Short Circuit and Coordination Studies: Why They Matter

Every electrical system is designed for normal days, but its protective devices are chosen for the worst day the building will ever have. A bolted fault on a main switchboard can release tens of thousands of amperes in a few cycles, and whether the equipment survives it depends on decisions made months earlier on the drawing board. That is the work of short circuit and coordination studies, a core part of the electrical engineering services we provide to owners, architects and developers.

Budlong has been designing building systems since 1958, and the lesson from decades of project work is consistent: studies done late are expensive, and studies done early are cheap insurance. This guide explains what the studies calculate, the codes behind them, how they connect to the rest of a full MEP design, and when to schedule them so they protect the schedule instead of delaying it.

1. What Are Short Circuit and Coordination Studies?

The short circuit study

A short circuit study is a calculation of the current that would flow if a fault occurred at a given point in the system. It is performed for every bus, switchboard, panelboard and major device, and it produces two families of numbers. The maximum values, with every source connected and every motor running, check that equipment can interrupt or withstand the fault. The minimum values, with sources out of service, check that protective devices will still sense and clear a low-level fault in time.

The coordination study

A coordination study, often called a protective device coordination study, overlays the time-current curves of series devices on one chart and checks that they operate in the right order. When a branch breaker trips on a short circuit, the feeder breaker above it should stay closed, and so should the main. Coordination is how engineers keep a single failed light fixture or motor from darkening an entire floor.

Why they travel together

The two studies answer different questions with shared data. The short circuit study asks whether a device is strong enough. The coordination study asks whether it is set correctly. A breaker that is strong enough but poorly set can still trip an entire building, and a well-set breaker that is under-rated can fail violently. For that reason we treat them as one scope, and we extend the same model to the arc flash analysis discussed in our guide to arc flash analysis and NFPA 70E compliance.

Also Read: Arc Flash Analysis and NFPA 70E Compliance

2. How the Studies Work

Building the model

The engineer starts from the single-line diagram and converts it into a network model: utility source, transformers, cables, bus duct, generators, motors and protective devices. Each element carries an impedance. The software, commonly a commercial power system package such as ETAP, SKM PowerTools or EasyPower, reduces the network to an equivalent impedance at each bus and solves for fault current using methods recognized in IEEE 551, the Violet Book on short circuit calculations.

A worked sense of scale

Consider a 1,500 kVA transformer with a 480 volt secondary and 5.75 percent impedance. Its full load current is about 1,800 amperes. Dividing by the per-unit impedance gives a fault current of roughly 31,000 amperes at the secondary terminals if the utility is treated as an infinite source. Real results are somewhat lower because utility and cable impedance reduce the value, and somewhat higher at motor-heavy buses because running motors feed current back into the fault.

Sources of fault current

• Utility: the serving utility provides available fault current and X/R ratio at the point of connection, usually as maximum and minimum values.
• Transformers: impedance limits the current they can deliver, which is why transformer data from the submittal matters.
• Generators: contribute for several cycles at a level set by subtransient reactance.
• Motors: induction and synchronous motors act as short-lived sources, with induction motors commonly modeled at several times their full load current for the first cycles.
• Inverter-based sources: solar and storage contribute only a limited multiple of rated current, but they must be modeled.

From fault current to curves

Once fault values are known, the engineer plots each device on a time-current chart: the long-time pickup, the short-time pickup and delay, the instantaneous trip, and any ground fault element. Fuse curves, relay curves and transformer inrush and damage points are added. The task is to shape settings so that a downstream device clears first with a safe margin, while still protecting cable and transformer damage limits.

3. Types of Calculations and Protection Schemes

Short circuit calculation methods

Engineers choose among several approaches depending on the project. Hand methods such as the per-unit or ohmic method suit small systems and sanity checks. Point-to-point methods give quick conservative estimates for panel-level work. Full software models, with duty-specific calculations for first-cycle, interrupting and 30-cycle conditions, are the standard for any building with a main switchboard, generator or medium-voltage source.

