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SCCR (short-circuit current rating) is how much fault current an assembly can withstand. AIC (ampere interrupting capacity, the industry shorthand for what the NEC calls an interrupting rating) is how much fault current a single protective device can safely interrupt. The two sound alike, but they apply to different hardware. Both have to be at least equal to the available fault current where the equipment is installed.
The third number, available fault current, isn't on any nameplate. You have to calculate it for the point where the equipment goes in, then check it against the AIC or SCCR.
On this page
- What is SCCR?
- What is AIC?
- SCCR vs. AIC: what's the difference?
- How do you determine SCCR for an assembly?
- Where does the NEC require these ratings?
- Can you just ask ChatGPT or Claude to run it?
- What AIC or SCCR rating do you need for the fault current?
- What if available fault current exceeds the rating?
- How do you document it?
- Getting per-point values without the spreadsheet
- FAQ
What is SCCR (short-circuit current rating)?
The SCCR rating is the maximum fault current an assembly can be connected to without sustaining damage beyond defined acceptance criteria. It applies to equipment as a whole: panelboards, switchboards, motor control centers, industrial control panels, HVAC equipment, elevator controllers.
The rating accounts for everything inside the enclosure, not just the main breaker. Terminal blocks, power distribution blocks, disconnects, contactors, and overload relays each carry their own rating, and the lowest one in the circuit governs the assembly.
The most common mistake in the field is reading the interrupting rating off the main breaker and calling it the panel's SCCR. This may not be true because the panel is only as strong as the weakest part inside it.
What is AIC (interrupting rating)?
The AIC rating is the maximum fault current an overcurrent protective device can interrupt and clear without failing. It applies to breakers and fuses specifically, and NEC 110.9 requires it to be at least equal to the current available at the device's line terminals. On the device label it usually reads "Interrupting Rating," "AIR," or "kAIR" (ampere interrupting rating). AIC, AIR, and interrupting rating all mean the same thing.
Two things to watch for:
- AIC is voltage-specific, and the rating drops as system voltage goes up. For example, a molded-case breaker might be marked 65 kA at 240V and 25 kA at 480V. Read the line that matches your system, not the largest number on the label.
- AIC is a device rating, not a panel rating. It tells you nothing about the terminal blocks or contactors sharing the enclosure.
SCCR vs. AIC: what's the difference?
| AIC (interrupting rating) | SCCR (short-circuit current rating) | |
|---|---|---|
| Applies to | Overcurrent protective devices: breakers, fuses | Assemblies: panelboards, switchboards, MCCs, control panels |
| Means | Fault current the device can interrupt and clear | Fault current the assembly can withstand |
| Set by | The device itself | The lowest-rated component in the circuit |
| Code section | NEC 110.9 | NEC 110.10 |
| Marked | On the device, per voltage | On the equipment nameplate or listing label |
Both get compared to the same number, the available fault current at that point in the system. A rating on its own doesn't tell you anything until you know what's available.
How do you determine SCCR for an assembly?
For a listed panelboard or switchboard, the SCCR is on the nameplate. For industrial control panels and machinery, it's determined under UL 508A Supplement SB, and it's less a calculation than a hunt: you go through every component in the power circuit, find each one's rating, and the lowest one is the rating for the assembly. That process is a topic of its own, and UL's guide to determining short-circuit current rating for machinery walks through it.
One thing to know from that method: a listed series-rated combination can let a downstream breaker be applied above its standalone interrupting rating, but only when the manufacturer has tested and listed that specific pairing. You can't make your own.
Where does the NEC require these ratings?
Available fault current appears throughout the Code, not just at the service:
| Section | Requirement |
|---|---|
| 110.9 | OCPD interrupting rating sufficient for the available fault current |
| 110.10 | Equipment SCCR adequate for the available fault current |
| 110.24 | Service equipment field-marked with maximum available fault current and calculation date |
| 408.6 | Panelboard and switchboard short-circuit current rating |
| 409.22 / 409.110 | Industrial control panels: not installed where available fault current exceeds the marked SCCR rating; the calculation and its date kept on file |
| 440.4(B) / 440.10 | Air conditioning and refrigeration equipment: same installation limit and documentation requirement |
| 620.16(B) / 620.51(D)(2) | Elevator control panels and disconnect marking |
| 670.3(A)(4) / 670.5 | Industrial machinery |
Every one of those sections boils down to the same two questions. What's available here, and is the gear rated for it?
