Engineers who have only worked with IEC-standard switchgear encounter a number of surprises when they first tackle a North American project. The dimensional conventions, protection philosophy, and CT installation approach differ enough from IEC practice that a direct translation of a proven design will almost certainly fail to meet specification. This guide covers the key engineering decisions for 15 kV metal-clad switchgear built to IEEE C37.20.2.

Why a Side-Panel Construction Strategy Does Not Scale for ANSI Projects

IEC metal-clad switchgear built for the Chinese domestic market commonly uses a full-height side panel that is shared between adjacent bays. It is efficient when the single-line diagram is standardised across a project, because one panel design covers hundreds of identical bays.

ANSI projects do not have that luxury. A typical North American substation single-line combines transformer feeders, capacitor bank controls, motor starters, tie breakers, metering bays, and bus tie arrangements in a single lineup. The current ratings, interrupting ratings, CT ratios, protection relay types, and auxiliary switch requirements are different on virtually every bay. If you design the structural side panel to accommodate all possible variations, you end up with hundreds of panel variants—each requiring its own part number, manufacturing routing, and field assembly drawing.

A more practical approach separates the front structure (which varies by function) from the rear structure (which is relatively consistent):

• Front module (arc-rated bays): Designed as interchangeable functional modules. Standard module heights correspond to 1/2-width breaker compartments, 1/4-width auxiliary truck compartments, and 1/4-width fuse or empty spare compartments. Any combination of these modules can fill a bay front without modifying the structural frame.

• Rear section: Side-panel construction with no more than ten structural variants covering all common one-line configurations. The rear carries the cable compartment, instrument transformer section, and bus connection—functions that vary less than the breaker arrangement.

For non-arc-rated lineups, a full-frame modular approach—using standard frame sections with defined module-height positions—achieves similar flexibility without the manufacturing complexity of hybrid construction.

Physical Envelope: Fixed Width, Variable Ampacity

IEEE C37.20.2 metal-clad switchgear at 15 kV is essentially fixed at 36 inches (914 mm) wide, regardless of the continuous current rating. This is not a coincidence; it reflects the phase spacing requirements of 15 kV insulation, the vacuum circuit breaker truck dimensions, and the through-type CT bore diameter requirements that have converged on that nominal width over decades.

The phase-to-phase spacing between primary stab contacts (the engagement points between the breaker truck and the fixed contacts in the breaker compartment) is 10 inches (254 mm). For 1,200 A and 2,000 A ratings this spacing is manageable—the stab contacts and CT bores at these ratings fit comfortably in the available envelope.

At 3,000 A and 4,000 A, the same fixed width and phase spacing creates real engineering challenges. The primary conductor cross-section, the contact system geometry, and the CT bore size all need to increase, but the 254 mm phase spacing and 914 mm overall width do not.

Insulation Requirements at 15 kV ANSI Voltage Class

The insulation coordination requirements for 15 kV ANSI class equipment are defined by IEEE C37.20.2 and the applicable switchgear standard. The key dielectric test levels are:

• Power-frequency withstand: 36 kV across the main circuit and 40 kV across the open isolation gap.

• Lightning impulse withstand (BIL): 95 kV across the main circuit and 105 kV across the open isolation gap.

These requirements are more demanding than their IEC counterparts at the same nominal voltage, which is relevant when adapting IEC-based vacuum interrupter assemblies or bushing designs to ANSI projects. An insulation system that passes IEC type tests at 12 kV Um will not necessarily pass ANSI insulation tests at the 15 kV voltage class without modification.

Through-Type Current Transformers: The Governing Constraint on Contact Design

The most architecturally significant difference between IEC and ANSI switchgear design is the current transformer location. In IEC metal-clad switchgear, window-type CTs are typically mounted in the cable compartment after the circuit exits the breaker truck contacts. The CT bore only needs to accommodate the cable or busbar, not the truck contact assembly.

In ANSI metal-clad switchgear, the standard practice is to mount low-voltage through-type CTs directly on the stab socket (the fixed contact socket in the breaker compartment), with the primary conductor passing through the CT core. This arrangement places the measurement point inside the breaker compartment, which simplifies the single-line diagram and satisfies the IEEE C37.20.2 requirement for front-accessible installation and maintenance.

