Fuse Coordination Screen Calculator

A fuse coordination screen compares the clearing time of an upstream fuse with the clearing time of a downstream fuse at the same comparison point. The calculation produces the required upstream time and reports whether the entered upstream clearing time provides the selected time margin over the downstream clearing time.

This check is used during preliminary branch-circuit and feeder protection review, especially when evaluating whether a downstream fuse is expected to clear a fault before an upstream protective device operates. It can help organize a protection study before detailed time-current-curve (TCC) work, fuse selection, interrupting-rating review, or field verification.

The calculator does not size conductors, determine AWG or kcmil, establish ampacity, calculate voltage drop, or select an overcurrent protective device. Its purpose is to compare two entered clearing times using an explicit margin assumption.

Clearing-Time Relationship

For a coordinated arrangement, the downstream protective device should generally clear first for the fault condition and comparison point being evaluated. The worksheet expresses that relationship as a time separation:

\(\displaystyle \text{Actual time margin} = \text{Upstream clearing time} - \text{Downstream clearing time}\)

The calculation then converts that separation into a percentage of the downstream clearing time:

\(\displaystyle \text{Actual margin percent} = \frac{\text{Actual time margin}} {\text{Downstream clearing time}} \times 100\)

A positive margin means the entered upstream clearing time is longer than the downstream clearing time. The entered Minimum margin establishes the screening threshold, not an approved coordination criterion.

Worksheet Inputs

Input	Unit	Electrical meaning
Upstream clearing time	s	Clearing time of the upstream fuse at the selected comparison point
Downstream clearing time	s	Clearing time of the downstream fuse at the same comparison point
Minimum margin	%	Time-margin assumption used to screen the entered clearing times

Both clearing-time inputs must represent the same comparison condition. In practical protective-device review, that commonly means using the same prospective current point and the same type of curve information for both devices.

Do not compare a downstream fuse’s minimum-melting time to an upstream fuse’s total-clearing time unless that comparison is intentionally required by the protection analysis. The time basis, current point, and device data must be aligned before the arithmetic has useful meaning.

Calculation Method

The calculator first determines the upstream clearing time required by the entered margin:

\(\displaystyle \text{Required upstream time} = \text{Downstream clearing time} \times \left(1 + \frac{\text{Minimum margin}}{100}\right)\)

It then calculates the actual separation between the two entered times:

\(\displaystyle \text{Actual time margin} = \text{Upstream clearing time} - \text{Downstream clearing time}\)

Finally, it calculates the actual margin as a percentage:

\(\displaystyle \text{Actual margin} = \frac{\text{Actual time margin}} {\text{Downstream clearing time}} \times 100\)

The Coordination comparison returns Meets entered screen when the Upstream clearing time is at least equal to the Required upstream time.

Calculation Example

Use the following entered values:

Field	Entered value
Upstream clearing time	0.25 s
Downstream clearing time	0.1 s
Minimum margin	100%

Required upstream time:

\(\displaystyle 0.1\text{ s} \times \left(1 + \frac{100}{100}\right) = 0.2\text{ s}\)

Actual time margin:

\(\displaystyle 0.25\text{ s} - 0.1\text{ s} = 0.15\text{ s}\)

Actual margin:

\(\displaystyle \frac{0.15\text{ s}}{0.1\text{ s}} \times 100 = 150\%\)

Result	Value
Required upstream time	0.2 s
Actual time margin	0.15 s
Actual margin	150%
Coordination comparison	Meets entered screen

The entered upstream clearing time of 0.25 s exceeds the required upstream time of 0.2 s. The two entered clearing times therefore satisfy the selected 100% screening margin.

Protection-Study Use

A fuse coordination time-margin calculation is typically part of a broader electrical distribution workflow:

• Establish available fault current and the comparison current or currents.
• Review downstream branch-circuit protection before upstream feeder protection.
• Obtain the applicable fuse time-current characteristics and identify the correct curve basis.
• Compare device behavior across the fault-current range, not only at one selected point.
• Confirm equipment short-circuit current rating, fuse interrupting rating, available fault current, and protective-device application requirements.
• Coordinate the final protective-device selection with conductor ampacity, terminal rating, equipment limitations, motor starting characteristics, and the installation design.

For motor circuits, transformer primary and secondary protection, long feeders, or systems with significant available fault current, a single time-margin comparison can be especially incomplete. Fault current can vary with conductor length, transformer impedance, source configuration, parallel conductors, and operating configuration. Voltage drop and conductor impedance may also affect the fault current available at downstream equipment, even though they are not inputs to this calculation.

TCC and Field Limits

Coordination comparison in this worksheet confirms only the entered arithmetic: the upstream clearing time is at least the calculated required value based on the entered Minimum margin.

It does not approve fuse coordination, TCC behavior, interrupting rating, protection settings, selective coordination, device compatibility, or code compliance. Final design review must use the applicable manufacturer data, complete time-current curves, available-fault-current information, equipment ratings, project specifications, and AHJ requirements.