I have an ISSPro pyrometer installed in the turbo inlet; the thermocouple is connected to the gage by about 26' of #14 stranded.

Because the ISSPro is designed to be on the turbo outlet (the thermocouple is brass), I've gotten a Stewart Warner set with a stainless steel thermocouple. I intended to get the SW hookup wires, but its longest is 20', and they offer no alternatives.

But since the ensemble is (I think) a voltage device, there should be nothing critical about wire length since there's no appreciable voltage drop due to the very minimal current. Neither ISSPro nor SW have provided me with the wire (i.e., resistance) specs, but I would expect the set to work properly with standard wire.

Any counterpoint or suggestion would be welcomed!

"...very minimal current."
#14 seems very large (size) for minimal current??? I know nothing about this situation, but my electrical rule of thumb is: when lengthening an original wire, go up at least one size - in this case, #12 gauge. Is the wire simply telling the gauge the temp?
Or is the amp draw in the wire moving the temp gauge?? Or is the resistance (heat) in the wire moving the temp gauge? If so, wire size/length may affect the gauge reading.
(I say again: I know nothing about this installation)

A thermocouple generates a small electrical potential (voltage) from heat applied to a bimetallic probe, which varies with temperature. There's one in every propane-fuelled device in the motorhome, which shuts off the gas if the flame goes out.

The gage is a very sensitive voltmeter, calibrated to read out in temperature rather than volts. Voltage devices are those that respond to the voltage in the signal - like a telephone, a modem, phono input on an audio amplifier - but draw little or no current. I used #14 stranded because that's what I had in stock.

What I find odd is that ISSPro and SW both sell "leads" (hookup wires) for their pyrometers for $50 or more, yet I can find no rationale for that price.

But I'd like to know if I'm overlooking something...

The reason for their "leads" is the wire is of special material that will not generate any voltage at a by-metalic junction. The wire has to be of the same material composition as the leads coming out of the thermocouple or there will be induced voltage where the junction of the extension wire is. Almost all thermocouple require compatible wire or a compensating junction if not, the accuracy will be compromised.

I don't remember but I think lead wire is Nichrome or similar composition.

Thanks, Ed, that would explain the issue (although the ISSPro leads are plain old copper if I'm translating the very sparse instructions correctly). However, that would be odd in that the leads, being electrically isolated and on complementary conductors, the net voltage shouldn't be affected. Nichrome would make sense (and it's very expensive).

Banks does have pyrometers with longer leads - but I don't need 55'! I may be forced into that, as neither ISSPro nor SW is being very cooperative.

I ended up getting a VDO ensemble as they have a 30' lead.

In discussions with the distributor, he got One Who Knows on the line, and he said pyrometer leads are indeed a special wire of some kind - VDO uses a proprietary name for theirs.