Sizing conductors of shielded wire for 4-20 loops?

[LWFB]

How do you determine the shielded wire size for a control or monitor loop for 4-20 signals. I have seen my company commonly use 18 ga in the field on temperature equipment for signals, and for short runs 22 ga but I need to determine maximum runs for each sized based on something other than voodoo.

thanks in advance for your responses

 

[Barry1961]

There may be factors besides the ampacity, resistance or impedance per foot for choosing a particular size of cable. Some of the factors could be standard cord grip range, common hand tool range, termination type/range at device, size range on intermediate plugs/pins, skill of people installing/repairing, minimize stocked cable types, what type the local distributor stocks, what the regional warehouse stocks, what types/sizes are available with UV resistant outer jackets, quality of shielding, resistance to ground and cost.

But if none of the real life factors are a concern you should be able to calculate the maximum operating distance if you know the resistance per foot, output impedance, input impedance and required range of input. I am guessing your frequency will not be a factor.

Barry1961

 

[dbaird]

Ohms Law: V=IR
On a 24 VDC circuit you don't want the voltage to drop more then 5-10% (21.6-22.8V at load).

Use the following online calculator:

http://www.currentsolutions.com/knowledge/vdrop.htm

David Baird

Sr Controls Designer
EET degree.
Journeyman Electrician.

 

[Walden]

I like the sound of 18 AWG! Over the years I have discovered, on site, just how much time it takes to strip the insulation off small cables without breaking the strands and then trying to get a cable ferrule to fit over the strands and get a good connection into the terminal. Life is a lot easier with 18 AWG!

 

[dpc]

We use #16, especially for single pair cable, and sometimes #18 for multi-pair cable.

 

[aolalde]

I like mpyke practical reason.

 

[HCBFlash]

You can't get appropriate cable from Belden, Alpha, probably others larger than 16ga last I checked.

Personally, I like 16 better than 18 in certain locations, but it's almost always too big for the terminals provided. Anything other than 18 will have somebody cursing you when they're installing, terminating, maintaining, or even removing it.

If distance is so great that loop resistance is an issue, it will be less of a concern than signal interferece and impedance characteristics that can't be calculated in advance.
,
,
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remember: An opinion is only as good as the one who gives it!

 

[sreid]

Also,the present Safety Regs (UL and CE) specify a mimimum conductor size of 18 ga even for control wiring (less likely to fuse or melt the insulation before the CB trips?).

 

[Compositepro]

Every 4-20 instrument will have a maximum allowable loop resistance for a given supply voltage. There will also be a maximum and minimum required supply voltage. Check the specs. of your instruments.

 

[waross]

To expand on Compositepro's advice;
I have had to do a few calculations for multiple instruments on a loop when we were exceeding the maximum voltage drop.
Each instrument will have a maximum voltage drop. If you can't find a figure, put the instrument on a calibrator inject 20 m.a. and measure the voltage across the instrument with a voltmeter.
Add the voltage drops of the instruments on the loop. Subtract this amount from the power supply voltage and that's how much voltage you have left for line loss. Use this value of voltage (The difference) and 20 m.a. to calculate the allowable resistance for the cabling.
Example;
Alarm relay 6 volts;
Transmitter 8 volts;
Controller 8 volts;
Total volts 22 volts.
Power supply 24 volts.
Available volts for voltage drop 2 volts.
Allowable loop resistance 100 Ohms
depending on what table I use that is between 12,000 ft. and 15,000 feet of #18 copper wire.
More important than the wire size is the sum of the voltage drops of the instruments on the loop.
Personally, I would use a bigger safety factor than 2 volts on a 24 volt power supply.
We had a runaway in an incinerator. The plant engineer thought that we should have got a signale from our high temperature alarm before the stainless steel thermo-wells melted.
He asked me to find out why.
It turned out that the loop was engineered properly, but that the alarm relay was way off spec. The voltage supply should have had enough voltage for the loop, but with the alarm relay having more resistance than it was listed at, the power supply did not have enough voltage to drive the current to 20 milliamps. The high temperature alarm setpoint was quite high, and regardless of the actual temperature, the loop was incapable of developing 20 m.a. or enough current to reach the setpoint.
The alarm relay was modified in the shop. When the input resistance was back on spec. it was recalibrated and put back in service. The loop was checked and was now capable of developing the full 20 m.a.
Subsequent runaways and alarms and shutdowns at the proper temperatures proved the repairs.
The points.
1> There is no fixed maximum length for a given size of 4-20 m.a. loop wire.
2> The sum of the device voltage drops is more important than the wire length.
3> If the sum of your device voltages is aproaching the power supply voltage don't hesitate to put the devices on the calibrator and check the actual voltage at 20 m.a. Not a bad idea before replacing devices also.
Hope this helps.