Prefabricated Electrical Buildings: Sizing the Shell Around Your Switchgear

Most first attempts at an electrical building start from the length of the switchgear lineup, and most of them come back wrong. The lineup sets one interior dimension. The building has to carry the working space around the equipment, a way to get a section out and back in, egress, headroom, cable entry, and the heat the gear throws off. This guide walks the equipment through to the envelope, then lists what a manufacturer needs from you before a quote means anything.

Last updated: July 25, 2026

The short version

A building sized from lineup length alone is usually too small. Working space, removal path, egress, and cable entry all add to it.
Heat loss per section, in watts or BTU/h from the equipment manufacturer, is the single most commonly missing number. Without it the HVAC is guesswork.
Ask early whether a supplier will quote a shell around equipment they do not supply. Selling the building and the gear together is a common preference, not a rule.
Transport limits and offload capability can redesign the building. Raise them before the layout is fixed, not after.
Write the scope split down: field terminations, commissioning, foundation, offload, and setting. Most disputes on these projects live in that gap.

What these buildings are, and what the names mean

A prefabricated electrical building is a factory-built structure that houses electrical equipment: switchgear, motor control centers, relay and control panels, variable frequency drives, and the supporting lighting, HVAC, and grounding that keep them working. It arrives as a finished shell, or as a shell with equipment already installed and wired, and it is set on a foundation at site.

The naming is not standardized, which matters when you are reading a specification or comparing two proposals. The same building gets different names depending on who wrote the document.

Term	How it is usually used
E-house	Common across industrial, utility, and renewables work. Usually implies the building was designed around a specific equipment lineup.
Power distribution center (PDC)	Often used where the building arrives with equipment already installed and wired. Note the term also returns automotive fuse-box results in search, so confirm which sense a document means.
Control house	Utility and substation usage, including municipal and cooperative work. Frequently a shell that receives owner-furnished relay panels and switchgear.
Prefabricated electrical building	The descriptive term. Says nothing about whether equipment is included.
Modular electrical room	Used broadly. Sometimes a factory-built room installed inside an existing structure, sometimes a complete offsite-built room, a skid-mounted room, or a rooftop unit. Ask which one a supplier means.

The distinction that actually changes your project is not the name. It is whether the equipment is included. A shell-only purchase means you furnish the switchgear and someone coordinates the interface between your gear and someone else's building. An integrated purchase puts both under one supplier, one purchase order, and usually one warranty. Decide which you are buying before you compare prices, because the two are not comparable.

Why lineup length is not a building size

A switchgear general-arrangement drawing gives you the equipment footprint. The building is that footprint plus everything the code, the maintenance crew, and the installer need around it. Two lineups with the same overall length can need visibly different buildings once the following are added.

Working space in front, and often behind. Rear-access equipment needs a rear aisle. Front-only equipment does not. This single question can change the building width substantially.
A removal path. A breaker or a whole vertical section has to come out someday, through a door, without dismantling the building.
Egress. Working space needs a way in and a way out, and long or high-current equipment carries additional requirements.
Headroom and the space above the gear. The code sets a minimum clear height and reserves the space directly above equipment for electrical use, which constrains where ductwork and lighting can run.
Cable entry. Bottom entry needs floor depth and a cable space. Top entry needs room above and a route that does not collide with the HVAC.
HVAC volume. The units have to fit somewhere, and where they sit affects both the envelope and the interior clear space.
Future sections. If the one-line shows a future breaker position, the building either holds space for it now or gets replaced later.

Practical test. Before you send a lineup length to anyone, mark up the general-arrangement drawing with the access side of every section, the heaviest single piece, and the door you expect that piece to come out of. If you cannot answer those three, the building cannot be sized yet.

Working space, headroom, and access

Equipment inside a prefabricated building is subject to the same electrical code requirements as equipment in a room built on site. Nothing about factory construction relaxes them. In the National Electrical Code, Article 110 is where the working space, headroom, dedicated equipment space, and entrance and egress requirements live, and equipment operating above 1000 volts is addressed separately from equipment at or below 1000 volts.

