## 4-20mA Distance limits

## 4-20mA Distance limits

(OP)

I need to send the speed reference from Drive A to Drive B over a distance of 800 Meters. Application is not very critical from the speed accuracy point of view. A +/- 10 RPM is good enough.

Option A: Use a field bus option but length is an issue. I am given to understand that I need to use Fiber Optic Cables to over come the distance, which add to the cost.

Option B: 4-20mA signal. I need a converter at both ends to convert the 0-10V Signal of Drive A to 4-20mA and 4-20mA back to 0- 10 V at Drive B since the drives can deliver/accept only Volts. The cable proposed to be used will be shielded one.

What is the maximum distance permissible with a 4-20mA option? Would I face any loss of signal over this distance?

Thanks!

Option A: Use a field bus option but length is an issue. I am given to understand that I need to use Fiber Optic Cables to over come the distance, which add to the cost.

Option B: 4-20mA signal. I need a converter at both ends to convert the 0-10V Signal of Drive A to 4-20mA and 4-20mA back to 0- 10 V at Drive B since the drives can deliver/accept only Volts. The cable proposed to be used will be shielded one.

What is the maximum distance permissible with a 4-20mA option? Would I face any loss of signal over this distance?

Thanks!

## RE: 4-20mA Distance limits

If memory serves me right (it's been awhile since I worked on those), somewhere around 500 ohms was typical. That would include the connecting cable both ways and the receiving unit(s) input impedance (could be more than 1 if you string several devices in series on the same loop).

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## RE: 4-20mA Distance limits

Most drives are fairly limited on the voltage they can deliver to drive current around the loop, but since you are using a pair of V/I converters you may be able to get one with a high compliance voltage. You need to apply Ohms Law and see what size conductor you end up with. Don't forget that there is 1600m of conductor in the loop. A fibre probably isn't going to be much different in terms of installed cost than a standard cable - installation labour is often the dominant cost with smaller cables and low core-count fibres.

Can you get a wireless Fieldbus solution? Consider what happens if (when) the link is lost - is there a safety issue?

Also consider a voltage/frequency converter. The signal should survive ok over that distance using little more than a telephone cable.

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If we learn from our mistakes I'm getting a great education!

## RE: 4-20mA Distance limits

Voltage/Frequency seems to be a workable option.

Let me discuss this with my client.

Thanks!

## RE: 4-20mA Distance limits

A 4-20ma circuit has a voltage source transducers and wire resistance. Each transducer will drop some voltage. From the voltage of the power supply, subtract the voltage demands of each transducer at 20ma. The voltage left will be the voltage available to overcome wire resistance. Use Ohm's law to find the maximum length. Lower resistance wire means longer circuits. Don't try to use 100% of the allowable resistance. The voltage ratings of the transducers may be subject to tolerances. I once had a high temp shutdown fail to operate. The loop was loaded to almost 100% of nominal capacity. The input resistor of one of the devices was 15% or 20% off spec. Given the real resistance rather than the nominal resistance, the power supply was unable to push 20 ma through the loop.

The high temp shutdown failed to operate.

Moral, test loop resistance and don't use 100% of the allowable loop resistance. The loop may fail if a device is replaced with an off spec device.

Bill

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"Why not the best?"

Jimmy Carter

## RE: 4-20mA Distance limits

I have run 4-20 mA loops over several miles. I think we used #10 AWG.

## RE: 4-20mA Distance limits

Keith Cress

kcress - http://www.flaminsystems.com

## RE: 4-20mA Distance limits

One other thing. Take away what the guidelines say about maximum lengths of 4-20mA and look at the installation quality of your VFD's. A VFD, as well as controlling the speed of your motor, creates high levels of RFI. If the installation is poor-and by that I mean the ability of the high frequency interference to find a low impedance path to ground, control cables are not sat right next to motor cables, cables cross at 90Deg etc.

If these and other recommendations provided by the VFD supplier are not adhered to then 8 or 80 metres is a challenge.

## RE: 4-20mA Distance limits

Line of sight is not absolutely essential with full 1 watt ISM band 900Mhz freq hopping (US situation) wireless. If there's some structural steel with openings (as opposed to solid steel walled buildings in the direct path) to bounce the signal, then 800m could very well be doable with wired in/wired out wireless.

