01What Instrument Transformers Do in Switchgear and Substations
An instrument transformer scales current or voltage to a level that meters, relays and monitoring equipment can use. A current transformer (CT) reproduces primary current as a standardized lower secondary current. A potential transformer (PT), also called a voltage transformer (VT), reproduces system voltage at a lower, measurable level.
These are not general load-supply transformers. They do not serve a building’s lighting, motors or other loads; they provide signals for measurement and protection. CTs and PTs can be installed inside switchgear, around or connected to buswork, or outdoors in substations. They may also be sold with a larger substation equipment lot. For other equipment categories, see the full guide to electrical transformer types.
California Transformer Buyer considers CTs and PTs from California facilities, utilities, contractors and property owners who have authority to sell the equipment. Utility-owned equipment located on private property cannot be sold by the property owner.
02How to Identify a CT, PT or VT from Its Form and Nameplate
Start with the equipment label and nameplate, not its size or apparent power rating. Look for CT, PT or VT markings; current or voltage ratios; burden and accuracy or protection class; polarity marks; rated voltage and BIL; and a connection or vector diagram. A CT may be a window or bar type, a bushing CT, or a molded assembly. A PT/VT may be a dry, resin-molded or oil-filled unit.
The examples below are illustrative, not a specification for every unit. Ratings and construction vary by manufacturer, application and voltage class. Photograph the complete nameplate so the ratio and class can be read together.
| Type | Typical nameplate clue | Common physical form |
|---|---|---|
| Current transformer (CT) | 200:5 A or 1000:5 A ratio | Window, bar or bushing |
| Current transformer (CT) | 1 A secondary may be specified | Molded or other design-specific form |
| Potential/voltage transformer (PT/VT) | 12,000:120 V example ratio | Resin-molded or oil-filled |
| Either type | Burden, class, BIL and polarity | Indoor or outdoor construction |
Why kVA Is Usually the Wrong First Search
Unlike a distribution transformer, an instrument transformer is usually specified by ratio, VA burden and accuracy or protection class, rather than a general kVA rating. The burden range is design-specific. If a listing or record shows only kVA, provide the actual nameplate so the unit can be identified correctly.
03Where CTs and PTs Are Used Across California
CTs and PTs support utility metering, protective relaying and power-system monitoring in substations and switchgear. They are also found in industrial and commercial electrical systems where operators need to measure high current or voltage indirectly and feed instruments or protective devices.
California systems include differing distribution and subtransmission voltages. For example, PG&E documents 12 kV and nominal 21 kV distribution configurations, while SCE has 4 kV, 12 kV and 16 kV distribution systems; actual equipment must be matched to the specific installation. Those system voltages are context, not a substitute for the CT or VT nameplate ratio. Utility and metering requirements can also affect whether a unit is suitable for reuse.
Municipal utilities, public power agencies and other operators have their own equipment standards and approvals. California Transformer Buyer is not affiliated with or approved by any utility. Sellers working with a public agency can also review information for utility and municipal equipment sellers.
04Construction, Insulation and Voltage-Class Variations
Instrument transformers may be indoor or outdoor units, with insulation and enclosure suited to their rated voltage and environment. Common designs include resin-molded or other dry-insulated CTs, window or bar CTs, bushing CTs, and molded or oil-filled PTs/VTs. The nameplate and visible form help identify the design, but do not establish its internal condition.
Insulation classes span low voltage through high-voltage transmission applications. A small-looking unit may still have a high system rating, so avoid judging capability by dimensions. Record rated voltage, BIL, ratio, frequency, burden, accuracy class and protection class where shown; include manufacturer and serial number if legible.
When instrument transformers are part of a larger station sale, document them separately from the power transformer and switchgear. A buyer of substation transformers and station equipment can assess the broader equipment lot, but the CT/PT details still matter.
05What Affects Resale Value of a Used Instrument Transformer
Resale value depends on whether the specifications match a practical reuse need and whether the unit’s identity and condition can be verified. Ratio, secondary rating, burden, accuracy or protection class, rated voltage and insulation class are central details. A commonly needed configuration with complete records may be easier to evaluate than a unit with missing or unreadable documentation.
Physical condition also matters. Note visible damage, cracked or chipped insulation, corrosion, signs of moisture, oil leakage, missing parts or altered leads, without opening or disturbing the equipment. Age, manufacturer, quantity, indoor or outdoor design, and whether the unit is used or new-surplus can further affect buyer interest.
For instrument transformers used in utility metering or protection, approvals and application compatibility are important. A buyer cannot assume a unit will be accepted by a utility or fit a particular relay or meter simply because the voltage class is similar. Test reports, service records and the intended application help establish what is being offered.
