UL 891 vs UL 1558: What Changes in the Quote

If you have quoted low voltage distribution equipment, you have met the two numbers that sit on the first page of the specification: UL 891 for a switchboard, UL 1558 for switchgear. UL 891 vs UL 1558 reads like a question about standards. At the estimating desk it is a question about money, because the same one-line diagram can be built either way and the two prices are far apart.

I spent two days reading how eight large owners answer that question in writing: two federal agencies and six universities that publish the electrical rules their designers must follow. They do not agree with each other. One draws the line at 1,000 amps, one at 1,200, one at 1,600, one at 2,000. Two of them ask for something that is neither product as most people picture it: a switchboard, listed to UL 891, that carries a rating and a breaker borrowed from switchgear.

This article is for the person who has to turn that first page into a number. It covers the differences that reach the price, where each owner draws the line, the hybrid that sits between the two products, and the seven sentences in a specification that tell you which one you are pricing. It uses the same documents-first method as our guide on how to quote switchgear, and most of the documents here are ones that guide did not use.

In brief

• A switchboard to UL 891 and switchgear to UL 1558 can serve the same one-line diagram. The difference is in the breakers, the way they are mounted, how long the assembly can carry a fault, and how much steel separates one part from the next.
• The line that matters most is the short-time rating. Switchgear is tested to carry a fault for 30 or 60 cycles so that a breaker downstream can clear it first. A switchboard is tested for three.
• Owners draw the line in different places. The Department of Defense starts switchgear at 1,200 amps with large feeders. The Department of Veterans Affairs prefers power circuit breakers from 1,600 amps. Dartmouth wants drawout devices from 1,000.
• Some owners ask for a hybrid: a UL 891 switchboard with drawout mains built to the power circuit breaker standard, and in one case a 30 cycle rating. That is a third product, and it has its own price.
• Seven sentences in the specification settle which one you are quoting. The title of the section is not one of them.
• The choice moves more than the lineup price. It moves the room, the bus, the spare spaces, the lifting gear and the safety options.

UL 891 vs UL 1558: the differences that reach the price

The clearest public comparison I found was written by two electrical engineers at CDM Smith, William McGugan and Tyler Roschen, for Consulting-Specifying Engineer in September 2022. The first three columns of the table below follow their description of the two standards. The last column is mine: where each difference lands in a quote.

Their summary of cost is careful, and worth repeating in the same spirit. The two products can be configured in so many ways that a like-for-like comparison only works for designs of similar size and purpose. Even so, the extra material in compartmented switchgear makes the switchboard the significantly cheaper option in general. They give no ratio, and I will not invent one. Your own price book is the only place that number lives.

Two rows in that table do most of the work at the estimating desk, so they get a closer look: the breaker row and the fault withstand row.

UL 1066 vs UL 489, in plain words

A molded case breaker, the UL 489 kind, is a sealed unit. All of its parts sit inside one housing of insulating material. If it fails, it is usually replaced whole. An insulated case breaker is a larger member of the same family with a stored-energy mechanism, which makes it behave more like a power breaker while keeping the sealed construction.

A low voltage power circuit breaker, the UL 1066 kind, is built to be maintained. It is almost always drawout, it is not sealed, and its parts can be replaced one at a time. It costs more, it is larger, and it needs a cell to live in. When a specification names UL 1066 or its companion, IEEE C37.13, for any breaker in the lineup, that breaker is the expensive kind, whatever the section is called.

The difference between switchboard and switchgear that matters most

The difference between switchboard and switchgear that moves the most money is a rating most quotes never state: how long the assembly can carry a fault before something has to open.

A cycle is one sixtieth of a second on a 60 hertz system. A switchboard to UL 891 is tested to withstand its short-circuit current for three cycles, a twentieth of a second, and it has no short-time rating beyond that. Switchgear to UL 1558 is tested for four cycles and then carries a short-time rating of 30 or 60 cycles, half a second or a full second. The CDM Smith authors give an example of what that looks like on a breaker: able to interrupt a 100 kA fault within four cycles, and able to sit through 65 kA for 30.

Why would anyone want a breaker to sit through a fault? Because of the way a well-designed system clears one. When a fault happens on a feeder, the breaker nearest the fault should open and everything upstream should stay closed, so that one bad circuit does not black out the building. Engineers call this selective coordination. It only works if the main breaker waits a moment to see whether the feeder breaker clears the problem. While it waits, the main breaker and the bus behind it carry the full fault current.

