Understanding Available Fault Current, AIC, Interrupting Ratings, and SCCR

Electricians regularly encounter terms such as Available Fault Current, AIC, Interrupting Rating, Ampere Rating, Bolted Fault Current, and Short-Circuit Current Rating. Because many of these values are expressed in amperes, it is easy to confuse them.

The simplest way to remember the difference is this:

Available Fault Current tells us how much current the electrical system can deliver during a fault. Interrupting Rating tells us how much fault current a fuse or circuit breaker can safely interrupt. SCCR tells us how much fault current equipment can safely withstand. Ampere Rating tells us how much normal current the device is designed to carry.

Available Fault Current — AFC

Available Fault Current is the maximum fault current that the electrical system can deliver at a particular point.

For example, suppose the calculated available fault current at a 480-volt switchboard is:

AFC = 31,500 amperes

That means a bolted short circuit at that location could theoretically produce approximately 31.5 kA of fault current.

Available fault current depends on several factors, including:

• Utility source impedance

• Transformer kVA

• Transformer percent impedance

• Generator contribution

• Motor contribution

• Conductor size

• Conductor length

• Conductor impedance

Fault current normally decreases as we move farther away from the source because conductors and other system components add impedance. Eaton notes that available fault current can vary considerably throughout a facility based on transformers, conductors, motors, and other system characteristics.

AIC — Ampere Interrupting Capacity

You will often hear electricians say that a breaker has a 10,000 AIC, 22,000 AIC, 42,000 AIC, or 65,000 AIC rating.

AIC is commonly used in the field to describe the amount of fault current an overcurrent protective device can safely interrupt. The more formal term used for circuit breakers and fuses is Interrupting Rating.

For example:

20-amp circuit breaker — 10,000 AIC

The breaker is a 20-amp breaker, but it can safely interrupt a fault of up to 10,000 amperes, subject to its voltage rating and listing.

Those two ampere numbers describe completely different things.

Interrupting Rating ( AIC)

The Interrupting Rating is the highest current, at the specified voltage, that a fuse or circuit breaker is identified to safely interrupt under standard test conditions.

The basic rule is:

Interrupting Rating ≥ Available Fault Current

For example:

Available Fault Current = 18,000 A

Breaker Interrupting Rating = 22,000 A

Acceptable

But:

Available Fault Current = 18,000 A

Breaker Interrupting Rating = 10,000 A

Not acceptable, unless the equipment is being applied under an approved tested series-rated combination or another specifically permitted arrangement.

A breaker should never be expected to interrupt more fault current than its applicable interrupting rating.

Ampere Rating of a Fuse or Circuit Breaker

The Ampere Rating is associated with the normal current that the fuse or circuit breaker is intended to carry and protect.

Examples include:

20-amp breaker

100-amp breaker

400-amp fuse

This is very different from interrupting rating.

A breaker marked:

100 A — 22 kA Interrupting Rating

essentially tells us two things:

100 A = its normal ampere rating.

22,000 A = the maximum applicable fault current it is designed to safely interrupt.

Circuit breakers also operate according to their time-current characteristics rather than simply opening the instant current exceeds the number printed on the handle. Continuous loading and equipment listing must also be considered when applying breakers.

The same basic concept applies to fuses. Fuse ampere rating and fuse interrupting rating are separate ratings. For example, many UL Class fuses are available in various normal ampere ratings while carrying interrupting ratings as high as 200,000 amperes.

Bolted Fault Current

A bolted fault assumes that conductors are connected together through an extremely low-impedance connection — essentially as though they were physically bolted together.

For a three-phase system, we frequently calculate a theoretical three-phase bolted fault.

Because there is very little impedance at the fault itself, the current is primarily limited by the impedance of:

• The utility system

• Transformers

• Generators

• Conductors

• Bus

• Other components between the source and the fault

Bolted fault current is commonly used when determining available fault current and evaluating equipment ratings.

It is important to understand that:

Bolted fault current is not the same as arcing fault current.

An electrical arc introduces additional impedance. Therefore, the current flowing through an actual arc can be different from the calculated bolted fault current. Fault-current calculations are also important inputs to more detailed protection and arc-flash studies.

Short-Circuit Current Rating — SCCR

The proper term is generally Short-Circuit Current Rating, or SCCR.

SCCR normally applies to equipment or an assembly, rather than describing the interrupting capability of an individual circuit breaker or fuse.

Examples include:

• Industrial control panels

• Motor control centers

• Switchboards

• Panelboards

• Machinery

• HVAC equipment

• Disconnect assemblies

SCCR represents the prospective short-circuit current that the equipment has been evaluated to withstand safely under its specified conditions.