Can you just ask ChatGPT or Claude to run it?
A general-purpose AI model is useful for learning the method, decoding a Code section, or sanity-checking how you've set up a problem. It's the wrong tool for producing the number you put on a label or pick a breaker against. That's not because the models are bad. It's how they work. They generate text one token at a time and don't do arithmetic unless they're hooked up to a calculator. They'll hand you a C value that looks exactly like the real one and isn't, and the same prompt can give you a different answer the second time you ask. You can't see any of that in the output, which reads just as confident either way.
This calculation is safety critical, and it has to be repeatable: same inputs, same result, every time, with the assumptions written down. That's also what 110.24 is asking for when it requires the calculation to be documented. A simple rule for any AI output here: if you can't reproduce the number by hand, or with software that gives the same answer every time, don't put your name on it.
What AIC or SCCR rating do you need for the fault current?
The AIC rating or SCCR rating you need is one that's at least equal to the available fault current at the point where the equipment goes in, so the job is getting that number. Start with the fault current at the source, then reduce it through each conductor run. If the utility gave you an available fault current or a meter reading, that's your starting value and you use it as-is. If not, calculate it from the transformer's kVA and nameplate impedance. Kopperfield's fault current calculator takes either input.
Full-load amps
Three-phase: FLA = (kVA × 1000) ÷ (V × 1.732)
Single-phase: FLA = (kVA × 1000) ÷ V
Fault current at the secondary
AFC = FLA ÷ (%Z ÷ 100)
Nameplate impedance carries a manufacturing tolerance, so to be conservative, use %Z × 0.90 (UL allows ±10%). Two adjustments depending on the system:
- On a 120/240V center-tapped secondary, run the numbers at the 240V winding. At the transformer terminals the 120V half-winding delivers about 1.5× that value, and it falls off faster than the 240V value as you move away from the terminals.
- On a three-phase system, a line-to-line branch (say the 480V pair of a 480Y/277V wye) sees 0.866× the three-phase value.
f factor
Three-phase: f = (1.732 × L × I) ÷ (C × n × V)
Single-phase: f = (2 × L × I) ÷ (C × n × V)
Multiplier and result
M = 1 ÷ (1 + f)
AFC at end of run = I × M
The factor of 2 applies to any single-phase branch, line-to-line or line-to-neutral. Repeat this for each run, feeding the result of one into the next as your new I.
L length in feet · I fault current at the start of the run · C conductor constant · n conductors per phase · V branch voltage
C values are published as a lookup table by conductor material, raceway, and cable type. If you keep a copy of Ugly's Electrical References on the truck, they're in the Short-Circuit Calculation section.
Worked example. A 300 kVA transformer, 208Y/120V secondary, 2% nameplate impedance. With the ±10% tolerance applied, the impedance used in the calculation is 2% × 0.90 = 1.8%, not 2%:
%Z = 2% × 0.90 = 1.8%, or 0.018
FLA = 300,000 ÷ (208 × 1.732) = 832.7 A
AFC = 832.7 ÷ 0.018 = 46,262 A, or 46.26 kA
At the unadjusted nameplate 2% it's 41.64 kA, but the conservative 1.8% number is the one to design against. Both assume an infinite primary source, which is the standard starting point when the utility hasn't given you a value.
Now check that against the equipment. A 22 kAIC breaker at those terminals is rated for less than half of what's available. A 65 kAIC device covers it. A panelboard marked 25 kA SCCR doesn't belong there either, no matter what its main breaker is rated for.
Downstream, the numbers drop. Conductor impedance knocks the current down with every foot of run, so the same job might have 46 kA at the main and under 20 kA at a sub-panel a couple hundred feet out. Gear that won't work at the service can be perfectly fine further down the line, which is why you need the number at every point and not just once for the building.
Downstream transformers work differently. A transformer partway down the system doesn't get the infinite-bus treatment above. Its secondary fault current is limited by whatever is actually available at its primary, combined with its own impedance. You can add a downstream transformer in Kopperfield's fault current calculator and it handles this for you.