Switchgear is built to do that waiting. A switchboard is not. A switchboard has to be protected by a breaker that opens at once on a large fault, which is why the instantaneous trip row in the table matters. A UL 489 breaker always has an instantaneous trip. A UL 1066 breaker may have one that can be switched off. The same authors warn about the combination that results when a UL 1066 breaker with its instantaneous trip switched off protects a UL 891 switchboard: a fault that lasts longer than three cycles can exceed what the switchboard was tested for.

For the estimator this turns an abstract standard into a reading task. Look for the sentences about coordination and trip settings. If the specification asks for a short-time delay on the main with the instantaneous trip switched off, or says the instantaneous function must be defeatable, it is describing switchgear behaviour, and a standard switchboard does not meet it no matter what the section title says.

Where eight owners draw the line

Large owners write their preferences down so that every design firm they hire gives them the same thing. Those documents are public, and they are the closest thing there is to a survey of how buyers settle UL 891 vs UL 1558. I read eight. The table gives each rule as closely as a short cell allows, with the paragraph it comes from.

Three patterns come out of those eight documents.

Some owners decide by size. The Department of Defense, the Department of Veterans Affairs, Dartmouth and Northwestern each name an ampere rating. They name four different ones. A 2,000 amp, 480 volt service is switchgear on a military base if it has large feeders, power circuit breakers by guideline at a veterans hospital, a switchboard with a drawout main at Northwestern, and drawout throughout at Dartmouth. What your last customer wanted is not evidence about this one.

Some decide by the job each breaker does. Pennsylvania and Yale both treat the mains and ties as the breakers that must be drawout and maintainable, and are more relaxed about the feeders. Pennsylvania goes further and keeps the feeder decision for itself: its engineering department tells the design firm, building by building, whether the distribution breakers are group mounted or drawout. If you are quoting a Penn job before that direction has been given, the honest quote prices one arrangement and says which.

Some decide by what the building does. Yale wants power circuit breakers for the entire substation in a utility plant. Houston wants them everywhere unless it approves otherwise. Michigan wants every breaker in a unit substation individually mounted and drawout. The Department of Veterans Affairs sorts its facilities into critical, essential and ancillary, and sets different rules for each class.

None of these documents is the specification you will be handed. A design firm edits the owner's standard into a project section, and edits leave gaps. But when a project section is silent, or says two things, the owner's standard is the first place to look for what was meant, and it is usually one search away.

The hybrid: a UL 891 switchboard asked to behave like switchgear

The most interesting of the eight is Northwestern's switchboard section, because it asks for a product that sits between the two columns of the first table.

Section 26 2413 is titled Switchboards and requires compliance with UL 891. So far, a switchboard. Then paragraph 2.1.F asks for a factory certified 30 cycle rating, which is the short-time rating that defines switchgear. Paragraph 2.3.B covers molded case breakers up to 1,600 amps to UL 489 and prohibits series rated equipment. And paragraph 2.4 covers main devices from 1,600 to 2,000 amps, which must comply with IEEE C37.13 and UL 1066, the power circuit breaker standards, and be a drawout insulated case power circuit breaker from a named product line or an equal.

Read as a bill of material, that is a switchboard enclosure, feeder breakers of the ordinary sealed kind, a drawout main of the switchgear kind, and a structure rated to carry a fault for half a second. Northwestern caps it at 2,000 amps. Above that, its separate switchgear section takes over, with UL 1558, mains and ties on a 3,200 amp frame at least, and a different named breaker line.

Northwestern is not alone. Dartmouth asks for drawout devices from 1,000 amps in switchgear and in switchboards alike. Pennsylvania allows group mounted feeders behind drawout mains. And the CDM Smith article notes that some switchboards are built with drawout UL 1066 breakers, and that these have to be laid out with the same clearances for withdrawing a breaker as switchgear.

So UL 891 vs UL 1558 is not a choice between two products. It is a choice among at least three:

• A switchboard: UL 891, fixed breakers to UL 489, three cycle withstand.
• A hybrid: UL 891, with drawout mains or ties to UL 1066 and, where the owner asks, a short-time rating on the assembly.
• Switchgear: UL 1558, drawout breakers to UL 1066 throughout, compartments, a short-time rating by definition.

Whether your product line includes the middle one, and up to what rating, is a catalogue question. It decides whether you can bid a section like Northwestern's at all without taking an exception. If you cannot, the place to say so is a line in the compliance matrix, with the paragraph number and what you offer instead, not a hope that nobody checks.

