How to Read a Terminal Block Derating Curve
Match the exact product and test configuration before reading the graph. Find the documented component ambient temperature, follow the trace that matches the conductor and loaded-position arrangement, and read the permitted continuous current. That result is one upper limit—not a universal ampacity, breaker setting, SCCR, or approval of the complete circuit.
Product-context image only. Photo: Zimin.V.G. / Wikimedia Commons, CC BY-SA 4.0; cropped and darkened. The photograph is not a derating test and proves no current, temperature, conductor, accessory, or approval condition.
Read four gates before you read one ampere
A plotted current is usable only when the curve, the installed configuration, and the design record refer to the same thermal case.
Identify the exact component
Record the full catalog number, revision, terminal point, and—where relevant—the exact plug/header pair. A family photo or similar pitch is not enough.
Read the title, legend, and test note
Confirm axes, units, conductor, loaded contacts, mating state, mounting, upper-limit temperature, and whether the line is a base curve or published derating curve.
Use the approved local ambient
The x-axis is commonly the air temperature surrounding the component. It is not automatically conductor temperature, terminal surface temperature, room setpoint, or enclosure rating.
Cap the result at every other ceiling
A curve can never raise the terminal, conductor, jumper, mating interface, equipment, approval, or system limit. The lowest verified applicable value governs.
The base-curve/0.8 method discussed below is a connector derating method used for pluggable products only when the manufacturer identifies that basis. Fixed support-mounted terminal blocks under IEC 60947-7-1 are normally verified through product ratings and temperature-rise tests. Do not import a connector curve or correction factor.
A curve answers a steady-state thermal question
A current-temperature curve expresses the permitted continuous current for a stated product configuration as ambient temperature changes. Current produces losses and temperature rise. As the air around the component becomes warmer, less thermal margin remains before the component's stated upper-limit temperature is reached.
The measured component or contact temperature helps the manufacturer construct the base curve; it is not normally the value a designer enters on the x-axis. Use the temperature variable named on the graph and the project's documented local condition.
- It can limit continuous current. It does not establish conductor ampacity by itself.
- It can distinguish tested loaded-position arrangements. It does not prove an untested partial-loading pattern.
- It can bound one component configuration. It does not set breaker size, SCCR, inrush, surge, or PE fault performance.
- It can support a design record. It does not validate the finished enclosure or listed panel alone.
Does this exact terminal even have a published curve?
A missing curve is not permission to borrow one. Classify the product before choosing the evidence path.
Feed-through, PE, fuse, disconnect, or multi-level block
Product data may give current, conductor, temperature-rise, rail, accessory, and approval conditions without a current-temperature graph. Use that exact evidence; do not import a pluggable connector curve.
Terminal plus removable plug or header
Manufacturer curves may separate 2-, 5-, 10-, or 15-position arrangements, conductor options, and mating configurations. Match the complete mated system and every test note.
Board connection, pitch, conductor, and component family
PCB-terminal derating data can depend on board interface, geometry, pitch, conductor, and product family. Do not transfer DIN-rail mounting assumptions to the board application.
I/O module, drive, power supply, or removable plug
The equipment manual may impose terminal-point and module-level current/temperature restrictions below a generic connector value. The host equipment documentation is part of the limit.
When no matching curve is published, use the exact product's other ratings and installation conditions. If the application requires temperature-dependent evidence, request OEM application guidance or qualified end-product validation. Never derive a curve from nominal current.
Read the legend and test note before the line
The example below is deliberately hypothetical. It teaches the reading motion; none of its values is a SENTOP or manufacturer product rating.
Illustrative current-versus-ambient-temperature chart
Illustrative only: do not copy the plotted values, traces, or position labels into a design. Read the official manufacturer graph for the exact product.
Component ambient temperature
Confirm the manufacturer's definition and the documented local thermal basis. Do not substitute a room thermostat or conductor temperature.
Permitted continuous load current
Read the stated unit and label. The curve cannot raise a lower catalog, conductor, accessory, mating, or approval current.
