulation. Limiting the number of circuits can help ensure the conduit fill percentage remains within acceptable limits.

Complexity and Troubleshooting

A conduit packed with many circuits can be more difficult to troubleshoot and maintain. Identifying and working on a specific circuit becomes harder.

Voltage Drop

While not directly related to the number of circuits, running many circuits in a long conduit can exacerbate voltage drop issues, especially if the loads are significant. Keeping the number of circuits lower might help keep voltage drop within acceptable limits.

Local Amendments/Specifications

It's possible that a local jurisdiction has added stricter rules or specific requirements for MWBCs or conduit fill. Project specifications might also include such limitations. Always check local codes and project documents.

Harmonic Loading

As mentioned, nonlinear loads (such as electronic devices, fluorescent lighting with electronic ballasts, and variable-frequency drives) produce harmonic currents that can overload the neutral conductor, even if the balanced current is low. The NEC requires the neutral conductor to be considered a current-carrying conductor in some circumstances when harmonics are present. A limit on the number of MWBCs in a conduit could be a way to mitigate this issue.

Practical Installation Issues

Trying to pull too many conductors through a long or complex conduit run can be physically difficult, increasing the risk of damaging the insulation.

In summary

While the NEC doesn't generally impose a direct limit of 3 circuits per conduit for MWBCs, the combination of derating requirements, conduit fill rules, and practical considerations often makes it a reasonable practice. Furthermore, the potential for harmonic loading and the increased complexity of a conduit packed with circuits can also make it a sensible approach.

Always consult the NEC, local codes, and project specifications before deciding on the number of MWBCs to run in a conduit.Recommendation:

Calculate the ampacity requirements, derating factors, conduit fill, and potential harmonic loading before deciding how many MWBCs to run in a single conduit. If you're unsure, it's always best to err on the side of caution and limit the number of circuits. Consult with a qualified electrician or electrical engineer for guidance. Flag for review