10 kA Breakers on a 19 kA Bus: The Panel That Cannot Interrupt
A small shop takes 208Y/120 V, 3-phase service from an 800 A service point where the utility reports 26,000 A available. A new 200 A subpanel is installed 120 ft away; the calculated bolted fault current at its bus is 19,400 A. The contractor purchased a 200 A panelboard marked 10 kAIC with 10 kAIC branch breakers, reasoning that "it's only a 200 A panel." A rooftop unit fed from the panel has a nameplate reading: minimum circuit ampacity 34 A, maximum fuse size 40 A, short-circuit current rating 5 kA. The electrician proposes pulling larger feeder conductors to "bring the fault current down" instead of changing the panel.
1. Is the 200 A panelboard acceptable at this location? Explain the governing principle.
Read the worked answer
No. The equipment must be able to interrupt or withstand the fault current available at its terminals.
- Available at the panel bus: 19,400 A
- Panel and branch breakers marked: 10,000 A
Every overcurrent device must have an interrupting rating at least equal to the maximum fault current available at its line terminals, and the panelboard's rating is limited by its weakest component. A 10 kA breaker on a bus with 19.4 kA available can be destroyed - or fail to clear - during a fault, which is exactly the failure mode that produces an arc flash rather than a trip.
The fact that it is "only a 200 A panel" is irrelevant. AIC rating is set by the available fault current at that point in the system, not by the size of the panel or of the feeder. A 100 A panel a few feet from a big transformer can need a 65 kA rating.
Two compliant paths:
- Replace the panel and breakers with equipment rated at least 22 kA (the next standard rating above 19.4 kA). This is the direct, defensible solution.
- Use a listed series-rated combination. A series rating lets a lower-rated downstream breaker be protected by a specific current-limiting upstream device, but the combination must be listed and labeled for that exact pairing by the manufacturer - same brand, specific catalog numbers, specific upstream device. It cannot be inferred from catalog data or assumed because the upstream breaker is "fast."
Also verify the service equipment itself: 26,000 A is available at the service point, so the 800 A service gear must be rated at least 35 kA (or be a listed series combination).
2. Evaluate the electrician's proposal to increase the feeder conductor size to lower the fault current.
Read the worked answer
It is the wrong tool for the problem, and it will not work.
Why it fails quantitatively. Available fault current falls with conductor impedance, and for a 120 ft run the impedance is very small. Doubling the conductor area roughly halves resistance, but the fault current is dominated by the transformer and service impedance upstream of the panel. Increasing the feeder from, say, 3/0 to 500 kcmil changes the total loop impedance by only a few percent, moving 19,400 A to roughly the high 18 kA range - nowhere near the 10 kA the equipment needs, and not even below the 22 kA rating the replacement equipment will have.
Why it fails in principle. Adding impedance to reduce fault current is a legitimate engineering technique only when it is deliberate, calculated, and reflected in the study - a line reactor, an isolation transformer, or a long calculated run. Upsizing a feeder to chase a fault-current number inverts the purpose of the feeder: you are paying for copper to make the voltage drop better and expecting it to make the fault current better at the same time.
What the electrician should do instead: order a study-grade calculation of available fault current at each point (including the utility's contribution and any motor contribution), then select equipment rated for it. Larger conductors remain a good idea for voltage drop on a 120 ft run - just do not book the AIC savings against them.
3. After the panel is corrected, is the rooftop unit acceptable as delivered? What is the significance of "maximum fuse size 40 A" and "SCCR 5 kA"?
Read the worked answer
No - the rooftop unit as delivered is not acceptable at 19.4 kA available.
SCCR 5 kA. The unit's internal components (contactors, terminal blocks, the control transformer) are rated to withstand only 5,000 A of fault current. At 19,400 A available, a fault inside the unit can arc and destroy the equipment rather than being cleared safely. Fixing the panelboard solves the panel's problem and does nothing for the rooftop unit - the equipment rating has to be checked at the point where the equipment is connected, and each piece of equipment has to stand on its own SCCR.
Options:
- Specify a rooftop unit with an SCCR of at least 22 kA. Cleanest and usually the answer when the unit has not shipped yet.
- Use a listed series-rated combination - for example, specific current-limiting fuses upstream that are listed with that equipment to raise its effective rating. The fuse class, the manufacturer, and the equipment must all be part of the listing.
- Add impedance (isolation transformer or reactor) to reduce the available fault current below 5 kA. Expensive and rare for an RTU.
"Maximum fuse size 40 A" is a separate instruction and it is mandatory. It tells you two things:
- The branch-circuit protective device must not exceed 40 A. The unit's minimum circuit ampacity is 34 A, so a 40 A device is correct; a 50 A or 60 A breaker sized "to be safe" violates the listing and voids the unit's warranty.
- The word fuse means a fuse. On this nameplate, a circuit breaker - even an HACR-rated one - is not what the manufacturer permits. If the nameplate instead reads "maximum fuse or HACR breaker," a breaker is acceptable. Read the exact wording and follow it.