4-20mA 0-10V

From what cable length should you prefer 4-20mA to 0-10 V (because of the voltage drop? Or is it more a mather that the current loop does not take electrical interference? What can the voltage drop be for long cables?

Thanks

it is more than voltage drop... to be precise, voltage drop is much BIGGER if you use current signal...

why? it's simple - voltage drop is proportional to current in the circuit and even 4mA (which is bottom of the range) is WAY MORE than any current you will see in 0-10V circuit (tipical impedance of 0-10V input channel is usually 6-digit number).

but the thing is that voltage drop in 4-20mA doesn't matter because current is the same in all parts of the circuit - unless voltage drop (circuit resistance) is so excessive that your sensor simply cannot bring signal to a 20mA level even if it tries. somple solution - do the math (or masure) and ensure that cable has conductors of sufficient size.

the bigger factor is noise imunity and current loops are less sensitive to noise (low circuit impedance).

now it also depends how much noise you have in particular enviroment etc.

since you are looking for some rule of thumb number, here are some from my experience:
current loops - pretty much any distance (I had many loops longer than 1km)
voltage inputs - in my case the longest ones reached some 50m (but i would prefer current if run is over 10m).

i never had issue with either of them. when the distance is concern, properly used (!) twisted pair cables with proper shield and 0.75-1.0 mm^2 or even larger conductors do the trick (usually pick bigger wire size just for some mechanical resistance for pulling etc.).

Good Info
Another question that relates to this.
Have a installation that we will be doing in China, and some of the 4-20ma signals will be about 30-40 feet away.
The China plant engineering people are asking what size wire we are planning for this.
In the best, I have always used Beldon 18awg shielded cable, but the sensors is usually only a few feet away from the panel.
They said they use 24 awg for all 4-20 ma wires.

Is there a certain quide or wiring code that deals with 4-20ma signals and conductor sizing, etc.
I did some searches on the web and this site, but nothing useful yet.

The limitation on wire size is wire resistance and how that contributes to the total loop resistance.

The question arises, how much resistance can the transmitter or analog output point (that produces the 4-20mA current signal) drive with a given power supply?

For analog outputs, it is usually clearly spec'd out on a spec sheet: 500 ohms or 600 ohms at 24Vdc,

For loop powered transmitters, there's a graph that shows power supply voltage vs. loop resistance.

The major contributor of loop resistance is the (internal or external) dropping resistor for the analog input, which is typically 250 ohms, but I've seen 1, 10, 25, 66.6, 100, 250 & 500 ohms used as dropping resistors.

If more than one analog input is on the current loop, then wire resistance can become an issue. In fact, whether two or more input devices will work is an issue itself, because of the combined resistances.

The wire resistance does play a role for very long wire runs, because even the low resistance of copper wire adds up.

I would not hesitate to use 24G wire for a 30 or 40 ft run.

Dan

They said they use 24 awg for all 4-20 ma wires.

Click to expand...

Irondesk, I am sure you know that 24 AWG is smaller wire than 18 AWG. Be sure to get a written statement from the customer saying that "24 AWG is approved to use for all 4-to-20 miliampere signals". If any signal loss (or wire breakage) problems arise later, pull out the statemenet and pass it around again.

irondesk40 said:

Good Info
Another question that relates to this.
Have a installation that we will be doing in China, and some of the 4-20ma signals will be about 30-40 feet away.
The China plant engineering people are asking what size wire we are planning for this.
In the best, I have always used Beldon 18awg shielded cable, but the sensors is usually only a few feet away from the panel.
They said they use 24 awg for all 4-20 ma wires.

Is there a certain quide or wiring code that deals with 4-20ma signals and conductor sizing, etc.
I did some searches on the web and this site, but nothing useful yet.

Click to expand...

I always try to stick with 18AWG or 20AWG Belden 2 wire shielded for all 4-20ma signals. Have had passive loops up to 100ft away with no issues and active loops up to 200 feet away with no issues.