One tested configuration
A line may represent pole count, adjacent loaded contacts, conductor, mating pair, orientation, or product variant. Never interpolate between different traces.
No extrapolation
Stay inside the plotted range. If the actual temperature or current falls outside it, request manufacturer guidance or qualified engineering review.
In the cited connector method, permissible load current is 0.8 times the base current—a 20% reduction. Do not describe that as an 80% reduction, apply it again to an already published derating curve, or transfer it to fixed support-mounted terminal blocks.
Eight steps from a graph to a controlled design limit
The method separates evidence collection, graph reading, current-path review, and engineering release. It does not turn the graph into a wiring or test procedure.
Classify the component
Identify fixed feed-through, PE, fuse/disconnect, pluggable rail, PCB, or equipment terminal. Record the exact terminal point and full part number.
Find the authoritative source
Use the current manufacturer data sheet, installation document, drawing, approval record, or application note. Reject a reseller screenshot or look-alike curve.
Capture chart conditions
Record title, axes, units, legend, revision, base-versus-derated status, conductor, loaded contacts, mating state, mounting, and upper-limit basis.
Establish local ambient
Use the approved component-location thermal basis. Account for enclosure heat sources and terminal density through that local ambient plus manufacturer spacing/orientation limits.
Select the matching trace
Match the exact product, copper conductor condition, mating hardware, and documented loaded-position arrangement. Do not choose a favorable lower-count line by assumption.
Read within the plotted range
Move from ambient to the same trace and then to current. Preserve the graph's resolution, round conservatively, and never extrapolate at either end.
Map the current path
Check the clamp, internal path, conductor, bridge, plug/header or PCB interface, upstream common segments, and downstream terminal/equipment limits.
Record and release
Document the source/revision, selected point or permitted interpolation, ambient basis, conductor, loaded contacts, accessories, other ceilings, decision, and reviewer.
Interpolate only when the manufacturer or approved project method permits it. Use adjacent points on the same trace, keep the graph's limited precision, and round conservatively. Never interpolate between different pole counts, conductor sizes, mating pairs, products, or test methods.
Match configuration details that change the thermal result
Multi-position pluggable products may publish separate traces for different simultaneously loaded contacts. Count them using the manufacturer's definition. Visible openings, unused pins, separate potentials, and mechanically adjacent blocks are not automatically equivalent to the legend's “positions.”
Also match the test conductor's material, cross-section, construction, preparation, and number of conductors. IEC 60947-7-1:2025 is scoped to copper conductors; a copper-based product curve does not authorize aluminum. A larger conductor is not automatically an approved substitute, even when it may remove heat differently.
- Exact SKU and revision: pitch, geometry, contact metal, housing, and mating half can differ.
- Loaded-contact pattern: use the documented actual or accepted bounding arrangement.
- Mounting and accessories: rail, orientation, covers, separators, bridges, and spacing can matter.
- Upper-limit temperature: it is the exact curve's test/material-system value—not automatically ambient rating, insulation RTI, conductor rating, or permissible case temperature.
A curve reading is an upper bound—never a rating increase
Map every continuous-current segment. Then verify incomparable requirements such as SCCR, PE fault behavior, voltage/insulation, and protective-device coordination separately.
Curve plus declared product ceiling
Use the matched trace, maximum load or rated current, contact configuration, temperature condition, and applicable certification/use-group data.
Material, size, insulation, routing, grouping
The terminal curve does not establish conductor ampacity. Conductor bundling/grouping is a separate calculation; do not invent a generic terminal grouping factor.
Own rating and aggregate upstream current
The first segment of a jumpered row can carry the sum of downstream branches. Check exact compatibility, rating, pitch, and segment current.
Plug, header, board, or host equipment
A mated contact, PCB path, I/O module, drive, or equipment terminal can impose a lower continuous-current limit than the visible block.
Feed-in, fuse, disconnect, covers, separators
Accessories can add an independent rating or configuration condition. Never assume the base terminal's ampere value transfers to every accessory.