Duty types

First-cycle values check bracing and closing capability. Interrupting-time values, often at one and a half to four cycles, check breakers and fuses. Thirty-cycle values apply to time-delayed devices and relays. A thorough report states which duty each result represents.

Coordination strategies

There are several ways to arrange protection, and each carries trade-offs in cost, continuity and arc energy.

Protection devices and their curves

Molded case and insulated case breakers with electronic trip units give the most adjustable curves. Power circuit breakers suit large service equipment. Fuses coordinate predictably when ratio rules are respected. Relays with ANSI device numbers 50 and 51 for phase overcurrent, 50G and 51G for ground, and 67 for directional overcurrent dominate medium-voltage systems.

4. Code and Standards Requirements

National Electrical Code provisions

The National Electrical Code, published by NFPA as NFPA 70, makes the studies an engineering necessity even where it never uses the word study. The sections that matter most:

• 110.9: equipment must have an interrupting rating at or above the current available at its line terminals.
• 110.10: protective devices, impedance and component short-circuit ratings must allow the system to clear a fault without extensive damage.
• 110.24: for service equipment in other than dwelling units, available fault current and the date of the calculation must be field marked, and the marking must be updated when modifications change the value.
• 240.86 and 240.87: series ratings and arc energy reduction for larger circuit breakers.
• 517.26, 620.62, 700.32, 701.32 and 708.54: selective coordination for healthcare essential systems, elevators, emergency systems, legally required standby systems and critical operations power systems.

California adoption

California enforces the California Electrical Code, Part 3 of Title 24, which adopts the NEC with state amendments. The code is published through the International Code Council and administered by the California Department of General Services building standards program. Always confirm the adopted edition and local amendments. Our primer on Title 24 and the California energy code explains how the parts fit together.

Referenced standards

The IEEE color book series underpins the engineering method: IEEE 551 for short circuit calculations, IEEE 242 for protection and coordination, and IEEE 399 for power system analysis. Device ratings follow product standards from UL, including UL 489 for molded case breakers, and from ANSI and IEEE for power breakers.

Selective coordination is a code requirement for life safety and emergency systems. Elsewhere it is a design choice, and the owner should make that choice knowingly.

Also Read: Electrical Engineering for Safety, Reliability and Efficiency

5. Applications by Building Type

Office, retail and mixed-use

Commercial buildings typically have a utility service, a main switchboard, several distribution panels and many tenant panels. The main risk is under-rated tenant equipment on a strong service, and the study confirms where series ratings are acceptable. Our commercial electrical services team runs the preliminary calculation before tenant improvement drawings are issued.

Healthcare

Hospitals have the strictest coordination needs because the life safety, critical and equipment branches must keep operating through downstream faults. Projects such as Cedars-Sinai Medical Center show how many levels of distribution can sit between the utility and a patient room. See our health facilities experience for more.

Mission critical and data centers

Uptime depends on fault isolation. Data center designs use multiple sources, static transfer devices and fast-acting protection, and the studies must cover every operating configuration. Our work in mission critical facilities and the article on power distribution for data centers describe the architecture.

Aviation, transit and laboratories

Airports combine very large services, generators and sensitive baggage and terminal loads. Laboratories and industrial plants add large motors and process equipment. Both are covered by our aviation and industrial and laboratory practices.

Need a short circuit and coordination study?

Our electrical engineers can scope the right level of study for your project, from a preliminary calculation to a full selective coordination report.

Talk to a Budlong electrical engineer

6. Design Process and Deliverables

Stage by stage workflow

The studies belong in several project phases, not one. Early estimates guide equipment choices, and final data closes the loop before energization.

What the report should contain

• One-line diagram with bus names matching the drawings and equipment labels.
• Input data tables, with assumptions and missing data clearly flagged.
• Short circuit results for each bus, with the duty type identified.
• Equipment evaluation comparing available fault current with each interrupting or withstand rating.
• Time-current curve plots grouped by logical coordination paths.
• A recommended settings table for every adjustable device.
• A list of deficiencies and the proposed fixes.