Motor contribution. Running motors feed current back into a fault for the first few cycles, roughly four times their full-load amps at the point of connection. On most branch and feeder work it's negligible. It matters when there's a lot of connected motor load sitting close to the equipment you're rating, and in that case an engineered short-circuit study is the right tool for the number.
What if available fault current exceeds the rating?
Then the equipment can't go in as specified. You have four options:
- Specify a higher AIC or SCCR. Simplest, and usually the most expensive.
- Use a listed series-rated combination. A tested upstream device protects lower-rated downstream breakers. Current-limiting fuses and breakers work the same way, by cutting the let-through energy the downstream gear actually sees.
- Raise source impedance. A higher-impedance transformer delivers less fault current, at the cost of voltage regulation. Easy at design time, hard once the gear is already installed.
- Add conductor impedance. A longer run, or a smaller conductor where ampacity still allows, brings the downstream value down.
Which one makes sense depends on the job and what's already in the ground. High-fault and multi-source systems usually need an engineered short-circuit study rather than a point-to-point calculation.
How do you document available fault current?
NEC 110.24 requires service equipment at other than dwelling units to be field-marked with the maximum available fault current and the date the calculation was performed. The marking has to have "sufficient durability to withstand the environment involved," which in practice means an engraved Lamicoid (phenolic) placard or an industrial vinyl label, not a paper sticker or a marker note on the cover. When a modification changes the available fault current at the service (a transformer swap, a service upgrade, a change on the utility side), the value has to be verified or recalculated and the marking adjusted to match.
The 110.24 field marking: the maximum available fault current in amps and the date it was calculated, on an engraved placard.
What often gets missed is that 110.24(A) also requires the calculation itself to be documented and made available to anyone authorized to design, install, inspect, maintain, or operate the system. The label alone doesn't satisfy the section, and reviewers regularly ask for the math behind it. If you have the per-point values on paper with the method and assumptions stated, that's a two-minute conversation.
Getting per-point values without the spreadsheet
You can do all of the above by hand. It's tedious, though: an f factor and a multiplier for every run, with a C value looked up for each conductor size and material along the way. Transpose one constant and the whole downstream chain is wrong.
Start from a utility-supplied value or the transformer's kVA and %Z, add each run and piece of equipment in the chain, and Kopperfield returns the available fault current at every point, ready to compare against the AIC and SCCR on the gear you're specifying. Every calculation exports as a branded PDF with the per-point table, the method, and the assumptions printed on it, matching the headers on your load calcs and panel schedules. That's what you hand the inspector, and it's where the number on the 110.24 label comes from.
Your first two fault current calculations are free with a free Kopperfield account. Unlimited calculations are included in Commercial Pro, alongside commercial load calcs, commercial panel schedules, and 3-phase voltage options.
👉 Try the fault current calculator
Frequently asked questions
Is AIC the same as SCCR?
No. The AIC rating is the interrupting rating of a protective device, the fault current a breaker or fuse can clear. The SCCR rating is the withstand rating of an assembly. Both must be at least the available fault current at that point, but they are different ratings on different hardware, required by different Code sections.
Does the main breaker's interrupting rating give me the panel's SCCR?
No. This is the most common mistake. SCCR accounts for every component in the assembly, and the lowest-rated component in the circuit sets the number.
Can a panel's SCCR be higher than the interrupting rating of a breaker inside it?
Only through a listed series rating. On its own, a low-AIC breaker caps the assembly. In a tested and listed series combination with a specific upstream device, the combination rating governs and the downstream breaker can be applied above its standalone rating.
What if a component isn't marked with an SCCR?
For industrial control panels, UL 508A Table SB4.1 provides default values for unmarked components. Defaults are conservative, so a marked component usually gives you a better number.
Do I need this on residential work?
NEC 110.24 field marking applies to other than dwelling units, so a single-family home generally doesn't require the label. The comparison against equipment ratings still applies wherever equipment is installed, and some jurisdictions ask for the calculation regardless.
Is there any commercial work that's exempt from the 110.24 label?
One exception: industrial installations where conditions of maintenance and supervision ensure only qualified persons service the equipment. Everything else at other than dwelling units gets the marking.