Enclosure thermal design and approvals
The component curve does not prove the finished assembly's worst-case steady-state temperature, listing, short-circuit, or environmental suitability.
I_design ≤ min(I_curve, I_product_max, I_conductor_continuous, I_bridge, I_mating_interface, I_other_continuous_limit)Verify SCCR, short-time withstand, PE/rail fault performance, voltage and insulation, and protective-device requirements separately. Those are different quantities and do not belong in this minimum-current equation.
Six shortcuts that create unsupported current limits
Each shortcut hides a different missing condition. The correction is to improve the evidence—not add an arbitrary percentage.
Copying nominal current
A catalog ampere value beside “see derating curve” is incomplete. Record the chosen trace, ambient, configuration, and lower applicable ceilings.
Choosing the 2-position trace
A dense energized assembly does not become a two-position test merely because the circuit of interest uses one contact. Match the documented loaded arrangement.
Applying 0.8 twice
Where a manufacturer already published the corrected derating curve, another 0.8 multiplier double-counts that method. Keep separate project margins separate.
Using the room thermostat
Remote room temperature may not represent air around a component near drives, power supplies, transformers, dense terminals, or enclosure hot spots.
Extending the line
Extrapolation is prohibited above or below the plotted range. At low ambient, the conductor, product maximum, or approval current can still cap the result.
Using the curve as SCCR or breaker data
Steady-state current-temperature performance does not establish protective-device size, fault interruption, SCCR, PE performance, or short-time withstand.
The fixed-terminal IEC 60947-7-1 temperature-rise specimen described by Phoenix uses five terminals on a rail. That is not a “5-position derating curve.” The 2-/5-/10-/15-position traces discussed for a series-loaded plug-in arrangement belong to a different connector-style test context.
Where the default reading path changes
These scenarios are decision examples, not product ratings or field instructions.
One circuit sits inside ten continuously loaded positions
Do not read the favorable two-position line. Map the actual energized pattern, match the manufacturer trace or written bounding case, then check conductor and mating plug limits.
Small branches create a larger common-feed current
Each branch terminal may carry only its own load, while the feed-in and first jumper segments carry the sum. Record current per segment and check every exact accessory.
No current-temperature graph is published
Use the exact rated-current, conductor, temperature-rise, accessory, and approval data. If the project needs a temperature-dependent limit, request OEM guidance; do not borrow a plug-in or PCB curve.
The assembled panel can still require its own thermal evidence
Documenting a component curve is not end-product validation. Where the panel, machine, or equipment procedure requires a worst-case steady-state thermal check, validate the actual assembly under the approved method with the intended loads, enclosure, accessories, conductor routing, ventilation, and source conditions.
Do not turn this article into a live measurement method. Electrical work and energized diagnostics require the site's electrical safe-work process and qualified personnel. For U.S. workplaces, OSHA 1910.333 sets the deenergization, lockout/tagout, and verification boundary where applicable.
Use terminal-block temperature-rating guidance for the wider difference between ambient, temperature rise, operating limits, conductor temperature, and material data.
Turn the read-off into a reviewable decision
A number copied from a graph is not a release record. Preserve the evidence so another engineer, buyer, or quality reviewer can reproduce the result.
All conditions match
Exact product, curve, trace, ambient, conductor, loading, accessories, and every other continuous-current ceiling support the design current.
Approval has stated controls
Acceptance depends on a specific conductor, loaded-position pattern, ambient ceiling, accessory, mating half, spacing, or documented current cap.
Evidence is missing
The curve revision, trace mapping, ambient basis, conductor, accessory rating, or exact product configuration is not yet verified.