- Ratio and secondary rating, including 1 A or 5 A CT output
- Burden and metering or protection accuracy class
- Voltage rating, BIL, insulation type and indoor/outdoor design
- Condition, completeness, quantity and available test records
- Utility, metering or protection application requirements
06What to Send an Instrument Transformer Buyer for an Offer
To sell an instrument transformer, provide enough information to distinguish the unit and assess its condition. Send clear photos of the nameplate and overall equipment, plus the quantity, California location and whether it is used, removed, spare or new-surplus. Take photos only from a safe position and only where you are authorized to access the equipment.
Include available test reports, maintenance or service records, ownership information, and any utility or metering documentation. For oil-filled units, share existing fluid and PCB records rather than sampling or draining the equipment. If records are incomplete or PCB status is unknown, state that plainly; testing and documentation should be handled with qualified environmental professionals.
Call or text Rocky at (951) 403-5738, or submit details through the transformer sale information form. The review process and what to expect are described in how a written offer and sale are handled. Reviewing a transformer costs nothing and creates no obligation.
- Nameplate and overall photos
- Quantity, condition and equipment location
- Test reports and service records, if available
- Ownership authority and site-access details
- Existing fluid or PCB documentation, if applicable
07Removal Planning and CT Secondary Safety
Instrument transformers may be integrated into energized switchgear or a substation bus. Sellers should not disconnect, move, open or otherwise work on them. De-energizing and disconnecting must be handled by the owner’s qualified electrical contractor or the serving utility, with the required isolation, testing and grounding. Backfeed from solar, batteries or generators must also be considered.
A CT secondary must not be opened while its primary carries current; doing so can create a hazardous voltage. Keep equipment in its existing condition and coordinate all electrical work through qualified personnel. California Transformer Buyer can state in the written offer who handles loading, rigging and freight before pickup is scheduled.
For large station lots, access, lifting arrangements and coordination with other equipment can affect removal planning. Review the scope and responsibilities in advance through transformer removal and pickup information.
08Why California Sellers Bring CTs and PTs to California Transformer Buyer
Facilities and contractors may have individual spare CTs or PTs after a switchgear replacement, a project surplus lot, or instrument transformers included with a substation sale. California Transformer Buyer reviews used, pulled, surplus and new-surplus equipment from sellers throughout California, from single units to mixed lots.
A written offer states the purchase terms, what is included, who handles loading or rigging and freight, and payment terms and timing. No purchase is guaranteed; the offer depends on the details and condition reviewed. Call or text (951) 403-5738, or use the contact page to request an offer.
FAQInstrument Transformers (CTs & PTs) FAQ
Do instrument transformers have a kVA rating like distribution transformers?
Usually, no. CTs and PTs are generally identified by ratio, VA burden, accuracy or protection class, voltage rating and insulation details rather than by a distribution-style kVA rating. If a record lists kVA, send a nameplate photo so the exact equipment and its intended function can be checked before treating it as a CT or PT.
Can I sell a CT without its matching meter or relay?
Possibly. A CT may have resale interest on its own, but the buyer needs its ratio, secondary rating, burden, accuracy or protection class, voltage and condition. Whether it can be reused depends on the new application and any utility or metering requirements. Include any compatibility records or test reports you already have.
Can California Transformer Buyer purchase a PT or CT that is still installed?
The equipment may be reviewed while it is installed, but the seller must not disconnect or move it. Only the owner’s qualified electrical contractor or serving utility should perform isolation and disconnection. Any offer should clearly state who handles rigging, loading and freight, and removal must be coordinated around the site’s electrical and access requirements.
What if the instrument transformer nameplate is unreadable?
Send clear photos of the full unit, any remaining labels, terminal markings and nearby equipment identifiers, along with the location and quantity. Do not open the unit or disturb wiring to find additional information. Existing drawings, purchase records, test reports or asset records may help establish the ratio and class, but uncertain details should be identified as unknown.
Do you buy oil-filled PTs with unknown PCB status?
Share the existing fluid and PCB documentation and state when status is unknown. Do not drain or sample the unit yourself. PCB classification and handling depend on testing and qualifying documentation, and California rules for PCB-containing oil can affect its management. Sellers should rely on qualified environmental professionals for testing and determinations; this information is not legal advice.
Can a seller include CTs and PTs in a larger substation equipment lot?
Yes, include the instrument transformers in the lot inventory and photograph them separately where safely accessible. Their ratios, classes, condition and records should remain identifiable apart from the power transformer or switchgear. Provide total quantity and identify which equipment is new-surplus, used or removed so the offer can describe what is included.