Cable Lug Size Chart: How to Match Lug, Conductor and Bolt Without Guesswork

Sizing a cable lug comes down to three measurements: the conductor cross-section in mm², the strand class of that conductor, and the stud or bolt diameter at the terminal. Get those three right and the lug fits, crimps correctly, and passes inspection. Miss any one and you get a joint that either won't crimp properly or won't carry its rated current.

This guide gives you a working size chart for the 4–630 mm² range, explains the details that catch even experienced installers, and shows how to read a lug designation like "SC-120-12" without opening a catalogue.

What You'll Get From This Page

• A reference chart linking conductor size, lug barrel dimensions and bolt holes

• The difference strand class makes (and why two "95 mm²" cables need different lugs)

• Hole size and palm width rules for busbar and breaker terminations

• The five sizing errors that appear most often in the field

Reading a Lug Designation

Most IEC-market lugs follow a simple code: series – conductor size – hole diameter.

So SC-120-12 is an SC-type copper tube lug for 120 mm² conductor with a 12.5 mm palm hole (fits an M12 bolt). DTL-1-240-16 is a single-hole bimetallic lug, 240 mm² aluminium conductor, M16 stud. Once you know the pattern, a materials schedule almost writes itself.

The Working Size Chart (Copper SC-Type, 4–630 mm²)

Dimensions below are representative of SC (JGK) series tube lugs manufactured to IEC 61238-1. Always confirm against the specific datasheet before ordering — barrel bores vary slightly between manufacturers.

Three observations that make this chart more useful than most:

• Barrel bore is the controlling dimension. The conductor must slide in fully with light resistance. If you need to hammer it, the strand class is wrong. If it rattles, the crimp will not reach full compression.

• Length grows faster than diameter. An SC-630 is five times longer than an SC-4. Check bending space in the panel gland zone before specifying large lugs — the straight length behind the palm is a frequent clash point in compact switchgear.

• Multiple hole options exist for mid-range sizes. SC-240 comes in M12, M14 and M16 hole versions. Order the hole that matches the terminal stud; drilling out a lug palm in the field removes plating and reduces contact area.

Strand Class: The Detail That Breaks the Chart

Two cables can both be marked "95 mm²" and have different physical diameters. IEC 60228 defines conductor classes:

• Class 1 — solid conductor

• Class 2 — stranded, compacted (the usual power cable)

• Class 5 — flexible, fine-stranded (welding cable, panel wiring, EV and battery leads)

• Class 6 — extra flexible

A Class 5 flexible 95 mm² conductor is visibly fatter than a Class 2 compacted one, because fine strands pack less densely. Standard tube lugs are bored for Class 2. Push a Class 5 conductor into one and strands fold back or shear off — each lost strand is lost ampacity.

The rule: for Class 5/6 conductors, specify lugs bored for flexible conductor, or step up one barrel size after verifying with the manufacturer's cross-reference. In our production experience, mismatched strand class is the single most common reason customers report "the lug is the wrong size" when the mm² marking matches.

Aluminium and Bimetallic Sizing Differences

Aluminium conductors carry less current per mm² than copper, so aluminium cables in the same circuit are typically one or two sizes larger. Two sizing consequences:

1. Aluminium barrels are longer and thicker-walled. Aluminium's lower yield strength requires more crimp length to achieve equivalent pull-out force. A DL-150 aluminium lug is noticeably longer than an SC-150. Panel space calculations should use the aluminium lug dimensions, not copper equivalents.

2. Bimetallic lugs are sized by the aluminium conductor. A DTL-240 takes 240 mm² aluminium cable; its copper palm and hole options follow the terminal side. When replacing a copper feeder with aluminium (a common cost-reduction change), re-size the conductor for ampacity first, then pick the DTL lug for the new aluminium size — not the old copper size.

Material grades matter here too: quality aluminium lugs use 1070-grade aluminium (99.7% purity, annealed), and the copper palms of bimetallic lugs use T2/T3 electrolytic copper. Purity is not a luxury — lower-grade aluminium alloys have both lower conductivity and worse crimp behaviour.

Palm Hole and Bolt Rules

The palm end has its own sizing discipline:

• Hole clearance is +0.5 mm nominal. An M12 bolt uses a 12.5 mm hole. More clearance shifts the lug under short-circuit forces; less makes assembly a fight.

• Don't upsize holes to "standardise" bolts. A 16.5 mm hole under an M12 bolt head leaves a ring of unclamped palm; contact pressure concentrates unevenly and resistance rises. If stock rationalisation matters, standardise the stud sizes at the design stage instead.

• Double-hole palms (DTL-2 type) exist for a reason. Long palms on high-current terminations rotate under electromagnetic force during faults. Two bolts prevent rotation and double the clamped area. Utility specifications commonly require double-hole lugs above 300 A on flat busbar — check yours.