Switchboard vs switchgear UL listing: seven sentences that decide which one you price

The title of the section is the least reliable sentence in it. A section called Switchboards can describe a hybrid, and a section called Switchgear can be met by something simpler than the title suggests. These seven sentences are the ones that settle the switchboard vs switchgear UL listing question, with a place each one appears in the eight documents.

1. The standard named for the assembly. UL 891 and NEMA PB 2, or UL 1558 and IEEE C37.20.1. The Defense criteria put this in two adjacent paragraphs, 3-2.3 for switchgear and 3-2.4 for switchboards, and there is no doubt which is which.
2. The standard named for the breakers. UL 489 or UL 1066. Northwestern's switchboard section names both, in paragraphs 2.3.B and 2.4.A, for different breakers in the same lineup. Check every breaker class separately.
3. The word drawout, and the rating it starts at. Dartmouth starts it at 1,000 amps in paragraph 1.2.D.2 and repeats a 1,200 amp rule in 2.1.C. A threshold like that changes how many cells you need.
4. A short-time rating on the assembly. Northwestern's 30 cycle rating in a switchboard section, or its switchgear rule in 26 2300 paragraph 2.2.C that the short-time and short-circuit ratings match the highest rated breaker in the lineup.
5. Individually mounted or group mounted. Dartmouth requires individual mounting from 400 amps. Pennsylvania allows group mounting for feeders. Individual mounting means more sections for the same number of circuits.
6. Barriers and compartments. The Defense criteria ask for switchboard sections completely isolated from each other by vertical steel barriers, and for hinged fronts. Northwestern's switchgear section asks for barriers between main and tie compartments carried to the rear of the section, and for the line bus isolated from the load bus at each main and tie.
7. Whether the instantaneous trip can be switched off. Michigan requires the instantaneous function on mains and ties to be defeatable. That one word says the assembly behind those breakers must be able to carry a fault while they wait.

Go through a section with those seven in hand and you will know which of the three products it describes. You will also know whether it describes one product consistently, which is the subject of the next section.

When the page says one thing and describes another

Specifications are assembled from older specifications. Words travel with the paragraphs they came in, and the result does not always describe a single product. The owner standards show the same wear that project documents do.

Houston's master specification is titled Low Voltage Switchgear, cites IEEE C37.20.1 and UL 1558, and then says in paragraph 2.2.A that the switchgear shall be metal-clad. Metal-clad is the term the standards use for medium voltage gear with drawout breakers and insulated bus. The low voltage product is metal-enclosed. Michigan's guideline uses the same word for its low voltage breakers. Nobody will be confused by this in practice, but it tells you that the sentence was not written while looking at the standard it cites.

Yale's standard is a sharper case. It references UL 1558 for the secondary unit substation, and it describes the main and tie breakers as true drawout, insulated case breakers. On the description in the first table, those two do not go together: insulated case breakers belong to the UL 489 family, and UL 1558 switchgear takes UL 1066 breakers only. A manufacturer reading that paragraph has to ask which one governs, the standard cited or the breaker described, because they lead to different cells and different prices.

The Defense criteria describe a switchboard with more separation than the base product has: sections fully isolated by steel barriers, and hinged fronts. Those are options on most product lines, and they cost money. A quote that prices the base switchboard against paragraph 3-2.4 is short by those options.

We found the same kind of mismatch in an issued project document, a 15 kV specification that describes fused switches under one standard and then asks for breaker tests under another. It is written up in the switchgear guide linked at the top. The response is the same every time:

• Ask the question early, in writing, with both readings stated.
• Price one reading, and choose it on purpose.
• Write the reading you priced into the quote, with the paragraph number.

A question that names two paragraphs and two prices gets answered. A quote that silently picks the cheaper reading gets a rejected submittal three months later, after the price is fixed.

What the choice does to the rest of the quote

The lineup price is the obvious difference. The same documents show five quieter ones.

The room and the route

Switchgear is deeper and usually needs working space behind it as well as in front. The CDM Smith authors work an example under the electrical code's working space rules: a front and rear access lineup can need about twelve and a half feet of total depth, where a front access switchboard against a wall needs about seven. Michigan requires rear access to every unit substation and working space sized for it. Pennsylvania requires the design firm to document how the equipment will be moved into the building, and requires shipping sections sized to make the move. If your quote assumes standard shipping splits, it should say so. The route is the buyer's, and the split sizes are yours.

