You're right to question this – it's a nuanced situation!Here's the breakdown: Individual Transformer Impedance

Each transformer has 2% impedance based on its own kVA rating and voltage base (240V primary, 120V secondary individually). This 2% represents the internal impedance voltage drop as a percentage of the voltage when full rated current is flowing through the winding.

Transformer Bank Impedance

The 2% impedance is essentially unchanged when considering the 3-phase bank if you're calculating the short circuit current on the 120/208V side, and you're referencing it to the bank's 3-phase kVA rating.

Why paralleling the 120V windings doesn't change the overall bank impedance



Impedance is a per-unit value

The 2% impedance is expressed as a per-unit value. Per-unit impedance is defined as the actual impedance divided by the base impedance. Base impedance is defined as (Base Voltage)^2 / (Base kVA).

Paralleling doesn't affect the per-unit impedance

When you parallel the 120V windings within each transformer, you are effectively doubling the kVA rating of each transformer. Since the base kVA of each single phase transformer increases, its new base impedance changes, but the transformer's per-unit impedance remains the same.

Bank kVA Consideration

When calculating the short circuit current on the 120/208V side, you are now considering the total 3-phase kVA rating of the bank.

In summary


Use 2% for the transformer bank impedance, referenced to the bank's 3-phase kVA rating, in your short circuit calculations.

Important Considerations



Transformer kVA Rating

Make sure you are using the

total kVA rating of the transformer bank in your short circuit calculation. This is 3 times the kVA rating of a single transformer. X/R Ratio

Don't forget to obtain or assume a reasonable X/R ratio for the transformer impedance. This is crucial for accurate arc flash calculations.

Upstream Impedance

The transformer impedance is not the only impedance in the circuit. You also have to consider the impedance of the utility source, any upstream cabling, etc. All impedances must be accounted for to get accurate short circuit current values.

Software

If you're using arc flash analysis software (EasyPower, SKM, ETAP, etc.), you'll input the transformer data (including the 2% impedance) into the transformer component in the software, and the software will handle the calculations.

NEC 450.6 (A)

Consider the fault current withstand rating of the transformers. If the available fault current exceeds the transformer's withstand rating, protection upstream needs to be coordinated.

Example

Let's say you have three 50 kVA single-phase transformers providing 120/208V service. Total bank kVA = 3 50 kVA = 150 kVA* Use 2% impedance for the 150 kVA bank when calculating the short circuit current at the 120/208V bus.

In conclusion, for your short circuit calculations at the 120/208V side of the transformer bank, use the 2% impedance, but ensure it's referenced to the total 3-phase kVA of the bank. Always double-check your assumptions and data inputs to ensure accuracy in your arc flash study. Flag for review