Coordination with other disciplines

Early coordination keeps results from surprising the team. Space for larger switchgear, changes to the one-line, and generator sizing all flow from the studies. Our article on early MEP coordination explains how early data prevents rework, and the guide to BIM coordination and clash detection shows how equipment dimensions get locked in.

Also Read: Electrical Load Calculation for Commercial Buildings

7. Special Cases and Difficult Conditions

Generators and cogeneration

On-site generation changes the fault picture twice. A generator adds fault current when it is running, and its lower fault current relative to the utility can make fault clearing slower. Emergency generator replacement jobs, such as the City of Anaheim emergency generators project, rely on updated studies to confirm that existing breakers still interrupt and coordinate with the new source. Cogeneration plants like the San Antonio Community Hospital cogen plant add parallel operation with the utility, which adds modes to study.

Solar, storage and other inverter sources

Inverter sources limit their output current, so they rarely raise interrupting duty, but they can reduce the current seen by upstream devices and affect sensitivity. Models must include them and test minimum fault cases.

Existing buildings and utility changes

Utility upgrades quietly raise available fault current. A building that was safe in 1995 may have under-rated panels today. Any change to a service transformer, a nearby utility feeder, or a new on-site source should trigger a review. Our guide on planning for future load expansion outlines how to leave headroom.

High resistance grounding and ground faults

Resistance grounded systems limit ground fault current to a few amperes and allow continued operation through a first ground fault. Their coordination study centers on ground fault detection and alarms, not on tripping, but line-to-line fault coordination still applies.

8. How to Specify the Studies and What Drives Cost

What to write in the specification

A good specification section names the standards used, the software capability, the required deliverables, the timing of the study relative to submittals, and who pays for settings adjustments. It should also require the contractor to supply equipment data, and require the final study to be accepted before energization. Review our CD package guidance for how this fits the drawing set.

Who should perform the study

Studies can be performed by the design engineer of record, the equipment manufacturer, or an independent testing firm. Each option has trade-offs in independence and speed. For coordinated life safety systems, the engineer of record is best placed to understand the intent of the design, while manufacturers are best placed to supply accurate trip unit data.

Cost drivers

For a broader view of budgeting, see our article on how to estimate MEP engineering costs.

9. Best Practices, Common Mistakes and Maintenance

Best practices

• Request utility fault current data in schematic design, and confirm both maximum and minimum values.
• Specify a coordination goal for each system, and state it in the drawings.
• Require the final study to use approved submittal data, not catalog assumptions.
• Keep the model as a living file, and hand it over to the owner at closeout.

Common mistakes

• Assuming an infinite utility source and overestimating or underestimating results.
• Ignoring motor contribution on large mechanical or process buses.
• Setting instantaneous pickups so high that coordination looks perfect but arc energy is not controlled.
• Completing the study after equipment has shipped, leaving little room to fix problems.
• Leaving field markings out of date after a utility or equipment change.

Testing and maintenance

Settings on paper are not settings in the field. Testing and commissioning confirm that trip units are set as recommended, that ground fault functions work, and that relays operate as designed. Our building commissioning process includes a check of protective settings, and our commissioning services can verify them independently. Failures in power systems usually trace back to unverified settings, as discussed in why data center power systems fail.

Also Read: The Building Commissioning Process

10. Where Budlong Delivers This

Budlong staffs offices in Glendale, downtown Los Angeles, Camarillo and Fremont, and serves Los Angeles, Ventura, Orange, San Bernardino, Riverside and Santa Barbara counties. Our teams pair local utility knowledge with a portfolio that includes LAX bus charging and hospital cogeneration plants. Explore our MEP portfolio for more examples.

11. Related Reading

12. Frequently Asked Questions