The governing constraint is immediately obvious: the stab socket outer diameter must be smaller than the CT bore inner diameter, while the stab socket inner diameter must be large enough to accept the largest breaker primary contact diameter at the current rating being designed.

Dimensional limits in practice

Through-type CTs complying with IEEE C57.13, at the current ratings commonly specified for 15 kV switchgear, have a standardised body width of approximately 9.8 inches (249 mm) and a bore inner diameter of approximately 6.5 inches (165 mm). This sets the hard limit: the stab socket outer diameter must not exceed about 160 mm, with some margin for manufacturing tolerance and sliding clearance.

Standard IEC 3,150 A vacuum circuit breaker primary contacts use tulip-type contact assemblies with outer diameters typically exceeding 140 mm—which is the practical inner diameter limit for a stab socket that itself must fit inside a 160 mm bore CT. Adapting these to ANSI requirements requires either:

• A redesigned contact assembly with reduced outer diameter and higher contact force per unit area to compensate for the reduced surface area; or

• A completely different contact geometry that achieves the required 65 K temperature rise limit (10 K tighter than the IEC 75 K limit) with the constrained envelope.

Current Transformer Specification: Avoiding Over-Specification That Defeats the Design

IEEE C57.13 CTs for ANSI switchgear are available with accuracy classes of 0.3 for metering and 5P20 for protection—both of which are standard and achievable in the geometry described above.

Problems arise when specifiers request unusually high accuracy (0.1 class or better) or unusually large burden (more than 30 VA secondary) without checking that those requirements can be achieved within the 160 mm outer diameter constraint. Accuracy class and burden both drive core cross-section area and therefore CT physical size. A CT specified at 0.1 class, 50 VA at a 50:5 ratio may simply not fit inside the available bore—meaning either the specification is relaxed or the stab socket design has to change.

For most commercial and industrial applications, 0.3 class metering and 5P20 protection adequately support revenue metering and overcurrent protection without requiring a CT that exceeds the available bore size. Push back on specifications that call for 0.1 class or multi-burden CTs unless there is a documented measurement requirement that justifies the physical consequence.

Temperature Rise: A Tighter Limit That Changes the Engineering

IEEE C37.20.2 limits the temperature rise of main circuit components to 65 K above a 40°C ambient—giving an absolute maximum temperature of 105°C on primary conductors and contacts. The comparable IEC limit is 75 K, giving 115°C. That 10 K difference sounds minor but it significantly changes the contact design requirement.

At 3,000–4,000 A continuous, a 10 K tighter temperature rise limit demands a higher contact force, lower contact resistance, or larger contact area than the equivalent IEC design. With the contact outer diameter already constrained by the CT bore, the only degrees of freedom are contact force and material selection. The design solution typically involves higher spring preload, harder silver-faced contact tips with a higher fusion temperature, and careful attention to the contact stem material’s thermal conductivity.

Anyone adapting a proven IEC high-current contact design to an ANSI project should run the thermal analysis before assuming the design transfers directly—it usually does not.

Summary of Key Design Decisions

1. Use modular front compartments for arc-rated bays; limit rear structural variants to fewer than ten to control manufacturing complexity.

2. Account for the 914 mm fixed bay width and 254 mm phase spacing from the start—these drive every other dimensional decision.

3. Verify insulation coordination against ANSI test levels (95 kV BIL main, 105 kV BIL gap), not IEC levels, even when using IEC-sourced components.

4. Design the stab socket to fit inside a standard IEEE C57.13 CT bore (165 mm)—this is the governing geometric constraint.

5. Verify CT accuracy and burden requirements are achievable within the bore constraint before accepting the specification.

6. Design the high-current contact system against the 65 K temperature rise limit—do not assume an IEC-compliant contact design will pass ANSI tests without verification.

ANSI medium-voltage switchgear design is not dramatically more difficult than IEC design—but the differences are specific enough that each one will cost time if discovered during type testing rather than during design review.