I want to know what is the maximum length (in feet or meters) for instrument cable (4-20mA signals) between Control room and field instruments in temperatures up to 50oC? Cables in consideration are Instrument single-pair cable of 2.5mm2 core area AND Multi-pair cable with 1.5mm2 core area. Cables are armored and shielded.

I have searched through various standards but there is no reference available for maximum possible length (e.g. 2000 ft., etc.) for loops.

Although, cable data-sheets have conductor resistance values at 20oC from where voltage drops can be determined by using ohms law (V=IR), but conductor resistance changes with rise in temperature. Hence the theoretical values are no more true. A typical field transmitter can except voltage level as low as 21 Vdc.

Can somebody please guide me from standard reference or practical experience about maximum distance of these cables?

 

Sashamsi,

You can easily calculate the maximum distance if you look at your field instrument spec.
For example - maximum loop resistance at 24 VDC = 600 Ohms.
Subtract the resistance of your control system e.g. 250 Ohms.
Therefore you have 350 Ohms to spare.

Look up the resistance table for the cable in question (multiply by 2 for a pair).
My table says 1.1073 Ohms per 100m for 1.5 mm (2.2146 for pair), 0.676 Ohms per 100m for 2.5 mm (1.352 for pair).
So for 1.5 mm you can have 350/2.2146 x 100 m or 15.8 km. This is not allowing for additional resistance due to elevated temperature. In practice you would probably never go that far because the cost of cable would be much more than a simple wireless link, but I have seen about 10 km to a remote pump station via telephone wires.

As you pointed out the voltage at the transmitter drops with increased resistance but that's the beauty of 4-20 mA. All you need is enough voltage to power the instrument. Again you can work that out from the specs 600 Ohms x 20 mA = 12 V that you have to lose.

It's also possible to increase the distance by increasing the supply voltage, but I don't think you will need to do that.

Note: If you have 2 loads, e.g. a PLC and a chart recorder, that would be 500 Ohms allowing only 100 for cable resistance.

Regards,
Roy

 

Oops,

I should have been a bit more careful, not as familiar with metric sizes 2.5 & 1.5, they are for power aren't they?

For a typical transmitter load 600 Ohm @ 24VDC
600 - 250 (control system load) = 350 for cable
350 Ohms in a couple of common AWG sizes.

For 22 AWG 16.2 Ohm/1000ft/wire
350/(16.2 x 2) = 10,800 ft

For 18 AWG 6.39 Ohm/1000ft/wire
350/(6.39 x 2) = 27,386 ft

The resistance change from 20¡ãC to 50¡ãC is quite insignificant as it's related back to 0¡ãK.

R = Rref [1 + 0.004041 (50 - 20)]

So 22 AWG goes from 16.2 to 18.16 Ohms/1000ft/wire

Tip: If you are considering buried cable look at telephone twisted pair, it's a fraction of the cost. Alt 0176 for ¡ã.

Regards,
Roy

 

Hello Roy,

Very clear your explanation. How is maximum distance of cable affected if you have a hart transmitter? And what about if you have an intrinsic safety barrier?

Regards,

AC.