You're right to be skeptical. While transformers DO have impedance that limits fault current, it's not a magic "I'll only ever output my rated current" button. Let's break this down

Understanding Transformer Impedance
What it is

Transformer impedance (usually expressed as a percentage, like 5%) represents the opposition to current flow within the transformer itself. It's mainly due to the resistance and leakage inductance of the windings.

Why it's important

This impedance limits the fault current that can flow during a short circuit on the secondary side.

How it's expressed

The percentage impedance is the voltage drop across the transformer (expressed as a percentage of the rated voltage) when the transformer is delivering rated current. For example, a 5% impedance means that if you force rated current through the transformer, you'll see a 5% voltage drop across its windings.

Short-Circuit Current Calculation The fault current (I_fault) during a secondary-side short circuit is approximately calculated as follows:I_fault = (I_rated) / (Z_pu)Where: I_rated is the rated secondary current of the transformer. Z_pu is the transformer impedance expressed as a per-unit value (the percentage impedance divided by 100). For example, 5% becomes 0.05. Example

A 100 kVA transformer with a 240V secondary and 5% impedance: I_rated = 100,000 VA / 240 V = 416.67 amps Z_pu = 5% / 100% = 0.05 I_fault = 416.67 amps / 0.05 = 8333.4 ampsThat's a huge fault current – about 20 times the rated current. (This simple calculation doesn't account for all system impedances, but it gets the point across.)

Addressing Your Specific Questions


"If I put a 200 amp load on a transformer secondary rated at 100 amps with no protection, would the current somehow be limited by transformer impedance to 100 amps?" NO! The transformer impedance doesn't magically limit the load current to 100A. You're drastically overloading the transformer. Here's what would likely happen: Overheating

The windings would overheat due to excessive current flow. The heat generated is proportional to the square of the current (I²R losses).

Voltage Sag

The secondary voltage would drop significantly due to the increased current draw and the transformer's impedance. This might cause the connected equipment to malfunction or shut down.

Possible Failure

If the overload is sustained, the transformer insulation could break down, leading to a short circuit within the transformer itself, potentially causing a catastrophic failure (smoke, fire, explosion).

No Protection

Without overcurrent protection, nothing will automatically interrupt the excessive current flow until the transformer fails.2.

"By nature transformers are current limiting. Also it was mentioned that this could possibly make OVCP slightly less important." MISLEADING STATEMENT. It's true that transformer impedance limits fault current, but it's a very limited and unreliable form of "current limiting" for overload protection. OVCP (Overcurrent Protection) is ALWAYS ESSENTIAL. You absolutely need circuit breakers or fuses to protect transformers from: Overloads

As described above, sustained overloads will damage the transformer.

Faults (Short Circuits)

Even though the transformer impedance limits fault current, the fault current is still extremely high and can cause rapid and severe damage if not quickly interrupted.

Downstream Faults

A short circuit in a circuit downstream of the transformer will cause a high current flow in the transformer's secondary, which must be interrupted quickly by OVCP.

Why the Confusion?

The idea that transformers "limit current" can be misleading because it refers to fault current, not normal operating current. The impedance is there to prevent instantaneous destruction in the event of a short circuit, but it's not a substitute for proper overcurrent protection.

In summary: Transformers have impedance that limits fault current, but that fault current is still many times higher than the rated current. Transformer impedance is not a substitute for overcurrent protection (OCP). OCP (fuses or circuit breakers) is crucial to protect transformers from overloads and fault currents. Overloading a transformer without protection will cause overheating and potential failure.Think of it like this: the transformer impedance is like a weak brake on a speeding car. It slows it down a little, but you still need a real braking system (OVCP) to prevent a crash. Flag for review