Air vs. Gas Substations for Constrained Sites: Which Is Better

Sep 23, 2026

AIS vs. GIS Substations for Space-Constrained Sites

Summary: Building or upgrading an electrical substation comes with many challenges, including space constraints. When space is an issue, it must be weighed against the costs of building both AIS and GIS systems. Is one choice better than the other when space is a problem?

Nearly every utility executive and grid project manager eventually runs into a space-related problem. Maybe expansion is unavoidable. Perhaps a new facility needs to be connected or aging equipment needs replacing. Regardless of the project, space is very limited.

Space limitations are further exacerbated when land is scarce, expensive, or difficult to develop. This leaves decision-makers to look at two options: air-insulated substation (AIS) and gas-insulated substation (GIS) design. Which one is better when space constraints are a factor?

It is easy to assume that GIS is always the better choice. In addition, making the right choice can save millions of dollars in upfront capital, as well as prevent project delays and keep maintenance costs under control. But every situation is different. There is no black-and-white solution for every substation project.

The Basic Differences Between AIS and GIS

Day in and day out, a substation has but one goal: safely convert high-voltage electricity that residential and business customers can use. Both AIS and GIS systems rely on the same basic components to manage power: transformers, circuit breakers, switches, and control systems. The fundamental difference between the two types of design is how high-voltage equipment is insulated in order to prevent electrical arcing.

1. AIS Designs

AIS design is the traditional design that utilities have relied on for decades. It is what you see when you drive along a highway and come upon an open-air substation. Here is a brief summary:

• How it works - AIS designs rely on atmospheric air as the insulating medium around high-voltage conductors. Air is a modest insulator, but it has its drawbacks.
• Physical layout - Those drawbacks dictate that high-voltage components be spaced far apart. It could mean several feet or yards between them to prevent arcing between live components.
• Location - Nearly all AIS substations are built outdoors across large gravel footprints. This means a lot of land is required.

2. GIS Designs

GIS design is the more modern of the two. It relies on specialized gases to act as the primary insulator between components. Here is a brief summary:

• How it works - In a GIS design, high-voltage equipment is sealed in grounded metallic boxes filled with a gas like sulfur hexafluoride. Engineers have access to eco-friendly alternative gas mixtures as well.
• Physical layout - Because gas insulates more effectively and equipment is contained in metal boxes, components can be placed more closely together.
• Location - GIS-based substations can typically be built inside industrial buildings or compact outdoor enclosures that make them ideal for urban environments.

To the untrained eye, it is surprising that the choice of insulator has such a big impact on substation design, but it does. So when space constraints are an issue, engineers tend to favor GIS installations.

When AIS Is the Better Choice

GIS might ultimately prove the better choice for a space-constrained site. Yet there are legitimate reasons for going with AIS, even when space is an issue. If a developer has access to enough flat land to meet safety clearance standards, AIS design has its advantages.

1. Available Adjacent Land

If a developer can purchase enough adjacent land at an agreeable price, paying extra for compact GIS equipment may not make financial sense. AIS equipment relies on simpler off-the-shelf components that keep upfront equipment costs in check. The money saved could offset the cost of purchasing adjacent land.

2. Faster Build, Simpler Upkeep

Other compelling reasons include the fact that AIS substations can be built faster and their upkeep is simpler. All equipment in an AIS facility is fully visible and easily accessed. Technicians do not require specialized testing tools or the skills needed to manage sealed gas enclosures. In fact, local utility crews are universally trained on AIS repairs.

A Real-World Example

The real-world example of a suburban industrial park expansion illustrates why an AIS design might be chosen even when there are concerns about space. Imagine that the manufacturing company behind the project has access to three acres of adjacent space. Even though the parcel is pretty small, it can easily accommodate a typical outdoor AIS layout. By going with AIS, the builder saves time and money while also giving local utility crews a system they are familiar with.

When GIS Is the Better Choice

Sometimes circumstances make AIS design completely unfeasible. Engineers are left with the GIS model. To a lot of utilities and engineers, GIS is the alternative of last resort. They will spend the extra money on a GIS installation because there is no viable AIS design to pursue.

1. Extremely Scarce or Expensive Land

Buying multiple acres of land might not be financially feasible in areas where it is scarce or expensive. This is a common problem in urban areas. A GIS footprint reduces total land requirements by up to 90%, making land acquisition far more affordable.

2. Significant Environmental Risks

Certain environmental risks - like exposure to coastal salt air or heavy industrial dust - can make an AIS installation sketchy. GIS handles such environmental risks more effectively because all sensitive equipment is enclosed in protective metal boxes.

3. Permitting Requirements Introduce Additional Constraints

Permitting can be an issue in residential neighborhoods or historic zones. So when low visual impact is a permitting concern, developers may not have any choice but to go with GIS.

A Real-World Example

A real-world example illustrating the necessity of GIS would be a data center in a metropolitan area needing a new substation to double its power capacity. Surrounding land can cost millions of dollars per acre, and there isn't a lot of land left to work with. Where an AIS design would require at least 2.5 acres, a GIS design can be housed in a multi-story building. Although the GIS equipment is more expensive, the cost of not having to purchase expensive land offsets it.

Space constraints are one of the primary concerns when designing new substations and expanding their existing counterparts. Land availability and cost are such important factors that they are often the dividing line between AIS and GIS installations.

FAQs

What is the single biggest factor when choosing between AIS and GIS?

The physical space available for your footprint. GIS is the choice when space is limited and there is no way to overcome it. It requires up to 80% less ground area.

Is a GIS build always more expensive than its AIS counterpart?

Not necessarily. Although GIS equipment is more expensive, its cost might be offset by not having to purchase more expensive land.

Can an AIS substation still be built on a space-constrained site?

Yes, as long as there is enough flat land to meet the required clearances between high-voltage components. Developers still have to consider future expansion needs, however.

Which design model more easily accommodates expansion?

AIS designs are typically easier and less costly to expand. However, the caveat is having access to adjacent land. If land is expensive or scarce, expanding an AIS site might not be feasible.