🔌 Minimum Disconnect Ampacity
Sizing Breakdown
Full Calculation Table
| Component | Basis | Value | Result |
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How to Use the Disconnect Sizing Calculator
Choose fused or non-fused to apply the correct sizing rule.
Enter full load amps always, and locked rotor amps if sizing a fused disconnect.
Enter the maximum fuse or breaker size from the nameplate, if listed, as an upper limit check.
Click Calculate Disconnect Size to see minimum disconnect ampacity and recommended fuse size.
Understanding Disconnect Sizing
A disconnect switch is a code-required, visible, lockable means of cutting power to HVAC equipment for service, distinct from the circuit breaker at the panel. See the breaker size calculator for panel-side circuit protection, which uses a different sizing basis than the disconnect itself.
Non-Fused Disconnects Size From Running Load
A non-fused disconnect only needs to safely carry the equipment's running current, since it provides no overcurrent protection of its own, that job belongs to the breaker at the panel. Standard practice sizes a non-fused disconnect at 115% of full load amps, then rounds up to the next standard disconnect ampacity rating.
Fused Disconnects Need Both a Switch Rating and a Fuse Size
A fused disconnect combines a switch body rating with a separate fuse selected to protect the circuit, typically sized from locked rotor amps rather than full load amps alone, since the fuse needs to tolerate motor starting current without nuisance tripping. The equipment nameplate's maximum fuse or breaker size value acts as a hard upper limit that should never be exceeded regardless of calculated values.
Disconnect Location Requirements
Most electrical codes require the disconnect to be within sight of, and readily accessible from, the equipment it serves. This sizing calculator addresses ampacity only; confirm placement and sight-line distance requirements against the electrical code in your specific jurisdiction.
Frequently Asked Questions
A circuit breaker protects the circuit conductors from overcurrent and can trip automatically, while a disconnect switch provides a visible, lockable means to manually cut power to equipment for service, typically mounted right at the outdoor condensing unit. Both are required on most HVAC circuits, sized using different rules based on different nameplate values.
A non-fused disconnect switch is typically sized at 115% of the equipment's rated load current, or selected from the next standard disconnect ampacity rating above that value. Since a non-fused disconnect has no internal overcurrent protection, its rating only needs to safely carry the equipment's running current, not interrupt a fault.
A fused disconnect needs both an ampacity rating for the switch body and a separate fuse size selected to protect the circuit, typically based on locked rotor amps and the equipment manufacturer's maximum fuse size listed on the nameplate. This differs from a non-fused disconnect, which only needs a switch body rated to carry the running load safely.
Most electrical codes require the disconnect to be within sight of, and readily accessible from, the outdoor condensing unit or other equipment it serves, allowing a technician to see the equipment while operating the disconnect. Exact sight-line distance and accessibility requirements vary by jurisdiction, so confirm the local electrical code before final placement.