The part worth understanding before you talk to a building manufacturer is what the depth of working space actually depends on, because it is not a single number:

Nominal voltage to ground. Higher voltage, more depth.
What is on the other side of the working space. The code distinguishes among a few conditions, based on whether the facing surface is exposed live parts, a grounded surface, or neither. The same equipment can require different depths depending on what you put across from it, which is exactly the sort of decision a building layout makes accidentally.
Width and height of the working space, which are set independently of depth.
Whether equipment faces equipment. Two facing lineups is a different problem from one lineup against a wall.

We are not making the code determination for you. Adopted code editions and local amendments vary by jurisdiction, and the numbers change between editions. Read the article in the edition your jurisdiction has actually adopted, have the engineer of record apply it to your equipment, and confirm with the authority having jurisdiction. Nothing on this page is a compliance determination, a clearance approval, or an engineering judgment for your project.

For work on or near the energized equipment once the building is in service, the arc flash and lockout side of the problem is covered in our arc flash PPE guide and lockout/tagout guide.

Equipment removal, the requirement everyone skips

Working space is about operating and maintaining equipment safely. Getting a failed section out of the building is a separate problem, and it is the one most often left until the building is already fabricated. A drawn-out breaker, a transformer section, or a full vertical structure has to leave through an opening that exists.

Identify the largest removable piece and its weight, not the lineup total. That piece sets the door or removable-panel size.
Decide how it moves. Rolled out on the floor, lifted by a hoist, or pulled through a removable wall panel are three different buildings.
Check the path, not just the opening. A door wide enough for the section is no help if a turn inside the building is not.
Personnel doors are not equipment doors. Specify both, and say which is which. Door swing direction and panic hardware are separate questions driven by egress, not by removal.
Ask the equipment manufacturer what they require. Some lineups have removal and maintenance clearances in their own installation instructions that exceed anything in the code.

This is the cheapest problem to fix on a drawing and one of the most expensive to fix in the field.

What else drives the envelope

Once the interior arrangement holds, four more inputs decide what the building actually is.

Heat loss. Every energized section rejects heat into the room. The equipment manufacturer publishes or can supply heat loss per section. Combined with your site design temperatures and solar gain, that sets the cooling load, the redundancy scheme, and whether you need a backup unit. This is the number most often missing from an RFQ, and no honest supplier can size HVAC without it.
Floor loading and base structure. Section weights drive the steel base, the floor construction, and the lifting design. Concentrated loads matter more than the total.
Site structural criteria. Wind speed, exposure category, snow, seismic design category, and elevation determine the structure and often the sealed calculations. These come from the project location and the governing building code, not from a catalog.
Grounding provisions. The building provides the ground bus, pads, and penetrations for the grounding system the engineer designed. The building supplier furnishes provisions. Designing the grounding system is a different scope, and you should be explicit about who has it.

Fire-rated assemblies come up on some projects, usually driven by occupancy, separation distance, or an insurer rather than by the electrical equipment itself. If a rating is required, it needs to be in the RFQ, because it changes wall construction rather than being added later.

Transport limits and site access

A prefabricated building only helps if it can reach the site. Transport is a design constraint, not a shipping detail, and it regularly forces a building to be split into multiple sections with a field-assembled joint.

Road route width, height, and weight limits, plus permit thresholds along the actual route rather than in general
Overhead restrictions, bridges, and any low clearance between the plant and the site
Whether the building can be split, and where a joint is acceptable given the equipment inside it
Crane or offload capability at site, laydown area, and ground bearing for the lift
Who provides the crane, who directs the lift, and who is responsible for setting the building on the foundation

Offload is a frequent gap. A quote that includes freight does not necessarily include offloading, and a building sitting on a truck because nobody rented a crane is a schedule problem with a daily cost. Confirm in writing where the supplier's responsibility ends: at the plant, at the site gate, on the ground, or on the foundation.

