Thermocouple Color Codes | ANSI/ASTM Wire Chart

Thermocouple color code reference for replacement and quoting

Thermocouple color codes help identify the sensor type, polarity, and extension wiring before a replacement probe is built. In the common ANSI/ASTM convention shown below, the negative conductor is red, and the positive conductor color identifies the thermocouple type.

Use this page to verify Type K, J, T, E, N, R, S, B, and legacy Type C color markings. Confirm the installed wiring standard before manufacturing because IEC, BS, DIN, JIS, and older plant wiring conventions can differ from ANSI/ASTM colors.

Fast replacement checks

Red conductor is negative in ANSI/ASTM
Positive conductor color identifies the type
Extension-grade jacket color can differ from thermocouple-grade jacket color
Send photos if the installed wiring standard is uncertain

Polarity check: in this ANSI/ASTM reference, red is negative. Do not assume red is positive when replacing an installed thermocouple.

ANSI/ASTM Thermocouple Color Codes

The table below converts the legacy Thermometrics color-code chart and imported color-code illustrations into searchable text. Type C is included because it appears in the legacy Thermometrics chart; confirm the exact tungsten-rhenium specification and plant convention before manufacturing.

Type	Positive conductor	Negative conductor	Positive color	Negative color	Extension-grade jacket	Thermocouple-grade jacket shown in legacy assets
T	Copper	Constantan	Blue	Red	Blue	Brown
J	Iron	Constantan	White	Red	Black	Brown
E	Chromel	Constantan	Purple	Red	Purple	Brown
K	Chromel	Alumel	Yellow	Red	Yellow	Brown
N	Nicrosil	Nisil	Orange	Red	Orange	Brown
S	Platinum 10% rhodium	Pure platinum	Black	Red	Green	None established in legacy asset
R	Platinum 13% rhodium	Pure platinum	Black	Red	Green	None established in legacy asset
B	Platinum 30% rhodium	Platinum 6% rhodium	Gray	Red	Gray	None established in legacy asset
C	Tungsten 5% rhenium	Tungsten 26% rhenium	Green	Red	Red	Unjacketed conductors shown in legacy asset

Legacy Thermometrics color-code chart

The original chart is preserved here as a visual reference. The table above converts the same color-code data into searchable text for buyers, engineers, and crawlers.

For replacement work, send photos of the probe, connector, terminal strip, and wire jacket along with the operating temperature range. That gives engineering enough context to confirm type, polarity, lead wire, and insulation.

Thermometrics thermocouple color code chart for Types T, J, E, K, N, S, R, B, and C

Thermocouple-grade vs extension-grade color

Thermocouple-grade wire uses the actual thermocouple alloy pair in the measuring circuit. Extension-grade wire is matched for lower-temperature signal extension and often uses the type-specific jacket color shown in the table.

What to include with a replacement RFQ

Include the thermocouple type if known, wire colors, jacket markings, probe diameter, insertion length, lead length, connector style, junction style, and maximum operating temperature. Photos are helpful when the installed standard is uncertain.