Understand your electrical equipment
Commercial Panelboard Ratings and Capacity, Explained
Several numbers describe a panelboard. Each answers a different question, and none can replace a review of the whole installation.
Availability confirmed by property address and project.
What do the ratings tell you?
Use this as a conversation guide. It is not a sizing tool or permission to select equipment from a label alone.
| Term | What it describes | What it does not prove |
|---|---|---|
| Voltage and phase | The supply configuration the equipment must suit. | That a business has a particular voltage or three-phase service. |
| Ampere rating | An equipment or device current rating. | The spare capacity available for a new load. |
| Circuit spaces | Positions the equipment provides for devices. | That the supply has enough capacity or any breaker will fit. |
| Available fault current | The prospective short-circuit current at a particular point. | An equipment rating or normal operating load. |
| Interrupting rating (often called AIC) | A protective device’s ability to interrupt fault current at the stated voltage. | The rating of every other component or the entire assembly. |
| Short-circuit current rating (SCCR) | The assembly’s rated short-circuit capability under its stated conditions. | Permission to substitute devices or ignore the upstream system. |
Sources: Eaton — Panelboard and switchboard series rating information manual · U.S. DOL / GPO — 29 CFR 1910.303: General
Why 208Y/120 and 480Y/277 are different planning questions
In a 208Y/120 V wye system, the nominal phase-to-phase voltage is 208 V and phase-to-neutral voltage is 120 V. In a 480Y/277 V wye system, those values are 480 V and 277 V. Equipment has to match the actual supply and its intended connection.
Tampa Electric’s service requirements describe several nominal configurations and make availability dependent on the site and demand. A Tampa mailing address does not by itself establish the provider or voltage. Do not assume a commercial building has three-phase service.
Sources: Tampa Electric — Standard Electrical Service Requirements
Three fault-current questions to keep separate
How much current could the supply deliver into a short circuit here? Can the protective device interrupt that current at this voltage? Is the complete assembly rated for the applicable conditions? The answers must work together.
For a fully rated arrangement, protective devices are selected for the prospective fault current at their locations. A series-rated arrangement depends on an identified upstream/downstream combination and its conditions. Substitution can affect that combination; matching the ampere number is not enough.
The manufacturer’s published tables and equipment markings need to be checked for the exact installation. A series rating is not, by itself, a promise of selective coordination or uninterrupted operation of unaffected circuits.
Sources: Eaton — Panelboard and switchboard series rating information manual · U.S. DOL / GPO — 29 CFR 1910.303: General
Plan capacity from loads and operating needs
A useful proposal starts with the existing loads, the equipment being added and how it will be used. The upstream feeder or service may set a limit even when the panelboard has unused positions. A replacement panelboard alone does not create more utility capacity.
Separate today’s approved work from future allowances. Ask which future equipment is included in the assessment, what remains an assumption and whether the proposed scope includes upstream changes.
Check more than the number on the main breaker
Main-lug-only equipment receives its supply at lugs without an integral main breaker. Its protection and disconnect arrangements must be understood in the full system. It should not be described simply as “unprotected” because no main breaker is inside that enclosure.
The enclosure, environment, circuit identification, working access and specified protective devices also belong in the assessment. Exact-model instructions apply; this page does not approve a combination of components.
Sources: Eaton — Panelboard fundamentals · U.S. DOL / GPO — 29 CFR 1910.303: General
Questions to ask when reviewing the proposal
What supply information and load assumptions were used? Which panelboard and protective devices are included? How was the fault-current requirement addressed? Does the scope include upstream work, drawings, permits, inspections and circuit identification?
Also ask which existing records will be updated and who coordinates any shutdown. If a fact is not established, it should remain an open item rather than an assumption hidden inside the estimate.
Questions you may be asking.
Is AIC the same as the panelboard’s ampere rating?
No. A normal current rating and a fault-current interrupting rating describe different capabilities. They must not be compared as if they measure the same thing.
Is a higher SCCR the same as more usable power?
No. SCCR addresses short-circuit capability under the equipment’s stated conditions. Usable capacity depends on the assessed loads and the electrical supply.
Can any same-size breaker replace one in a series-rated system?
No. The identified combination and its conditions must be preserved or reassessed. The ampere number alone does not establish compatibility.
Sources: Eaton — Panelboard and switchboard series rating information manual · U.S. DOL / GPO — 29 CFR 1910.303: General
In Simple Terms
The ampere number tells you one part of what equipment is rated to carry. The spaces tell you where circuits can go. Neither tells you, by itself, how much additional equipment your business can run.
Fault-current ratings address a different situation: a short circuit. The supply’s possible fault current, the breaker’s ability to interrupt it and the panelboard assembly’s rating must be considered together. These are assessment questions for qualified professionals, not numbers you need to measure yourself.
Tell us what you already know and what you want to add or change. Existing drawings and equipment manuals are useful. Your team can explain what still needs to be confirmed and how that affects the equipment choice, price and work plan.
Your property. A clear next step.
Let’s talk about your panelboard project.
Share the planned equipment, any existing documentation and your operating needs. The team can identify which electrical information is needed for your particular project.
Clear scope before paid work.
A free estimate prices known installation or improvement work. Investigating an unidentified fault is paid diagnosis; the scope and fee are explained before testing begins.
We confirm availability for your service address and project. Tell us about operating hours and access needs; scheduling follows confirmation.
The price depends on the agreed scope, equipment, route, access and any drawing, permit or utility requirements.
Ask which written warranty applies to your proposal. Equipment-manufacturer coverage and installation-workmanship terms can differ.
Sources and further reading
Reviewed September 13, 2026. Confirm current requirements and exact-model instructions for the project.
- Eaton — Panelboard and switchboard series rating information manual
April 2020, revision R, pages 2–4. Series combination principles only; current exact combinations and applicable rules must be rechecked.
- U.S. DOL / GPO — 29 CFR 1910.303: General
July 1, 2025 edition, paragraphs (b), (f), (g). General-industry listing, ratings, identification and working-space principles. Annual compilation; not a complete current legal determination.
- Tampa Electric — Standard Electrical Service Requirements
Printed page 29; revision date not independently established. Nominal service configurations and site-dependent availability; no guarantee of a specific address voltage or provider.
- Eaton — Panelboard fundamentals
Undated. Terminology, main-lug arrangements and equipment families. Published generic maximum ratings are not universal limits.