Who is responsible for what

On a shell-only purchase the responsibility split is where projects go wrong, because the building supplier, the equipment supplier, the engineer of record, the installing contractor, and the authority having jurisdiction each own a different piece and none of them owns the interface by default.

Building supplier. The structure, the building's own electrical and mechanical systems, and typically structural calculations. Whether those calculations carry a professional engineer's seal valid in your state varies by supplier, and some state that they do not perform engineering services at all. Ask, and get the answer in writing.
Equipment supplier. The gear, its ratings, its heat loss data, its installation requirements, and its shipping splits.
Engineer of record. The one-line, protective device coordination, arc flash study, code application to the installation, and foundation design unless it is explicitly bought from someone else.
Installing contractor. Foundation work, setting, field terminations, and commissioning, unless the purchase says otherwise.
Authority having jurisdiction. Final code interpretation, inspection, and approval. Some states also require a modular building certification or decal and a third-party plan review before the building can be set. Find out early, because it has a lead time of its own.

Whoever coordinates the interface between your equipment and someone else's building is doing real work. Decide who that is, and price it, rather than assuming it comes free with one of the two purchase orders.

What to gather before you ask for a quote

Manufacturers publish guide specifications, which tell you how to write a specification for their product. What is much harder to find is the buyer-side list: the information you have to assemble before a quote can be anything better than a placeholder. This is that list.

Category	What to have in hand
Equipment	General-arrangement drawing, overall lineup dimensions, section-by-section weights, front-only or front-and-rear access, shipping splits, future expansion sections
Thermal	Heat loss per section in watts or BTU/h from the equipment manufacturer, plus your site design temperatures. Without this the HVAC is a guess.
Cable entry	Top or bottom entry, conduit or tray or bus duct, quantity and size, and which wall or floor area they land in
Building	Interior clear height, fire-rating requirements if any, finish requirements, lighting and receptacle expectations, grounding provisions
Site	Wind speed, snow and seismic design criteria, exposure category, elevation, foundation type, and who designs the foundation
Transport	Road route limits, permit constraints, overhead restrictions, laydown area, crane or offload capability, and who provides it
Schedule	Required on-site date, approval-drawing turnaround you can live with, and any funding or bid deadline behind it
Scope split	Field terminations, commissioning, foundation, offload, and setting. Write down who owns each one before you ask for a number.

If you are missing items, that is normal and it is still worth sending. A supplier who tells you which gaps actually block a number, and which ones can be carried as an assumption, is giving you useful information. A supplier who quotes anyway without asking is giving you a number that will move.

Working on an electrical building and want a second set of eyes on the RFQ?

Send the equipment general arrangement and the site criteria you have. We will tell you which gaps actually block a manufacturer quote and which can be carried as assumptions, and route it to building manufacturers where that is the right fit. We do not perform engineering, size clearances, or make code determinations.

or call 714-248-6555 · email partners@usmadesupply.com

What drives cost and lead time

We are not going to print a price per square foot or a lead time in weeks. Where suppliers do publish general lead-time language, it is tied to their own product and configuration and does not transfer to a building designed around your equipment, and a figure that is wrong by half is worse than no figure. What is useful is knowing which decisions move the number, so you can tell whether a quote is high for a reason.

Cost moves most on:

Overall size, which the working space and removal decisions drive
Whether the building ships in one piece or several
HVAC capacity and redundancy, which follow from heat loss
Site structural criteria, particularly high wind and seismic
Fire-rated assemblies, if required
Sealed engineering, state certification, and plan review
Freight distance, permits, and whether offload is included

Schedule moves most on:

Approval-drawing turnaround, including your own review time, which is routinely underestimated
Equipment delivery, if the building cannot be finished without the gear
Fabrication queue at the time of release, not at the time of quote
State modular certification or third-party plan review, where required
Permit and route approval for an oversize shipment

Ask every supplier to break their lead time into quote turnaround, approval-drawing turnaround, and post-approval fabrication. A single number hides the part you can actually influence.

