# Random thoughts: 4-20mA loops



## Jlarson (Jun 28, 2009)

I'm sitting here designing a panel with a bunch of 2 wire 4-20 stuff going on and thought I'd share my usual methods.

For cable I usually use 18 or 16 gauge shielded tray cable (TC-ER)

Each loop gets a 100 ma fuse, this way if a transmitter fails shorted it doesn't take out the whole DC power supply. 

Each loop also gets a surge protector in the panel, and in some cases a full isolator. Depending on the conditions each transmitter might get local surge protector, they make packages that thread right into a 1/2 hub so its easy.

For transmitters with M12 connections I usually make my own cables with a field installable M12 connector ( I use ones made by tyco electronics)

If I have a Jbox near the transmitter (lots of times I'll have a box to pick up the M12 cable and go to conduit) I use faston terminals so I can break the loop to put my loop calibrator in. 

All I got for now


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## lefleuron (May 22, 2010)

What are we using this for? And what kind of environment?

Give us some nitty gritty details!

We find a lot of 18 Ga. 6 wire gets used for this, I really believe this is a bad idea. I think pinning over 4 of the conductors is just making a bigger antenna.


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## Jlarson (Jun 28, 2009)

lefleuron said:


> What are we using this for? And what kind of environment?
> 
> Give us some nitty gritty details!
> 
> We find a lot of 18 Ga. 6 wire gets used for this, I really believe this is a bad idea. I think pinning over 4 of the conductors is just making a bigger antenna.


All that is pretty much universal no matter what environment I'm working in, usually all that changes is the transmitters and somtimes there's an IS barrier in the panel if its a hazardous location.


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## Big John (May 23, 2010)

Larson, do you have a Fluke 771? I've been eyeballing one those for a while. I bought a cheap-ass adapter for a multimeter hoping it would have just enough accuracy to get by, but no dice.

Haven't quite convinced myself to throw down the $500 though.

-John


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## Jlarson (Jun 28, 2009)

Big John said:


> Larson, do you have a Fluke 771?


No, I budgeted some 773's for the next quarter probably.


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## HARRY304E (Sep 15, 2010)

Jlarson said:


> No, I budgeted some 773's for the next quarter probably.


 Make sure you post the ones that you throw away..:laughing:


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## micromind (Aug 11, 2007)

Jlarson said:


> I'm sitting here designing a panel with a bunch of 2 wire 4-20 stuff going on and thought I'd share my usual methods.
> 
> For cable I usually use 18 or 16 gauge shielded tray cable (TC-ER)
> 
> ...


I do it pretty much the same way. I have no problem using a multi-conductor cable from the control panel out to J-boxes (if there are any), and a terminal strip. I've used 20 gauge a lot; even 200' out. I've used quite a bit of 20/12 and 20/16. (6 pair and 8 pair). There's an overall shield, but the individual pairs are not individually shielded. 

4-20 is a pretty tough signal, in my experience it handles a lack of shielding well. Voltage signals on the other hand.......

I have a Fluke 771. Love it! Had it a little more than a year. No problems so far..... You need a bit of slack in the wire though, the hole in the jaw is a ways away from the edge of the jaw. But it sure is nice to not have to break the circuit in order to get a current reading. 

The jaw part can be removed from the rest of the meter, it has a 2' cord. I find I have to hit the 'zero' button quite a bit. It'll stay accurate while clamped around a conductor, but if you go to other conductors it doesn't always drop to 0 ma. 

It uses 2-AA batteries, they last a long time.


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## Jlarson (Jun 28, 2009)

Some stuff gets 20 gauge PLTC or ITC but more often then not I need something with a 600 volt rating so I go with TC. 


I can't wait till we get out first 773, I really need a new loop calibrator on my truck.


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## micromind (Aug 11, 2007)

Jlarson said:


> Some stuff gets 20 gauge PLTC or ITC but more often then not I need something with a 600 volt rating so I go with TC.
> 
> 
> I can't wait till we get out first 773, I really need a new loop calibrator on my truck.


My process calibrator is an Extech, I don't remember the model. It works well though.

The best thing about having something that can output a 4-20 is when someone else does the startup and something doesn't work, they ALWAYS blame me for it. Always. 

When they do, I just hook up the calibrator and prove that my stuff is good. 

Takes the wind right out of their sails!!

It's also good to be able to program loop powered instruments while they're easy to get to.


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