A fault in one rack branch should not turn into an outage across an RPP, PDU, or busway run. That outcome depends on selective coordination: the overcurrent protective device immediately upstream of the fault opens while the devices farther upstream remain closed. Eaton’s Bussmann series Low-Peak fuses can make that design task much simpler—but only when the published selectivity ratios are applied to the exact fuse families in the circuit.

The familiar 2:1 rule is therefore best treated as a powerful engineering shortcut with defined boundaries, not as a universal rule for every fuse combination. This article explains where it applies, where it does not, and how to document a coordinated data center distribution system.

Selective Coordination Is a System Property

Selective coordination is not a feature that belongs to one fuse in isolation. It is a relationship between every adjacent pair of protective devices along a circuit path—from the service or switchboard, through feeders and PDU/RPP branches, to the rack PDU or cabinet distribution unit (CDU). One unverified pair can become the point where an otherwise localized event spreads upstream.

The design objective is to coordinate for the full range of possible overcurrents, from overloads through short circuits, up to the limit stated in the applicable manufacturer data. Bussmann fuse selectivity tables generally cover overcurrents up to the lower of 200 kA or the fuses’ interrupting ratings. Available fault current, device interrupting rating, equipment SCCR, and selective coordination must still be checked as separate requirements.

When the 2:1 Rule Applies

For time-delay CUBEFuse (TCF) and Bussmann Low-Peak Class CC, J, L, and RK1 fuses, a minimum 2:1 upstream-to-downstream ampere-rating ratio is the standard published path to selective coordination. For example, a 200 A Low-Peak Class J feeder fuse can coordinate with a 100 A or smaller time-delay CUBEFuse when the applicable table confirms that 2:1 relationship.

The value of this method is repeatability. Designers can compare the installed ampere ratings with one published ratio instead of relying only on a visual overlap check of time-current curves. The result is easier to review, specify, commission, and preserve when a facility changes loads later.

When 2:1 Should Not Be Assumed

A blanket 2:1 statement can create a coordination error. Use the current Bussmann Selectivity Ratio Guide or a product-specific table when the circuit includes:

• Fast-acting CUBEFuse (FCF), SC fuses, or another fuse family with a published ratio other than 2:1.

• Fuses within the same case size, where the general table directs the designer to consult Eaton Bussmann series technical support.

• A fuse-to-circuit-breaker pair, adjustable trip units, ground-fault protection, relays, or other devices that require dedicated coordination data.

• A fault-current or voltage condition outside the limits of the published table or tested combination.

A Five-Step Coordination Workflow

• Draw every normal and alternate source path. Include UPS bypass paths, transfer switches, tie breakers, PDU/RPP feeders, busway plug-in units, and rack-level branch protection.

• Record the exact manufacturer, catalog family, voltage rating, ampere rating, and time-delay or fast-acting characteristic for every protective device.

• Calculate available fault current at each point and verify that every protective device has adequate interrupting rating and every assembly has adequate SCCR.

• Check each adjacent pair against the current selectivity ratio guide or tested coordination table. Document the required ratio, the as-designed ratio, and the applicable fault-current limit.

• Revalidate the study after changes to source capacity, transformers, conductors, busway, UPS equipment, fuse ratings, or downstream rack PDUs.

Rack-Level Example: Why the Exact Combination Matters

A common rack-level example uses an upstream CUBEFuse and downstream 20 A SC fuses. The general published upstream CUBEFuse-to-SC ratio is 4:1, which would point to an 80 A upstream fuse. Eaton’s data center application note also reports tested TCF-to-SC-20 combinations at specific system voltages and available fault currents—for example, a TCF40 or larger coordinating with an SC-20 up to 100 kA under the stated 415/240 V test conditions.

That result is useful, but it is not permission to generalize the 2:1 rule to every CUBEFuse/SC installation. The exact voltage, fuse catalog numbers, conductor arrangement, available fault current, and test conditions must match the published data used for the design.

Conclusion

The Bussmann 2:1 rule can turn selective coordination from a curve-matching exercise into a fast, auditable design check—when the circuit uses the fuse families covered by that ratio. Apply the published boundaries, verify every source path, and treat exceptions as engineering checks rather than assumptions. That discipline helps keep a branch fault at the branch and protects the uptime investment made everywhere upstream.

Models Mentioned

Sources

• Eaton Bussmann Series — Fuse-to-Fuse and Fuse-to-Breaker Selective Coordination, Application Note No. 10454

• Eaton Bussmann Series — QSCP Application Note No. 3148

• Eaton Bussmann Series — Data Center Circuit Protection, Application Note No. 10079