Okay, this is a complex question involving electrical safety and code compliance. You're right to be cautious and thorough. Let's break down how to size that bare ground wire to reliably trip a 400A breaker, considering the potential for high fault currents and conductor temperature.

Understanding the Goal

You want to ensure that in the event of a ground fault, sufficient current flows through the grounding conductor long enough to trip the 400A breaker. This is about both the magnitude of the current and the duration. The grounding conductor must be sized to handle this fault current without failing before the breaker trips.

Key Considerations1. Fault Current Calculation

The most critical factor is determining the available fault current at the location where the grounding conductor will be installed. A 400A breaker doesn't limit the fault current; it only limits the overcurrent it's designed to interrupt. The available fault current depends on: The utility's supply capacity (transformer size, impedance). The impedance of the conductors between the utility transformer and the point of the fault. The impedance of any transformers or other equipment in between.

You MUST perform a fault current study to determine the available fault current. This is typically done by a qualified electrical engineer using software like SKM Power Tools, ETAP, or EasyPower. Assuming the fault current exceeds 400A is a good start, but you need to know the actual fault current value. Let's assume the fault current is 10,000A for the sake of example.2. NEC (National Electrical Code)

The NEC provides the minimum requirements for grounding conductor sizing. It DOES NOT guarantee optimal performance, but it is the bare minimum.

NEC 250.122

This is the primary section for sizing equipment grounding conductors. It typically bases the grounding conductor size on the size of the overcurrent protective device (in this case, the 400A breaker). However, it also has provisions for increasing the size of the grounding conductor if necessary to handle the available fault current.

NEC 250.4(A)(5)

This section emphasizes that the grounding path must be permanent and continuous and have the capacity to conduct safely any fault current likely to be imposed on it.3.

Grounding Conductor Material and Temperature Rating



Bare Copper

While bare copper has a higher melting point than insulated conductors, using a temperature rating of 75°C or 90°C (whichever is more restrictive based on terminations) is common and conservative when sizing the grounding conductor using ampacity tables. This is because the NEC's ampacity tables are based on limiting the temperature rise to protect connected equipment and maintain safety. The temperature rating is important because it determines the ampacity you use from the tables.

In Conduit

The presence of conduit affects the heat dissipation of the conductor. Conduit can trap heat, especially if it's PVC.

Proximity to 70°C Wire

This is a very important point. If the grounding conductor is in the same conduit as 70°C insulated conductors, the weakest link rule applies. You must size the grounding conductor so that it does not cause the 70°C conductors to exceed their temperature rating during a fault. The heat generated by the grounding conductor during a fault can easily damage the insulation of the other conductors in the conduit.4.

Adiabatic Equation

To prevent the melting of the insulation or conductor, the maximum allowable temperature rise for the ground wire must be calculated. This can be accomplished by using the adiabatic equation, which takes into account the fault current magnitude, time duration, and the wire's thermal characteristics.

Adiabatic Equation



(I^2 t) / k^2 = CSA^2 Where: I = Fault Current in Amperes (rms) t = Fault Clearing Time in Seconds CSA = Cross-Sectional Area of Conductor in Circular Mils k = Constant based on conductor material: Copper (205), Aluminum (122)5. Breaker Clearing Time

The breaker clearing time is the time it takes for the breaker to interrupt the fault current. This is critical. A faster clearing time means the grounding conductor experiences the high fault current for a shorter duration, allowing for a smaller conductor size. You can obtain this information from the breaker's time-current curve (TCC). A typical 400A breaker might clear a high fault current in a few cycles (e.g., 0.0167 seconds for a 60Hz system). Be conservative in your estimate.

Sizing Procedure

Here's a step-by-step approach:1. Determine Available Fault Current

This is not optional. Hire an electrical engineer to perform a fault current study. Let's assume the study shows 10,000A available.2.

Determine Breaker Clearing Time

Obtain the time-current curve for your 400A breaker. Determine the maximum clearing time at the calculated fault current (10,000A in our example). Let's assume it's 0.02 seconds (1.2 cycles).
3.

Initial Grounding Conductor Size (NEC 250.122)

Look up the minimum grounding conductor size based on the 400A overcurrent protection device in NEC Table 250.122. For a 400A breaker, the minimum is typically #3 AWG copper.4.

Adiabatic Calculation (Temperature Rise Check)

Use the adiabatic equation to calculate the minimum conductor size needed to withstand the fault current for the duration of the breaker clearing time without exceeding a safe temperature. This will likely be much larger than #3 AWG.

Example

Using the example values above, copper conductor, and aiming for a maximum conductor temperature of 70C: I = 10,000A t = 0.02 seconds k = 205 (for copper) CSA = √((10000² 0.02)/205²) = 337.432 circular mils Using standard wire sized, round up to the smallest available size: 400 kcmil copper wire, with CSA of 400,000 circular mils5.

Compare and Select

Compare the grounding conductor size from the adiabatic calculation with the size from NEC Table 250.122.

Use the larger of the two. In our example, the adiabatic calculation resulted in 400 kcmil, which is much larger than the #3 AWG that NEC Table 250.122 initially suggested. Therefore, you would need a 400 kcmil copper grounding conductor.

Important Considerations and Recommendations Coordination Study

If you have a complex electrical system with multiple levels of overcurrent protection, a coordination study is highly recommended. This ensures that the correct breaker trips first during a fault, minimizing the stress on the grounding system.

Professional Engineering

This type of calculation requires expertise in electrical power systems.

Engage a qualified electrical engineer. They can perform the fault current study, review the breaker characteristics, perform the necessary calculations, and ensure that the grounding system meets all applicable codes and standards. Documentation

Keep detailed records of all calculations, assumptions, and the basis for your design decisions. This is important for future reference, maintenance, and potential liability.

Inspection

Have the installation inspected by a qualified electrical inspector to verify compliance with the NEC and local codes.

Grounding Electrode System

This discussion focuses on the equipment grounding conductor (the wire that connects to equipment). Don't forget the grounding electrode system (ground rods, water pipes, etc.), which is also critical for a safe and effective grounding system.

In Summary

Sizing a grounding conductor to handle high fault currents is a critical safety issue. A simple "look up the breaker size in a table" approach is often insufficient. You must determine the available fault current, consider the breaker clearing time, perform an adiabatic calculation, and ensure that the grounding conductor can safely handle the fault current without damaging other conductors or equipment. Consult with a qualified electrical engineer for this task. Flag for review