• Washer stack order: bolt head, spring or conical washer, flat washer, lug palm, busbar. The flat washer spreads the load; the conical washer maintains pressure through thermal cycling.

Five Field Errors and Their Fixes

1. Sizing by cable OD instead of conductor mm². Insulation thickness varies by brand and voltage class. Always read the conductor marking. Fix: keep a strand-count/mm² reference card in the tool kit.

2. Ignoring the die code. Each barrel size pairs with a specific hex die (often stamped on the lug). Using the "closest" die under- or over-compresses. Fix: crimp charts taped inside the tool case lid.

3. One crimp where three are specified. Larger barrels need multiple crimps in marked positions, starting from the palm side. A single centre crimp on an SC-240 leaves half the barrel loose. Fix: count the die position marks; crimp them all.

4. Cutting strands to fit a small barrel. Removing strands to force a conductor into an undersized lug reduces cross-section exactly at the highest-resistance point. This one shows up in thermal surveys within months. Fix: order the correct bore; flexible-conductor lugs exist.

5. Forgetting inspection windows. Quality tube lugs have a sight hole showing full conductor insertion. If the copper isn't visible in the window, the crimp zone is partly empty. Fix: make window check part of the QA sign-off.

A crimped joint done right needs no maintenance for decades. Every one of the errors above converts a maintenance-free joint into a scheduled inspection item.

Verification: The Twenty-Minute Batch Check

When qualifying a new lug batch or a new crimp tool, we recommend the check we run in-house:

Crimp three samples on scrap conductor of the correct class.

Pull test one — IEC 61238-1 class A joints should hold specified tension without slip.

Section one — the cut face should show near-void-free compaction between strands and barrel.

Measure resistance across one — compare against an equal length of uncut conductor; a good joint measures below 1.1× the reference.

Twenty minutes of bench work validates thousands of production joints. Cheap insurance.

Sizing for Special Cases

Three situations fall outside the standard chart and deserve their own rules:

Parallel conductors per phase. Large feeders often run two or more cables per phase. Each conductor gets its own correctly sized lug — never two conductors in one barrel, which cannot compact properly around both. Check the terminal pad length accommodates the stacked palms, and keep phase conductor lengths equal so current shares evenly; unequal sharing overloads one lug of the pair.

Reduced-neutral circuits. Four-core cables with a half-size neutral (e.g. 240/120 mm²) need two lug sizes per termination point. Schedule them as separate line items — the most common site shortage we see is the odd-size neutral lug, because someone ordered "four per termination" of one size.

Short-barrel and long-barrel variants. Where gland-to-terminal distance is tight, short-barrel lugs save 15–25 mm of straight length but take fewer crimps and hold less tension. They are legitimate components with their own test coverage — just not silent substitutes for the standard barrel on drawings that assume full pull-out strength.

Frequently Asked Questions

What size lug do I need for a 95 mm² cable?

A 95 mm² Class 2 copper conductor takes an SC-95 or DT-95 copper lug; 95 mm² aluminium takes a DL/AU-95; aluminium conductor onto a copper terminal takes a DTL-95 bimetallic. Then choose the hole to match the stud — typically M10 or M12 at this size. If the conductor is flexible Class 5, confirm the flexible-bore variant before ordering.

Can I use a lug one size bigger than the conductor?

No. An oversized barrel never reaches full compaction, no matter how many crimps you add, and the joint fails the resistance and pull-out criteria the size pairing was tested to. If the conductor falls between sizes (common with imperial-metric conversions), use the manufacturer's cross-reference table rather than rounding up.

Do hole sizes affect current rating?

Indirectly, yes. The rating assumes full palm contact under correct bolt pressure. The right hole with the right bolt and washer stack delivers that; an oversized hole reduces clamped area and concentrates pressure. Where two hole options exist for one lug size, either is electrically fine — provided the bolt matches the hole.

How do I size lugs for busbar connections rather than equipment studs?

Same conductor logic, but check palm width against the bar. The palm should not overhang the busbar edge; on narrow bars this sometimes forces the narrow-palm variant of the same conductor size. For high-current flat-bar work above roughly 300 A, prefer the double-hole (DTL-2 style) palm to resist rotation under fault forces.

Wrapping Up

Lug sizing is three numbers — conductor mm², strand class, stud diameter — plus the discipline to check barrel bore, die code and crimp count against the datasheet. The 10–630 mm² chart above covers the bulk of LV and MV work; the exceptions (flexible conductors, aluminium transitions, double-hole palms) follow the rules outlined in each section.

If you are preparing a cable schedule and want it cross-checked, MINGXU's team does this routinely for export projects: send the conductor list with sizes, classes and terminal details, and we return a lug schedule with exact part numbers, die codes and IEC 61238-1 documentation — covering copper SC/DT, aluminium DL/AU and bimetallic DTL/ACL series from 10 to 630 mm². It costs nothing to have a second set of eyes on the schedule before the order is cut.