The fundamental relationship is:

Equipment SCCR ≥ Available Fault Current

If equipment has an SCCR of:

10,000 A

but the available fault current where it is installed is:

28,000 A

the equipment does not have an adequate SCCR for that location unless an applicable tested/listed protection method raises the effective rating.

Eaton summarizes the principle clearly: equipment SCCR must be equal to or greater than the available fault current at the equipment's installation point.

Putting the Ratings Together

Consider a panel supplied from a transformer.

Available Fault Current: 18,500 A

Panel SCCR: 22,000 A

Main Breaker Interrupting Rating: 22,000 A

Main Breaker Ampere Rating: 400 A

These numbers tell us:

18,500 A AFC — what the electrical system could deliver during a fault.

22,000 A SCCR — what the panel assembly can safely withstand under its applicable rating.

22,000 A Interrupting Rating — what the breaker can safely interrupt.

400 A Ampere Rating — the breaker's normal current rating.

Therefore:

18,500 A AFC ≤ 22,000 A Interrupting Rating

and

18,500 A AFC ≤ 22,000 A SCCR

The fault-current ratings are adequate.

The 400-amp rating has almost nothing to do with the 18,500-amp short-circuit calculation.

That distinction is one of the most important concepts to understand.

How to Calculate Available Fault Current

A detailed fault-current study may consider utility impedance, transformers, generators, motors, conductors, and other sources. However, electricians can perform a useful basic calculation beginning with the supplying transformer.

Step 1 — Determine Transformer Information

You need:

• Transformer kVA

• Secondary voltage

• Single-phase or three-phase

• Transformer percent impedance

Percent impedance is normally found on the transformer nameplate.

Suppose we have:

75 kVA

208Y/120 V

Three-phase

5.75% impedance

Step 2 — Calculate Transformer Full-Load Current

For three-phase:

I = VA ÷ (√3 × Voltage)

Therefore:

I = 75,000 ÷ (1.732 × 208)

I ≈ 208 A

Step 3 — Calculate Maximum Fault Current at the Transformer Secondary

Convert transformer impedance to decimal form:

5.75% = 0.0575

Then:

Available Fault Current = Transformer Full-Load Current ÷ Per-Unit Impedance

Therefore:

208 ÷ 0.0575 ≈ 3,620 A

So the approximate maximum three-phase bolted fault current directly at the transformer secondary terminals is:

3.62 kA

This calculation assumes an infinite primary source, meaning we ignore the impedance of the utility system ahead of the transformer. Ignoring source impedance produces a conservative maximum calculation at the transformer secondary for equipment-rating purposes. Eaton identifies this approach as an accepted conservative fault-current calculation technique.

Step 4 — Calculate Fault Current Downstream

As we move away from the transformer, conductor impedance must be included.

For a three-phase system, determine the resistance and reactance — or appropriate impedance data — of the conductors between the source and the point being evaluated.

The basic process is:

1. Determine the source impedance.

For a three-phase system:

Zsource = VLL ÷ (√3 × Isc)

2. Determine conductor impedance for the one-way conductor length.

If impedance is given per 1,000 feet:

Zconductor = Z per 1,000 ft × Length ÷ 1,000

3. For a more accurate calculation, add resistance and reactance separately.

Rtotal = Rsource + Rconductors

Xtotal = Xsource + Xconductors

Then:

Ztotal = √(Rtotal² + Xtotal²)

4. Calculate the new three-phase fault current.

Isc = VLL ÷ (√3 × Ztotal)

Each additional section of conductor adds impedance and normally reduces available fault current.

Motor and generator contributions must also be considered when they can materially contribute to the fault. Professional fault-current studies and calculation software account for these additional sources and system characteristics. Eaton's FC² calculator, for example, performs point-to-point single- and three-phase available fault-current calculations through transformers and conductor runs.

The Four Numbers Electricians Should Remember

When looking at electrical equipment, ask four separate questions:

1. What is the Available Fault Current? How much current can the system deliver here?

2. What is the Interrupting Rating or AIC of the breaker or fuse? How much fault current can the protective device safely interrupt?

3. What is the SCCR of the equipment? How much fault current can the equipment safely withstand?

4. What is the Ampere Rating? How much normal load current is the fuse, breaker, or equipment intended to carry?

The fundamental safety relationship is:

Available Fault Current ≤ Protective Device Interrupting Rating

and

Available Fault Current ≤ Equipment SCCR

Once those concepts are kept separate, fault-current terminology becomes much easier to understand.

Always verify calculations, equipment listings, manufacturer instructions, applicable series ratings, utility data, and the edition of the National Electrical Code adopted by the authority having jurisdiction before making equipment selections.