It's OK to prefer a minimum gauge wire for "pulling wire" reasons.

But there's nothing wrong with 24 gauge wire in terms of sending or receiving the current signal.

Here's wire resistances:

Gauge-------Diameter (mm)-------Ohms/meter
18--------------1.024----------------------0.0204
20--------------0.8118--------------------0.0324
22--------------0.6439--------------------0.0515
24--------------0.5105--------------------0.0820

So over 40m, 24g wire has a resistance of 3.28 ohms x 2 (2 wires) or a total of 6.56 ohms, compared to the resistance of the 18g of 0.816 ohms x 2, or a total 1.632 ohms. Either total pales in comparison with the receiver load, typically 250 ohms.

It gets back to "How much will the current source drive?" 500 ohms? 600 ohms? Those are typical numbers in the industry. With a typical analog input burden of 250 ohms, that leaves lots of loop resistance for copper wire.

Dan

Maybe I am oversimplifying this, but I would think that voltage drop is irrelelevant in a 4-20ma circuit provided the source can supply enough voltage to power the loop.

I would have to agree with everything that DanW says:

"
It's OK to prefer a minimum gauge wire for "pulling wire" reasons.

But there's nothing wrong with 24 gauge wire in terms of sending or receiving the current signal.

Here's wire resistances:

Gauge-------Diameter (mm)-------Ohms/meter
18--------------1.024----------------------0.0204
20--------------0.8118--------------------0.0324
22--------------0.6439--------------------0.0515
24--------------0.5105--------------------0.0820

So over 40m, 24g wire has a resistance of 3.28 ohms x 2 (2 wires) or a total of 6.56 ohms, compared to the resistance of the 18g of 0.816 ohms x 2, or a total 1.632 ohms. Either total pales in comparison with the receiver load, typically 250 ohms.

It gets back to "How much will the current source drive?" 500 ohms? 600 ohms? Those are typical numbers in the industry. With a typical analog input burden of 250 ohms, that leaves lots of loop resistance for copper wire.

Dan"

I have run 4 - 20 loops over #24 single strand telephone wire for as much as much as 4 miles, which is about the limit. If you think of your loop transmitter as a constant current regulator, then the open circuit voltage is the ultimate limiting factor as to how far you can go. So if you start with 24 VDC open circuit and you want to push up to .020 Amps, 24 VDC / .020 = 1200 Ohms. So you must design a system that does not get near 1200 ohms, or you will run out of voltage at the high end of the control range. (It could become a 4 - 19 mA Loop)

Remember that your wire goes out and back and make certain of your available open circut voltage.

Check out the wire calculator at this link:

http://www.cirris.com/testing/resistance/wire.html


Best Regards,

Bob A.

Bob A. said:

I have run 4 - 20 loops over #24 single strand telephone wire for as much as much as 4 miles, which is about the limit. If you think of your loop transmitter as a constant current regulator, then the open circuit voltage is the ultimate limiting factor as to how far you can go. So if you start with 24 VDC open circuit and you want to push up to .020 Amps, 24 VDC / .020 = 1200 Ohms. So you must design a system that does not get near 1200 ohms, or you will run out of voltage at the high end of the control range. (It could become a 4 - 19 mA Loop)

Remember that your wire goes out and back and make certain of your available open circut voltage.

Check out the wire calculator at this link:

http://www.cirris.com/testing/resistance/wire.html


Best Regards,

Bob A.

Click to expand...

Great link
I have always used 18awg, but I double checked some of the papers that the Chinese engineers sent, and it did state that all analog wires be 24awg, next week will contact them and make sure that what they really mean, is that 24awg is the minimum they will accept.
Then as suggested, save all email to as the saying goes, " cover your a_*.

One additional thought, would be to acknowledge their attempts to be "thrifty" but counter with the thought that they would only have to break one wire sometime in the lifetime of the installation to make up for the difference in cost. Finish by putting the responsibility for this aspect squarely on them by disavowing any warranty responsibility for analog loops thus wired.