Dan

## RE: 4-20mA Distance limits

Steve

## RE: 4-20mA Distance limits

5 miles each way = 10 miles round trip = 50,000 ft.

For 18AWG use your number 37 ohms/1000 ft.

Resistance = 37 ohms/1000ft * 50,000 ft = 1850 ohms.

Now, we expect the circuit to carry 20 milliamps.

V = IR = 0.020A * 1850Ohms = 37 volts used by the wire alone.

A 0-1V input device has input impedance of 1V/20milliamps = 50 ohms. A 0-5V input device has input impedance of 5V/20milliamps = 250 ohms. (1850 ohms doesn't sound trivial compared to 250, does it?)

We need 1-5 voltage for just a single receiving device. Let's say 5 volts.

And we need approx 12vdc left accross the transducer for it to operate correctly.

Power supply requirement is 12vdc + 5VDC + 37VDC = 55VDC.

Typical power supply might be 24vdc. What happens when you use a 24vdc power supply to drive the above system? You couldn't even push the current through the wire alone, let alone leave enough voltage for the transducer to operate correctly.

But , let's assume there is a special power supply, even though you haven't said a special power supply is needed for these long distances. Give it the full 55vdc power supply needed to power the full 20 milliamps through all that resistance.

Now what happens when we reduce the signal to 4 milliamps? The (37 + 5) = 42 drops to 42*(4/20) = 8.5. The voltage accross the transducer is now 55 - 8.5 = 46.5VDC.

What happens when you put 46.5VDC on a transducer?

http://celesco.com/faq/420.htm

I think you'll need a special transducer to go along with your special power supply.

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## RE: 4-20mA Distance limits

Steve

## RE: 4-20mA Distance limits

My apologies.

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## RE: 4-20mA Distance limits

I am happy to overlook the misunderstanding that prompted your reply.

Bill

--------------------

"Why not the best?"

Jimmy Carter

## RE: 4-20mA Distance limits

^{2}LV power cable because he needs to run an especially long loop: it would work ok but the cost would be outrageous. Just because something which works in theory doesn't make it a good solution.----------------------------------

If we learn from our mistakes I'm getting a great education!

## RE: 4-20mA Distance limits

Steve

## RE: 4-20mA Distance limits

I recreated now where my approx 500 ohm number from my very first post came from. 24vdc power supply. 12vdc transducer minimum leaves 12vdc for the rest of the circuit. 12vdc/0.02A = 600 ohms. Round down to the nearest round number(500) as correction factor to account for engineer's fading memory.

Let's go back to the original post:

I assume 24 volt power supply in drive A

Drive B will take 10 volt at 20 milliamp, leaving 14 volts for the cable and the transducer A.

I assume transducer A requires around 12 volts, and that leaves 2 volts for cable.

The calculations provided above 1600m total ft of 18AWG at conservatively elevated temperature gives approx 40 ohms or 0.8 volts

So, of 2 volts available, the cable voltage drop takes up 0.8 volts and we still have some margin. If we tripled the length of the cable, we would be in a situation where we would need to start looking at bigger cable or higher voltage supply.

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## RE: 4-20mA Distance limits

Steve

## RE: 4-20mA Distance limits

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## RE: 4-20mA Distance limits

## RE: 4-20mA Distance limits

In order to use a current loop

At Drive A you will need a 4 wire 0-10/0-20 transducer. Either way you will only loose a few volts e.g. 5 (not 12 as you would with a 2 wire transmitter) so assuming you just have a 500 Ohm resistor at B you have 9 V for line loss.

If you use a loop powered 0-10/4-20 transducer at A you would also need a transducer at B but this would be 250 Ohms so now you have 14 Volts for line loss. But now you can also use a higher power supply voltage e.g. 48 V giving you 38 for line losses.

Not all transducers are 250 Ohms some are 50 Ohm or less.

Another option would be fiber optics thus eliminating any noise problem and need for conduit.

http:

Roy

## RE: 4-20mA Distance limits

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## RE: 4-20mA Distance limits

## RE: 4-20mA Distance limits

"800 meters should be no problem, #18 wire has a resistance of less than 7 ohms per 1000 feet, # 16 has less than 5 per 1000"

Steve

## RE: 4-20mA Distance limits

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