The bus

The Department of Veterans Affairs asks that the horizontal bus be fully rated for the length of the lineup. Michigan asks that the vertical bus in each feeder section be rated for the breaker cell frame sizes, including spares and spaces, not for the breakers actually installed. Both rules add copper that a breaker list will not show you. Our busbar calculator turns a rating and a lineup into pounds of bar if you want to see how much.

The spare space that is not empty

The Defense criteria require 15 percent empty space and 15 percent spare capacity in all switchgear and switchboards. The Department of Veterans Affairs asks for 25 percent space for additional devices. Michigan goes further: blank compartments are not permitted, and every space must be fully bused, fitted with neutral current sensors that match the breaker frame, and ready to accept a breaker. A space at Michigan is most of a cell with no breaker in it. It has a price, and the price is not zero.

Lifting, racking and safety options

Drawout breakers have to be lifted out. Dartmouth requires a lift trolley for drawout breakers of 800 amp frame and larger. Yale requires a trolley lifting mechanism on each substation. The Defense criteria ask the designer to evaluate arc resistant construction, infrared viewing windows, a remote racking mechanism that lets the operator stand at least 20 feet away, and shutters in the drawout compartments. Houston requires remote racking and arc resistant construction unless it approves otherwise. Each of these is a line on the quote, and each is easy to leave off when the breaker schedule is the only thing being read.

One manufacturer

The Defense criteria require the switchgear and its circuit breakers to be the product of the same manufacturer. If you build assemblies around another maker's breakers, that sentence decides whether you can bid without an exception.

How to quote when the specification does not say

Sometimes the section really is silent. It gives a one-line diagram, a bus rating and a breaker schedule, and never names a standard for the assembly. Five steps, in this order:

1. Find the owner's standard. Universities, federal agencies, transit agencies and many cities publish theirs. A short search often answers the question the project section left open.
2. Read the breaker descriptions. Drawout, stored energy, UL 1066, C37.13 and 100 percent rated are switchgear words. Molded case, group mounted and UL 489 are switchboard words. A mix of the two is the hybrid.
3. Read the coordination requirement. A study that needs short-time delays on the mains needs equipment that can carry a fault for that long.
4. Check the room on the drawings. A room with seven feet of depth was not drawn for rear access switchgear, whatever the section says.
5. Price one product, name it, and send the question. State the standard the assembly is listed to and the standard each breaker class is listed to, in the first lines of the quote.

That last step sounds too simple to matter. It matters because two honest manufacturers reading a silent section can price different products and both believe they complied. The buyer then compares a switchboard price with a switchgear price and awards the lower one. Whether that is your quote or your competitor's, somebody finds out at the submittal. We wrote about that judgement, and when the equal is worth arguing for, in or approved equal: when to bid and when to walk away.

If the lineup is double ended, with two mains and a tie, the same reading has a second half: how the two sources are transferred and kept apart. That is covered in the companion article on main-tie-main switchgear.

Why a quoting company wrote this

Mavlon builds software that reads a customer's specification and drawings when the request for quote arrives. It lists every requirement that changes the price, ties each one to its page, and flags the ones that disagree with each other, such as a section that cites one standard and describes the breakers of another. Your estimator decides what to do about each flag, and the quote is drafted in your own pricing rules. The seven sentences in this article are the kind of thing it is built to find. If your team quotes switchboards or switchgear from specifications, the documents in this article are a fair test of any tool, including ours.

Sources

• U.S. Department of Defense, UFC 3-520-01, Interior Electrical Systems, 16 January 2025, paragraphs 3-2.1 to 3-2.4.
• Department of Veterans Affairs, Electrical Design Manual, PG 18-10, 1 March 2025, paragraph 5.3.4.
• University of Pennsylvania, Design Standards 26 10 00, Power Distribution System and Equipment, revision July 2019.
• Northwestern University, Section 26 2413, Switchboards, and Section 26 2300, Low-Voltage Switchgear, issued 29 March 2017.
• University of Michigan, Design Guideline 26 11 00, Unit Substations, February 2023.
• University of Houston, Master Specification 26 24 14, Low Voltage Switchgear, August 2020.
• Dartmouth College, Design and Construction Guidelines 26 24 13, January 2023.
• Yale University, Design Standards, Subdivision 16300, Transmission and Distribution, revision 3, April 2009.
• William McGugan and Tyler Roschen, CDM Smith, Comparing low-voltage switchgear and switchboards, Consulting-Specifying Engineer, 28 September 2022.

Found a mistake? Write to atishay@mavlon.co and we will fix it.