Okay, let's break down the best way to approach this and address the code considerations.

Understanding the Situation 277/480V Generator

This is the output voltage of your generator.

400A ATS

Your Automatic Transfer Switch handles the switching between utility power and generator power.

2 x 120V Circuits

The generator needs two dedicated 120V circuits for its own operation (likely for control, monitoring, or essential accessories).

Transformer & Load Center

You're planning to use a transformer to step down the 480V to 120/240V and feed a small load center.

Tap Location

You're asking about tapping into the load side of your service (which, during normal operation, would be powered by the utility) to feed the primary of the transformer.

The Best Approach (and why)

The cleanest and most code-compliant solution is to tap the transformer BEFORE the ATS.

Here's why and how:1. Reliability

The generator needs those 120V circuits regardless of whether the ATS has switched over. If you tap after the ATS on the load side, the transformer and its 120V circuits will only be powered when the utility is feeding the load. If the generator needs to start for any reason before the ATS switches over, it won't have power to its 120V circuits.2.

ATS Functionality

Tapping on the load side of the ATS for essential generator circuits could potentially interfere with the ATS's monitoring or control functions. It's best to keep those circuits isolated.3.

Code Compliance (NEC)



700.12(E) - Emergency System Circuits

If the generator is part of an emergency system, the branch circuits powering the generator's essential controls must meet the requirements of Article 700. These typically require wiring methods and protection similar to the rest of the emergency system.

450.3 - Transformer Overcurrent Protection

The primary and secondary of the transformer must be properly overcurrent protected. We'll discuss this in detail below.

Detailed Steps & Considerations1. Location of Tap

Find a suitable location

BEFORE the ATS, directly from the 480V service. This might be in the main service panel or a junction box dedicated to the generator system.2. Overcurrent Protection (Primary)

This is critical. You must protect the transformer primary. You will need a suitably sized circuit breaker or fuses at the tap point. The size of the breaker/fuses is determined by NEC 450.3.

450.3(B) - Primary Protection

The NEC allows for the primary overcurrent protection device to be rated up to 125% of the transformer's primary current, if the overcurrent device next inline protects to the equipment ampacity.

Calculate the primary current of the transformer: Determine the kVA rating of your transformer. You need to choose a transformer that is adequately sized to handle the load of the two 120V circuits PLUS a safety factor (25%). Use the formula: `Primary Current (I) = kVA 1000 / (Voltage √3)` (Voltage = 480V) Example: If you need a 1 kVA transformer: I = 1 1000 / (480 1.732) = 1.2 Amps Allowable breaker size = 1.2A 1.25 = 1.5A (Use the next standard size up 2A) Use an appropriately sized circuit breaker or fuse at the tap point, based on this calculation.3.

Transformer Selection

Choose a 480V primary to 120/240V secondary transformer. The kVA rating should be large enough to handle the connected load on the 120V circuits, with a safety margin. A small transformer, perhaps 0.5 kVA or 1 kVA, would likely be sufficient for just two circuits powering controls.
4.

Transformer Installation

Mount the transformer according to the manufacturer's instructions. Ensure proper ventilation.5.

Load Center (Secondary Panel)

Install a small load center connected to the secondary (120/240V) of the transformer.6.

Overcurrent Protection (Secondary)



450.3(C) - Secondary Protection

This is also very important. The secondary of the transformer must have overcurrent protection as well. Determine the secondary current of the transformer: Use the formula: `Secondary Current (I) = kVA 1000 / (Voltage)` (Voltage = 240V) Example: If you are using a 1 kVA transformer, the secondary current would be 4.16A. There are two ways to provide secondary protection:

Secondary Breaker

Install a main breaker in the load center, sized appropriately for the secondary current.

Feeder Breakers

Install individual breakers for each of the two 120V circuits. Ensure the sum of the breaker ratings does not exceed the calculated secondary current. If you're using individual breakers, size them appropriately for the conductors.7.

Wiring Methods

Use appropriate wiring methods for the voltage and environment. Consider using conduit for protection.8.

Grounding

Properly ground the transformer and the load center. The grounding electrode conductor (GEC) for the transformer should connect back to the main service ground.9.

Conductor Sizing

Size the conductors appropriately for the calculated currents, taking into account voltage drop.10.

Labeling

Clearly label all components, including the transformer, load center, and the breakers feeding the transformer primary.

Tap Rules (NEC 240.21) and Your SituationThe "tap rules" (NEC 240.21) allow you to run conductors of a reduced size for a short distance from an overcurrent device (OCD) to the equipment they supply. The rules specify requirements regarding length, ampacity, and termination.The 10-foot tap rule is one of the tap rules. You only need to apply the tap rules if you are using a smaller gauge wire after the breaker.

Answers to Your Specific Questions

"Can we tap wire off load side of service to primary side Transformer with small panel on secondary?"

It's generally not recommended to tap from the load side of your ATS to the primary side of the transformer "Would I need to use the 10' tap rule on primary?"

You only need to apply the tap rules if you are using a smaller gauge wire after the breaker.

Important Considerations

Coordination with ATS

Ensure that the installation of the transformer and associated wiring does not interfere with the proper operation of the ATS. Consult the ATS manufacturer's documentation.

Professional Installation

Due to the high voltages and code complexities involved, this work should be performed by a qualified and licensed electrician.

Local Codes and Inspections

Always comply with all applicable local electrical codes and obtain necessary permits. Schedule an inspection after the work is complete.

In summary:* Tap BEFORE the ATS, ensure proper overcurrent protection on both the primary and secondary sides of the transformer, select appropriately sized components, follow all applicable codes, and use a qualified electrician. This approach will provide a safe, reliable, and code-compliant solution for powering the generator's essential circuits. Flag for review