Let's break this down, as the NEC can be tricky and your specific scenario warrants careful consideration.

The Core Issue: 300.3(C) and Mixing Voltages

NEC 300.3(C) is the section you're referencing, and it generally allows conductors of different voltage systems to occupy the same raceway, cable, or enclosure. The key wording to pay attention to is: "All conductors of the circuit, including the neutral and equipment grounding conductor, shall be within the same raceway, auxiliary gutter, cable, or enclosure."Analysis of Your Situation

You have a 277V lighting circuit in an EMT raceway. You want to add a 120V circuit for a recirculation pump to the same raceway. Here's how to evaluate:1. Voltage Rating

Both 277V and 120V circuits are below the 600V rating of THHN wire. This is fine.2.

Insulation Type

You state you're using THHN wire, which is permitted for both voltage systems. Again, no problem.3.

The Most Important Requirement: SAME CIRCUIT! 300.3(C)(1) states:"Where more than one circuit is installed in a raceway, all conductors of all circuits shall have an insulation rating equal to at least the maximum circuit voltage applied to any conductor within the raceway."This does NOT allow you to install a COMPLETELY separate 120V circuit, in a 277V Circuit raceway.Why This Matters: Safety

The primary reason for this rule is safety. Mixed voltages in the same raceway or enclosure can increase the risk of insulation breakdown and potentially dangerous situations, especially if not properly managed.

Permissible

If the pump would have been supplied with 277 volts then this would have been permissible.

What to Do Instead

You have a few options, and the best choice depends on the specifics of your installation:1. Separate Raceway for 120V Circuit

This is generally the safest and most straightforward approach. Run a dedicated EMT raceway from your 120/208V panel to the mechanical room for the recirculation pump. This keeps the voltage systems completely isolated.2.

Existing 120V Raceways. You mentioned you have a box with 120V wiring already, the best option would be to use that. If there is room in that raceway.3. Dedicated Multi-Wire Branch Circuit

Consider running a dedicated multi-wire branch circuit (MWBC) from the 120/208V panel. MWBCs share a neutral, which can be used in a common raceway with the hots. However, MWBCs have specific requirements in the NEC (like handle ties on the breakers - 210.4) and can be problematic if not installed correctly. This would only work if you have other loads on that 120 panel in the mechanical room.
4.

Another method that will work, install a small 480 to 120 transformer near your existing 277 pull box and run wire from that transformer to the pumpImportant Considerations Ampacity and Conduit Fill

Even if you find a configuration that technically complies with 300.3(C), always ensure your raceway is properly sized for the number of conductors (conduit fill calculations) and that the conductors are properly sized for the load (ampacity).

Local Amendments

Remember that local jurisdictions may have amendments to the NEC. Always check with your local inspector or building department to ensure your installation meets all applicable codes and regulations.

In Summary

While NEC 300.3(C) allows for conductors of different voltage systems in the same raceway, the 120V pump circuit and the 277V lighting circuit are considered separate systems in your situation. Running a separate raceway for the 120V circuit is generally the best and safest approach. If you absolutely must* combine circuits, proceed with extreme caution and consult with a qualified electrical engineer or inspector to ensure full compliance with the NEC. Flag for review