Okay, this is a significant amendment to NEC 300.3(C). Let's break it down and discuss its implications

Understanding NEC 300.3(C) (The Baseline)
NEC 300.3(C) generally focuses on the mixing of conductors operating at different voltages or with different insulation requirements within the same enclosure, raceway, or cable. Its primary concern is safety – preventing faults from higher voltage systems impacting lower voltage systems, and ensuring proper insulation ratings are maintained. The NEC allows for some mixing under very specific and controlled conditions. Different systems or class 1, 2, or 3 circuits are permitted in the same raceway, cable, or enclosure where specifically permitted in Chapters 5, 6, 7, and 8.

San Francisco's Amendment: A Much Stricter Rule

The San Francisco amendment completely prohibits the mixing of conductors from the following sources within the same enclosure, cable, or raceway: AC and DC circuits

Regardless of voltage level (up to 1000V), AC and DC circuits cannot be mixed.

Separately Derived Systems

This refers to sources like generators, transformers, UPS systems, etc. If you have a transformer providing power to one set of loads and a generator feeding another, the conductors from those two systems cannot share a raceway.

Separate Services

If a building has two or more electrical services (e.g., due to high power demand), the conductors from those different service entrances cannot be mixed.

Separate Utility Meters

If a building has separate utility meters for different tenants or purposes, the conductors served from each meter cannot be combined in the same raceway, cable, or enclosure.

Implications and Considerations1. Increased Conduit and Enclosure Requirements

This amendment will almost certainly increase the amount of conduit, boxes, and other enclosures needed in electrical installations. Each "system" must have its own dedicated pathway.2.

Stricter Labeling and Identification

Proper labeling of raceways and enclosures will be critical to ensure that circuits from different systems are easily identifiable and are not inadvertently mixed during future modifications or maintenance.3.

Design Changes

Electrical designs in San Francisco must meticulously plan circuit routing to avoid running conductors from different systems within the same raceway, cable, or enclosure.4.

Inspection Scrutiny

Inspectors in San Francisco will likely pay close attention to conductor segregation during inspections to ensure compliance with this amendment.5.

Cost Increases

The need for more conduit, boxes, and labor will likely increase the overall cost of electrical installations in San Francisco.6.

Space Constraints

In dense urban environments like San Francisco, finding space for the increased number of raceways and enclosures can be challenging. This may necessitate creative design solutions.

Rationale (Possible)

The reasoning behind such a strict amendment could be several things: Enhanced Safety

Reducing the potential for accidental shorts or ground faults by completely separating different systems.

Reduced Confusion

Simplifying maintenance and troubleshooting by ensuring that each system has its own dedicated pathway.

Preventing Overloads

A faulty system could back feed into another, and the stricter rules could prevent this from happening.

Increased Reliability

Isolating systems could minimize the impact of a fault on one system affecting others.

Example

Imagine a building with both solar panels (DC) and standard AC power. Under the NEC, there might be very limited scenarios where some mixing of conductors could be allowed (with appropriate insulation, separation, etc.). Under San Francisco's amendment, it's a flat no. The DC conductors from the solar panel inverter and the AC conductors from the utility service must be in completely separate raceways, cables, or enclosures throughout the building.

In Conclusion

This San Francisco amendment to NEC 300.3(C) is a significant* departure from the national code. It represents a much more conservative approach to conductor segregation and has substantial implications for electrical design and installation practices in the city. Anyone working on electrical projects in San Francisco must be acutely aware of this local rule and its impact on their work. Flag for review