Available Fault Current and HVAC Control Panel SCCR
A 480 V, three-phase service has 35,000 A available fault current at the service terminals. A 150 A feeder supplies a packaged rooftop HVAC unit. The feeder breaker is a 150 A molded-case breaker with a 10 kA interrupting rating, installed in the service panel. The rooftop unit has a factory-installed control panel marked SCCR 5 kA and a 150 A non-fused disconnect. The unit nameplate shows a minimum circuit ampacity of 104 A and a maximum overcurrent protective device of 150 A. The installing contractor says the 150 A breaker is fine because the unit has a 150 A maximum fuse/breaker size and the disconnect is within sight.
1. Identify the short-circuit current rating problems in this installation.
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There are two independent violations. First, the 150 A breaker has a 10 kA interrupting rating, but 35 kA is available at its line terminals. Under 110.9, the breaker must have an interrupting rating at least equal to the available fault current, unless it is part of a listed series-rated combination. Second, the HVAC control panel is marked 5 kA SCCR, which is far below 35 kA. The 150 A maximum overcurrent device marking on the unit nameplate is for conductor and equipment overcurrent protection; it does not establish an SCCR. The non-fused disconnect also needs an adequate SCCR or must be included in a listed combination. The 104 A minimum circuit ampacity and 150 A maximum device are otherwise consistent.
2. What compliant solutions are available? Discuss series rating and current-limiting fuses.
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Replace the 10 kA breaker with one rated at least 35 kA, or use a listed current-limiting fuse arrangement. For the 5 kA panel, a listed series-rated combination may be used if the fuse and panel combination is tested and marked for the available fault current. A current-limiting Class J fuse ahead of the panel is common, but the combination must be listed; you cannot assume that any 200 kA fuse protects a 5 kA panel. Another option is to have the panel manufacturer upgrade the SCCR, or to add an isolation transformer to reduce fault current. The disconnect must be included in the rating. Document the utility available fault current and the listing.
3. The contractor proposes simply changing the 150 A breaker to a 150 A, 22 kA breaker because it is close enough. Is that acceptable? What if the actual available fault current is 22 kA after utility transformer replacement?
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No. A 22 kA breaker is not acceptable where 35 kA is available; close enough is not a valid basis. If the utility replaces the transformer and provides written data showing the available fault current is actually 22 kA at the breaker terminals, a 22 kA breaker could be acceptable for the breaker. The 5 kA control panel would still be unprotected unless it is part of a listed series-rated combination or its SCCR is upgraded. Available fault current changes with transformer size, impedance, and service conductor length, so recalculate rather than guess. Also verify the panel bus bracing and the disconnect rating.