Thermocouple Insulator Sizes by Wire Diameter (mm)

The bore has to be bigger than the wire, with room for the bore tolerance. Our standard twin-bore insulators have bores from 0.5 to 1.8 mm, four-bore from 0.4 to 1.6 mm, which takes noble-metal wire (0.35 and 0.5 mm) and, in twin-bore, base-metal wire up to 1.38 mm with at least 0.3 mm to spare. For 2 or 3 mm base-metal wire you need a single-bore tube per leg or a larger insulator made to drawing.

The table below takes the wire sizes WIKA, Rössel and ASTM list, plus three that buyers named on our own orders this year, and gives the smallest standard size of ours each one passes through.

Insulator size by wire diameter

Smallest GY Alumina standard insulator for each thermocouple wire diameter
Wire Ø (mm)	Where this wire size comes from	Twin-bore, OD × bore	Four-bore, OD × bore	Single-bore, OD × bore
0.25	30 AWG	3×0.8 (0.55)	3×0.6 (0.35)	2×1 (0.75)
0.33	28 AWG	3×0.8 (0.47)	4×1 (0.67)	2×1 (0.67)
0.35	S, R, B (WIKA, Rössel)	3×0.8 (0.45)	4×1 (0.65)	2×1 (0.65)
0.5	S, R, B (WIKA, Rössel)	3×0.8 (0.3)	4×1 (0.5)	2×1 (0.5)
0.51	24 AWG	4×1 (0.49)	4×1 (0.49)	2×1 (0.49)
0.6	Named on our 2026 orders	4×1 (0.4)	4×1 (0.4)	2×1 (0.4)
0.8	Named on our 2026 orders	5×1.2 (0.4)	5×1.2 (0.4)	3×2 (1.2)
0.81	20 AWG	5×1.2 (0.39)	5×1.2 (0.39)	3×2 (1.19)
0.9	Named on our 2026 orders	5×1.2 (0.3)	5×1.2 (0.3)	3×2 (1.1)
1	K, N, J (WIKA); base metal (Rössel)	6×1.4 (0.4)	6×1.3 (0.3)	3×2 (1)
1.38	Base metal (Rössel)	8×1.8 (0.42)	8×1.6 (0.22, tight)	3×2 (0.62)
1.63	14 AWG	8×1.8 (0.17, tight)	–	3×2 (0.37)
2	Base metal (Rössel)	–	–	4×2.5 (0.5)
3	K, N, J (WIKA); base metal (Rössel)	–	–	6×4 (1)
3.25	8 AWG	–	–	6×4 (0.75)

All sizes in mm, nominal. Each cell is the smallest of our standard sizes whose bore is at least 0.3 mm bigger than the wire; the figure in brackets is that gap (bore minus wire). "Tight" means no standard size leaves 0.3 mm and the one shown leaves less. We used 0.3 mm because it is the largest bore tolerance written on our 2026 orders (±0.15 to ±0.3 mm); it is a starting point for this table, not a factory rule.

Where a cell says tight, a bore at the low end of its tolerance could grip the wire. Either go one size up, or write the wire on the order and let us check it. Some buyers already do: instead of a bore tolerance, their 2026 orders named the wire that has to pass through the bores (0.6, 0.8, 0.9 mm). If fit is what you care about, that is the clearer way to write it.

For wire thicker than 1.8 mm the standard list only has single-bore tubes, one per leg. Larger twin-bore insulators, other layouts up to 7 bores and flat twin-bore insulators are made to drawing.

AWG sizes in millimetres

ASTM E230, the US thermocouple standard, sizes wire by AWG gauge. It lists six gauges in its table of suggested upper temperature limits for protected thermocouples (in closed-end protection tubes). Thinner wire gets a lower limit: for types K and N, 1,260 °C at 8 AWG against 870 °C at 24 AWG.

AWG thermocouple wire sizes in mm, with the ASTM E230 suggested upper limit for types K and N
AWG	Wire Ø (mm)	K and N, suggested upper limit (°C)
8	3.25	1260
14	1.63	1090
20	0.81	980
24	0.51	870
28	0.33	870
30	0.25	760

ASTM E230, as reproduced on Tempco catalog page 14-103. ASTM gives these limits as a guide, not as absolute values or a promise of service life.

So the temperature you need decides the wire, and the wire decides the bore. A type K probe in 8 AWG wire (3.25 mm) is past our standard twin-bore list; the same probe in 20 AWG (0.81 mm) fits our twin-bore 5×1.2 with 0.39 mm to spare.

Which grade for the insulator

Rössel's handbook puts base-metal thermocouples in C 610 insulating rods and noble-metal ones in C 799. JUMO goes further for platinum thermocouples: both the insulating rod and the protection tube must be its KER 710 grade, because in a reducing atmosphere as little as 0.2% SiO₂ in the ceramic can form brittle platinum silicides. What the class numbers mean is in C 799, C 610 and C 530.

We quote insulators by Al₂O₃ content: 99.5% (C799-class) as standard, or 99.7%, 99% or 95%. Tell us the thermocouple type and the quotation confirms the grade.

Our standard insulator sizes and the tubes they fit

Every size below has its own page with a drawing. The last column is the smallest of our standard protection tubes the insulator goes into, with at least 1 mm between insulator OD and tube ID.

GY Alumina standard insulator sizes and the smallest standard protection tube each fits
Bores	OD × bore (mm)	Fits our protection tube
1	2×1	6×4
1	3×2	6×4
1	4×2.5	8×5
1	5×3	10×6
1	6×4	10×7
1	8×5	15×10
1	10×4	15×11
1	12×7	20×15
2	2×0.5	6×4
2	3×0.8	6×4
2	4×1	8×5
2	5×1.2	10×6
2	6×1.4	10×7
2	8×1.8	15×10
4	2×0.4	6×4
4	3×0.6	6×4
4	4×1	8×5
4	5×1.2	10×6
4	6×1.3	10×7
4	8×1.6	15×10

Nominal sizes. Standard sizes are usually in stock; lengths are cut to order, from 10 to 4000 mm. On 2026 orders, multi-bore insulators were toleranced ±0.2 to ±0.5 mm on the OD and ±0.15 to ±0.3 mm on the bore.

What to send for a quotation

Wire diameter and thermocouple type.
Number of bores, OD and length. If you don't have a size yet, the wire and the protection tube ID are enough to start.
The wire that must pass through the bores, or a bore tolerance.
Quantity and how often you reorder.

Standard sizes usually ship in about 3 days for small orders and about 7 days for bulk. Other sizes are made to drawing, usually in 25–30 days, from 100 pieces. See the insulator page for photos and bore layouts, or request a quote.

Sources

WIKA data sheet TE 65.80, thermocouples with ceramic protection tube
Rössel-Messtechnik, Praxis der Thermoelemente (German), Table 9 and insulating rods
JUMO data sheet 90.1000 (German), KER 710 and KER 610
Tempco catalog page 14-103: ASTM E230 suggested upper temperature limits for protected thermocouples, by wire gauge
Our own figures: the factory's standard-size list, 2026 orders and test report, as recorded in our product catalog.

Thermocouple insulator sizes: which bore for which wire diameter