Domestic content on federally funded work

If the project is federally funded or federally assisted, a domestic-content requirement may reach both the building and the equipment inside it, and it may reach them under different tests. This is one of the more misread areas in the category, because a building that contains electrical products is not a single thing as far as these rules are concerned.

The funding source decides which rule applies. Direct federal procurement, federally assisted infrastructure work, and specific grant programs are governed differently. Get the funding source before you get anyone's compliance letter.
Different parts can fall under different categories. A steel structure and a manufactured electrical product are not evaluated the same way, and whether a given piece of electrical equipment is reached at all depends on the program and on how it is classified rather than being automatic. Ask the manufacturer how they classify the building you are actually buying, and on what basis.
Certification is project-specific. A general statement that a company manufactures domestically is not a certification. What a project needs is a signed document naming the products and the project. Ask what a supplier will actually sign before you rely on it.
Waivers exist and are project-specific, granted by the funding agency rather than by a supplier.

We do not state that any product is compliant with a domestic-content requirement. That conclusion belongs to the manufacturer's signed certification read against your project's specification and funding source. Anyone telling you a product qualifies without seeing both is guessing.

For how the rules differ from one another, start with BABA vs BAA vs AIS, then the detail pages on Build America, Buy America and American Iron and Steel.

Frequently Asked Questions

What is the difference between an E-house and a power distribution center?

The terms overlap and neither is standardized. In practice an E-house is a building designed around a specific electrical equipment lineup, and a power distribution center more often arrives with equipment already installed and internally wired. Because usage varies by manufacturer and by region, treat both as descriptions rather than definitions and confirm the scope in writing.

Can I buy just the building and install my own switchgear?

Yes, and this is a real procurement pattern, particularly on utility, municipal, and cooperative control-house work where the owner furnishes relay panels and switchgear. It is less common in industrial work, where suppliers frequently prefer to sell the building and the equipment together under one purchase order and one warranty. Ask directly whether a supplier will quote a shell around equipment they do not supply, because not all of them will.

Why can a building manufacturer not size the building from my lineup length?

Lineup length sets one interior dimension. The building also has to hold the working space in front of and behind the equipment, a path to get a section out and replaced, door swings and egress, headroom above the equipment, cable entry room, and whatever the HVAC needs. Two lineups of identical length can require noticeably different buildings once those are added.

What working clearances apply inside an electrical building?

The same electrical code requirements that apply to equipment in a conventional room apply to equipment in a prefabricated building. NEC Article 110 sets the working-space, headroom, dedicated-space, and entrance and egress requirements, with depth driven by the voltage and by what is across from the equipment. Equipment above 1000 volts is handled separately. Use the code edition your jurisdiction has actually adopted, and confirm with the authority having jurisdiction, since adopted editions and local amendments vary.

Who stamps the drawings?

This varies by supplier and by state. Some building manufacturers provide structural calculations sealed by a professional engineer registered in the project state, and some explicitly do not perform engineering services. Ask which states a supplier can seal in, whether the seal covers the foundation as well as the building, and whether the state requires a modular building decal or third-party plan review.

How long do these take?

We do not quote a lead time here. Where suppliers publish general timing, it is tied to their own product and configuration, and a building designed around your equipment moves with approval-drawing turnaround, equipment delivery, and the fabrication queue at the time of release. Ask each supplier for their current quote turnaround, approval-drawing turnaround, and post-approval fabrication time as three separate numbers, and treat the approval loop as the part most often underestimated.

Does an electrical building have to be domestically produced on federally funded work?

It depends on the funding source and on how the rule classifies each part. A building shell and the electrical products inside it are evaluated under different tests, and whether a given electrical product is reached at all depends on the program and the classification rather than being automatic. The applicable rule also differs between direct federal procurement and federally assisted work. Get the specification language and the funding source, then get a project-specific certification from the manufacturer. No one can tell you a product qualifies from a catalog page alone.