Okay, let's break down your situation and the potential implications. You've correctly identified the key points: Voltage Drop

You're exceeding the NEC's recommendation for voltage drop (generally 3% for branch circuits and 5% overall), but your load (the fixtures) is likely still functioning correctly at the lower voltage.

Universal Voltage Fixtures

The fixtures are designed to handle the voltage range you'll be delivering, so you won't damage them.

Wire Size Economics

Upsizing wire dramatically increases cost with diminishing returns in voltage drop reduction, especially over long distances.

Circuit Breaker Clearing Time

This is the crucial consideration you need to address more thoroughly.Here's a more structured approach to evaluating your design:

1. Voltage Drop - Deeper Dive NEC Recommendations vs. Requirements

Remember, the NEC's voltage drop guidelines in informational note No. 1 to 210.19(A)(1) and 215.2(A)(3) are recommendations, not absolute requirements. You can technically exceed them as long as it doesn't create a safety hazard or prevent the equipment from functioning properly. Document in your design notes why you are exceeding the recommendation and why it's acceptable in this case (fixture voltage range, cost considerations, etc.).

Fixture Performance at Low Voltage

While the fixture accepts the voltage, consider if there's any impact on light output or efficiency at the lower voltage. Check the fixture manufacturer's specifications if possible. If there's a significant reduction in light output, that might be a reason to reconsider. However, with the modern high-efficiency lighting available today, this is rarely an issue.

Real-World Voltage Fluctuations

Consider that the 277V feed to your site might also fluctuate. If the supply voltage occasionally dips, your final voltage at the far fixture could drop even further. Monitor voltage with a multimeter for a period of time to verify the consistency of the voltage being supplied.

2. Circuit Breaker Clearing Time - The Critical Issue

This is where you need to focus your attention. Excessive voltage drop can impact the performance of overcurrent protection devices (OCPDs) during a fault. Here's why: Fault Current Reduction

A large voltage drop reduces the magnitude of a short-circuit or ground-fault current at the end of the circuit. This lowered fault current might be insufficient to trip the circuit breaker within its specified clearing time.

Impedance Increase

The resistance and reactance of the conductors increase impedance along the fault loop path.

Coordination Issues

If the breaker doesn't trip quickly enough, it could cause damage to the conductors, equipment, or even create a fire hazard. It also impacts selective coordination if you have downstream breakers.

How to Evaluate Clearing Time


Calculate Fault Current

You MUST perform a short-circuit current calculation. This is non-negotiable. Use software (like SKM Power Tools, EasyPower, ETAP, or even a simplified point-to-point calculation) to determine the available fault current at the end of the circuit (at the last fixture) under both short-circuit and ground-fault conditions. This calculation must include: Utility source impedance (available from the utility company) Transformer impedance (if applicable) Conductor impedance (your #8 and #10 wires, plus any feeder wires) Any other impedance in the fault path. Account for minimum voltage scenario.2.

Breaker Time-Current Curves (TCC)

Obtain the TCC for your proposed 15A or 20A breaker. The TCC shows how long the breaker will take to trip at different fault current levels.3.

Compare Fault Current to TCC

Overlay your calculated fault current value onto the TCC. Determine the clearing time for that fault current.4.

Compare to Conductor Damage Curves

Obtain the allowable clearing time for the conductors (#8 and #10) based on their ampacity and insulation type. Ensure the breaker clearing time is faster than the conductor damage time.

Important Considerations for Fault Current Calculation



Minimum Voltage

Use the minimum expected voltage at the start of the circuit (e.g., 277V - some margin) in your fault current calculation. This will give you the worst-case (lowest) fault current.

Ground-Fault vs. Short-Circuit

Perform calculations for both types of faults. Ground-fault currents are often lower than short-circuit currents and may be the limiting factor.

Length of Run

Fault current is affected by the length of the run, so perform the calculation at the last fixture where the fault current will be at its lowest.

Software vs. Manual

Using software is highly recommended for accuracy and speed. Manual calculations are complex and prone to error.

3. Mitigating Clearing Time Issues

If your calculations show that the fault current is too low to trip the breaker quickly enough, you have several options: Increase Conductor Size

As you noted, this can be expensive.

Dedicated Ground Conductor

Ensure a dedicated equipment grounding conductor (EGC) is run with the circuit conductors. This provides a low-impedance path for fault current to return to the source and help improve breaker clearing time.

Ground-Fault Protection

Consider using ground-fault protection (GFP) devices. These are designed to detect very low-level ground faults and trip the circuit quickly. GFP is especially important for long runs where ground-fault current might be too low for a standard breaker to detect. You may be required to install it per NEC 215.10.

Current-Limiting Breaker

Use a current-limiting circuit breaker. These breakers are designed to interrupt high fault currents very quickly. However, they are typically more expensive.

Lower Breaker Amperage

As you noted, dropping the breaker to 15A might help, but only if it doesn't cause nuisance tripping under normal operating conditions (inrush current from the fixtures). You must verify the load is below 80% of the breaker rating.

4. High Bay Lighting

The same principles apply to your high bay lighting circuits. Perform voltage drop and, most importantly, short-circuit/ground-fault calculations to ensure adequate overcurrent protection. The fact that you have 180 fixtures makes this even more critical.

Documentation is Key

Regardless of your final design, meticulously document everything. This includes: Voltage drop calculations Short-circuit and ground-fault current calculations Breaker time-current curves Conductor damage curves Justification for exceeding NEC voltage drop recommendations Rationale for your chosen circuit protection strategyIn Summary

While the NEC voltage drop recommendations are not mandatory, you must ensure that your circuit protection is adequate. That means performing thorough short-circuit and ground-fault current calculations and verifying that the breaker will trip quickly enough to protect the conductors and equipment. Don't rely solely on the fact that the fixtures can accept the voltage. Clearing time is the most important safety aspect to verify. Flag for review