Correct Conductor Marking

OkiePC said:

How about getting the wire number from the sheet/line number of the drawings? If there are 200 pages of drawings and I find a wire labeled "+24VDC" how do I find where it originates in the drawings?

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+1 Paul.
This is pretty much the standard for me and most projects.
Regardless of the panel/system size.
Works with everything.
Especially larger installs/systems where "+24V" or whatever means absolutely nothing.
If it were unmarked it would be as much help as being labeled just "something".

OkiePC said:

How about getting the wire number from the sheet/line number of the drawings? If there are 200 pages of drawings and I find a wire labeled "+24VDC" how do I find where it originates in the drawings?

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If I see something like this then the drawing I am going to look for is the power distribution drawing. You should have a page that shows where the power is converted from line voltage thru a transformer to the DC power supply.

I am not condoning the practice but if I saw that label I know where I would start in the drawings.

You Get What You Ask For (Or Is It You Can't Get What You Don't Ask For?)...

Mihaylov said:

Hi
I am working for machine manufacturer and I am responsible for electrical diagram designing and PLC & HMI programming. Machine wiring is performed by a subcontractor. He uses a method of wire numbering which seems wrong. ...

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Am I missing something here? When I'm responsible for electrical wiring diagrams, I assign the numbering on the drawing before releasing it to the guy wiring it. By putting the wires on the diagram, the sparky can't creatively number in any way he wants. If you don't tell somebody how you want something done, how are they suppose to read your mind? Vulcan Mind meld? ESP?

Over the years I've found the KISS (Keep It Simple Stupid) is always best.

First principle: All wires that are electrically identical should have the same wire number. For example, neutral should always be #20 (or whatever) on all sheets.

Second principle: Wires that may not be at the same voltage should not have the same number. When a conductor is terminated at a device that may change it's electrical state, the wire number leaving the device should change. This is true whether it is a NC contact or a PLC output or whatever. Terminal blocks do not change electrical state, so the wire number on both sides of the terminal block should be identical.

Third principle: Field wiring is not my responsibility, and I can't control it. The contractor can use any darn scheme he wants on his side of the field terminals.

I use the method Okie describes. I never repeat rung numbers, so each drawing sheet starts with a new beginning number. For example, sheet 1 has rungs 1 through 999, sheet 2 rungs 1000 to 1999, and so on. That takes care of the problem of which sheet to look at.

The problem of lost drawings is real. However, anyone trying to debug a panel or trace wiring using just wire numbers without a print is in deep doo doo anyway.

AS far as wire numbering goes. Today we are not restricted to numbers so I actually prefer a scheme using device id.function. . .Other systems that use plc io addresses as wire numbers are ok until the plc is changed, field wiring normally lasts a lot longer than a plc system, when the plc is updated all the io addresses could change.

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Great points. I like labeling according to function. The function of a device is probably the last thing that will change. Even if the pump or motor is swapped out, the function remains, so that the new equipment can use the same wire numbers.

It is not age that counts when making decisions, but the amount of experience. Most of us have figured out that there is not one perfect wire-numbering system that will work equally well for all situations. Even if there was one such system, many would still use other methods. If you are a contractor, you have to be flexible and adaptable.

Well, the guy who did the previous design liked your method of labeling wires, so 0v was actually labeled -12V.

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How do you know that it was really 0 volts? Did you measure it? Was it an ungrounded -12 volts? If so, the label was correct. A DC power supply has at least two terminals, labeled + and -. If you study electrical circuit theory, you will find out that the - terminal of a power supply or battery is every bit as hot in the negative direction as the + terminal is in the + direction. Calling -12 volts as 0 volts goes against the training of anyone who has been through Circuits 101. The only way that you can get away with wrong theory like this is to make the -12 volts go to 0 relative to Earth, then call it 0 because if you put your voltmeter across Ground and -12, it will read 0.

In reality, it is still -12 at the power supply. You can test it for yourself. If you put your + voltmeter lead on the ungrounded (-) terminal of the power supply, and your - lead on the (+) terminal, your voltmeter will show you the true voltage at that terminal (-12 volts).

Do you think your multi-meter lies to you, and it is really 0 volts? What would you have to do to make your meter read 0 volts at the -12 volt terminal? Yes, that's right, you would have to re-design the power supply so that it has dual voltages, with a center common 0 volt point, as many +12,0,-12 power supplies are built. If you measure across the +12 and -12 terminals of your new power supply with a voltmeter, now what would it read? Certainly not +12 or -12 volts. Do you want to add to the mistake by calling this new voltage +12 and -12 when it reality it is now a 24 volt power supply with a 0 center point? How is this more accurate than the original +12/-12 power labeled as such?

Do you see now that you are comparing apples and oranges?

OkiePC said:

How about getting the wire number from the sheet/line number of the drawings? If there are 200 pages of drawings and I find a wire labeled "+24VDC" how do I find where it originates in the drawings?

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sorry for the late reply on this one.
+24Vds would not be used in this wiring method.
It would be numbered according to the page sheet line no. as mentioned.
This is very important if there are three or more dc supplies.

+24Vds would not be used in this wiring method.
It would be numbered according to the page sheet line no. as mentioned.

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For me, that is the big disadvantage of the sheet-and-line number method. It obscures things, such as the power supply wires, that would have been perfectly obvious using some other wiring method. Another big disadvantage is how it works with project revisions. If there is a set of drawings for a previous revision, but you only have the set for the most current revision, then there could be hundreds of wires that you have no clue about where they are, or what they do. In other words, there is no "continuity" from one revision to the next. There are no common wire numbers, and none that get reused from project to project.

Do you think your multi-meter lies to you, and it is really 0 volts? What would you have to do to make your meter read 0 volts at the -12 volt terminal? Yes, that's right, you would have to re-design the power supply so that it has dual voltages, with a center common 0 volt point, as many +12,0,-12 power supplies are built.

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I wrote an untruth. In reality, the center point of a dual-voltage power supply is no closer to being "0 volts" than the negative terminal of a 24-volt power supply is 0 volts. The problem arises when people try to create a 0-volt point that defies the laws of physics. The center point of a +12-0- -12 volt power supply is the same thing as connecting two 12-volt power supplies in series, with one - terminal connnected to the + terminal of the other power supply. That does not make it 0 volts relative to either of the other two ends, only a center point that is negative relative to one end, and positive relative to the other end, and the voltages actually available are not +12 and -12, but +/-24 and +/-12. Think of two batteries hooked in series. The only thing you get is two voltage levels, but still no 0 volt terminal. If this center-point terminal is labeled as 0, that is just a shorthand convention, a little white lie to make labeling easier. It sure leads to a lot of confusion.