A verified limit is exceeded
Design current exceeds the applicable curve or another product, conductor, bridge, interface, equipment, or approval ceiling.
| Record field | What to capture | Why it matters |
|---|---|---|
| Product identity | Manufacturer, full SKU, revision, terminal point, lifecycle status, and exact mating half where relevant. | Similar pitch, color, clamp style, or family name does not establish the same contact or thermal system. |
| Curve evidence | Official source, revision/date, graph title, units, legend, selected same-trace point, and permitted interpolation method. | A cropped graph without test notes cannot be audited or reliably applied later. |
| Thermal basis | Defined component ambient, enclosure hot-spot assumption or approved measurement basis, spacing, orientation, ventilation, and nearby heat sources. | Remote room temperature or enclosure rating is not automatically the curve's x-axis condition. |
| Conductor and loading | Material, cross-section/AWG, construction, preparation, number of conductors, and simultaneously loaded-position pattern. | The conductor and adjacent energized contacts can be part of the published curve configuration. |
| Accessories and path | Bridge, feed-in, plug/header, PCB/equipment interface, fuse/disconnect, rail, covers, separators, and current per common segment. | A lower accessory or upstream aggregate-current limit can govern before the plotted terminal trace. |
| Separate system checks | Conductor ampacity, overcurrent protection, SCCR/fault duty, voltage/insulation, PE fault path, environment, approvals, and end-product thermal evidence. | These questions are not proven merely because the steady-state curve read-off is acceptable. |
Use the product standard and the curve method in their own scopes
A standard number identifies an evaluation framework. It does not make curves interchangeable or approve the finished panel.
| Reference | Relevant role | Do not infer |
|---|---|---|
| IEC 60947-7-1:2025 | Current product standard for support-mounted screw-type and screwless terminal blocks for copper conductors, including product rating and temperature-rise context. | It does not assign every fixed terminal a connector derating curve or authorize aluminum from copper data. |
| IEC 60947-7-4:2019 | Product-standard context for PCB terminal blocks, including product-specific current/temperature considerations. | A PCB curve is not evidence for a DIN-rail terminal or equipment plug. |
| IEC 60512-5-1 / -5-2 | Connector test methods used for current-carrying/temperature-rise work and current-temperature derating where the manufacturer declares that basis. | The 0.8 multiplier is not a universal terminal, enclosure, grouping, code, or design-margin factor. |
| UL 1059 / component record | North American terminal-block component evaluation under stated markings, ratings, categories, and Conditions of Acceptability. | A UL logo or curve alone does not approve every conductor, accessory, configuration, use, or completed panel. |
| UL 508A Supplement SA | Current component-requirement cross-reference for industrial control panels; the official landing page identifies the October 2025 edition. | An older supplement PDF or one component value should not replace the current panel procedure and exact component record. |
| End-product procedure | Defines any required worst-case assembled-equipment thermal verification, measurement points, stabilization, uncertainty, and acceptance record. | A component curve is not an end-product temperature test or blanket compliance statement. |
For deeper rating fields, use the terminal-block specification guide. For fault-duty boundaries, use the terminal-block SCCR guide.
For U.S. work, OSHA 29 CFR 1910.333 requires exposed live parts to which an employee may be exposed to be deenergized before work on or near them unless the employer demonstrates a permitted exception. Deenergized circuits must be controlled under the applicable lockout/tagout procedure and verified by a qualified person before they are treated as deenergized. Energized work is restricted to qualified persons using work practices appropriate to the hazard.
Ask for the curve context—not a cropped graph
Send the exact product, conductor, loading, accessories, local thermal basis, application, and approval needs. Ask the supplier to state whether the published curve actually bounds the intended steady-state configuration.
Please provide the current official current-temperature derating curve for terminal [full SKU / terminal point], including the chart legend, test basis, product revision, conductor condition, loaded-position arrangement, mounting/mating configuration, component-ambient definition, upper-limit basis, approved accessories, and all related current, voltage, fault, and approval limitations. Please confirm whether it bounds our stated continuous-load pattern.
Terminal block derating curve FAQ
Use these short answers as decision boundaries, then return to the exact manufacturer graph and product record.
What are the x- and y-axes on a terminal block derating curve?
In the connector method discussed here, the x-axis is commonly component ambient temperature and the y-axis is permitted continuous load current. Always verify the labels, units, definitions, and test notes on the exact graph.
Is nominal current the same as a derating-curve reading?
