Okay, let's break down the grounding and bonding requirements for your UPS setup and address your questions, based on the 2020 NEC.1. UPS as a Separately Derived System (SDS)

Yes, your assessment is correct. Because the input neutral is isolated from the output neutral inside the UPS, it classifies as a Separately Derived System (SDS). This means the output of the UPS needs to be treated as a new power source regarding grounding and bonding.

2. Bonding the Output Neutral

You are on the right track with bonding the output neutral. According to NEC 250.30(A)(1), for a single-phase SDS with line-to-neutral loads, you must bond the neutral conductor to the enclosure or grounding electrode conductor at the SDS. This creates a low-impedance path for fault current.

Internal Jumper

Installing an internal jumper between the output neutral and the chassis, if the manufacturer allows it and it is done safely and according to instructions, would be one way to comply with 250.30(A)(1).

Crucially

Check the UPS manufacturer's documentation or contact their technical support. They may have a specific method or location for this bonding. Tampering with the UPS internals without authorization could void the warranty and create safety hazards.

Compliance with 250.20

Bonding the neutral in this way helps comply with 250.20, which addresses the general requirements for grounding AC systems. Specifically, 250.20(B) would require that the neutral be grounded if the supply is a separately derived system and the system is designed to have a grounded neutral.

3. Grounding the Output Neutral and the EGC vs. GEC Issue

Here's where it gets a bit more nuanced. Your instinct about using the EGC as a GEC is correct – it's generally prohibited by NEC 250.121(A). The equipment grounding conductor (EGC) exists to provide a low impedance path for fault current to return to the source. It is not intended to be the primary means of grounding the system.

The Correct Approach

You will not be using the EGC as a GEC. The EGC will do its job as an EGC, but it won't be the sole connection to ground. Because you are bonding the neutral conductor to the chassis of the UPS, you must connect the chassis of the UPS to a grounding electrode system or system bonding jumper per NEC 250.30(A).This means you will have a single GEC leaving the UPS chassis and connecting to the rack's ground system (your preferred method), or to the building's grounding electrode system. You only need one of these GECs. The EGC in the supply cable to the UPS is what will handle the fault current back to the main panelboard. The GEC will serve as a fault-clearing conductor for ground-faults on the load side of the UPS.Here's how to do it:1.

Bond the Neutral

As discussed, bond the output neutral to the UPS chassis internally, if and as permitted by the manufacturer. If the manufacturer prohibits this, the alternative will likely be to replace the output receptacle assembly with one where a connection is brought out for the neutral to be bonded to the chassis externally.2.

Grounding Electrode Conductor (GEC) to Rack Ground Busbar

Connect a GEC (sized according to NEC 250.66, but often a minimum of #8 AWG copper) from the UPS chassis ground terminal to the rack's ground busbar (RGB). This provides a dedicated grounding path for the SDS.
3.

Rack Grounding System

The RGB should then be connected to the Master Ground Busbar (MGB) of the room, and the MGB is connected to the building's Grounding Electrode System (GES). This creates a comprehensive and robust grounding network.

Important Considerations and Clarifications



Multiple UPSs per Rack

The above process must be done for each of the two UPSes per rack.

Redundancy

Redundancy in grounding is generally good, within limits. You don't want multiple paths for normal neutral current, but multiple paths for fault current are desirable.

Rack as Enclosure

Think of the rack as an extension of the UPS enclosure. The RGB should be well-bonded to the rack frame.

Telecommunications Grounding

Be mindful of any specific telecommunications grounding requirements for your equipment. ANSI/TIA-607 is a relevant standard. Your proposed solution should be compatible with these requirements.

Documentation

Keep thorough documentation of your grounding and bonding scheme. This is crucial for future maintenance, troubleshooting, and inspections.

Diagrams

While I understand the concepts, a diagram would certainly help me confirm that your specific implementation is correct and complete. A simple sketch showing the UPS, its input power source, the output receptacles, the rack, the RGB, the MGB, and the GES with all the grounding and bonding connections would be very beneficial. Please draw the input EGC and the output GEC clearly.

Disclaimer: This information is for educational purposes only. Always consult with a qualified and licensed electrician for any electrical work, and ensure that all work complies with local codes and regulations.* Flag for review