No. Nominal or rated current is declared under stated product conditions. A curve reading is a configuration-specific continuous-current ceiling at one ambient and trace condition. It cannot exceed another applicable product, conductor, accessory, interface, equipment, or approval limit.
What is the difference between a base curve and a derating curve?
A base curve is a precursor in the cited connector test method. The published derating curve may already include the method's correction for practical use. Read the exact title and note; do not assume every terminal uses this method.
Do I always multiply the curve by 0.8?
No. In the cited connector method, the manufacturer creates the corrected curve by multiplying base current by 0.8, a 20% reduction. Do not apply it again to an already published derating curve or transfer it to a fixed terminal block without explicit manufacturer instruction.
Which curve should I use when only some positions are loaded?
Use the trace or bounding condition the manufacturer documents for the actual simultaneously loaded arrangement. Do not assume a partly loaded dense assembly behaves like a two-position specimen. Request OEM guidance when the legend does not cover the pattern.
Does a jumper carry the current of all downstream circuits?
It can. An upstream bridge segment or feed-in terminal may carry the aggregate current of downstream branches even when each output carries only its branch current. Map current by segment and check the exact bridge and terminal data.
Can I interpolate or extrapolate a derating curve?
Interpolate only on the same trace when the manufacturer or approved project method permits it, and do not claim more precision than the graph supports. Never interpolate between different configuration traces or extrapolate beyond either graph boundary.
Can a derating curve set the circuit-breaker size or SCCR?
No. The curve is a steady-state thermal constraint for one component configuration. Overcurrent protection, conductor sizing, SCCR, short-time withstand, fault coordination, and equipment rules require separate evidence and calculations.
What should I do if no derating curve is published?
Use the exact product's other ratings and installation conditions. If the application requires temperature-dependent continuous-current evidence, request manufacturer guidance or use the approved end-product engineering validation path. Do not generate a curve from nominal current.
Can I reuse a curve from a similar terminal block?
No. Geometry, contact system, conductor condition, material, pole count, mating arrangement, mounting, and test method can differ. Use the curve only for the documented product/configuration or obtain written manufacturer confirmation.
Standards and manufacturer evidence
Product pages illustrate method and configuration. They do not create cross-family equivalence.
- IEC 60947-7-1:2025. Support-mounted terminal blocks for copper conductors. Official IEC page.
- IEC 60947-7-4:2019. PCB terminal blocks and their product-standard context. Official IEC page.
- IEC 60512-5-1:2002. Connector current-carrying capacity and temperature-rise test reference. Official IEC page.
- IEC 60512-5-2. Connector current-temperature derating test-method reference where declared by the manufacturer. Official IEC page.
- Phoenix Contact electrical tests. Separates fixed-terminal temperature-rise testing from connector derating for plug-in terminal blocks. Manufacturer page.
- Weidmüller derating curves. Defines component ambient, base/derating logic, pole and conductor influence, and low-ambient current capping. Manufacturer page.
- Weidmüller terminal-block electrical testing. Explains fixed-terminal and pluggable-terminal evidence paths. Manufacturer page.
- Phoenix Contact ST 2,5/1P, item 3040012. Product-specific example with multiple loaded-position traces and separate approval data. Product PDF.
- Siemens terminal blocks. Official product catalog example containing multi-pole derating presentation for the specified 8WH5 product. Official PDF.
- UL 1059, Edition 6. Active terminal-block standard scope and component suitability boundary. Official UL page.
- UL 508A Supplement SA. Official current landing page for component requirements; identifies the October 2025 edition. UL resource.
- WAGO 2006-434 jumper bar. Example of an accessory with its own rating and compatibility context. Product page.
- OSHA 29 CFR 1910.333. U.S. electrical safety-related work-practice boundary. Official regulation.
Match the curve, map the path, and record the limit
Share the exact terminal or connector, official graph, conductor, loaded-position pattern, local ambient basis, accessories, design current, approval needs, and quantity. SENTOP can help screen the product and documentation match; final engineering and panel approval remain project-specific.
