Sizing Wires for Chassis

Discussion in 'General Electronics Chat' started by AutoNub, Feb 14, 2012.

  1. AutoNub

    Thread Starter Member

    Oct 14, 2011
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    Hello, everyone!

    I'm having a little difficulty recognizing the correct way to size wires. I looked at the American Wire Gauge table, but it seems overly simplified compared to some other sources such as this calculator. The calculator asks for the current, temperature limitations based on insulation type, conductors/strands, etc... and the AWG table is used to find wire gauges based solely on current requirements. I would tend to trust the linked calculator above over the AWG table, except it appears to only go as small as #14 gauge wire. I need to be able to calculate accurately and safely down to #28 gauge wire. I wish I knew the formulas used by the calculator or the standard formulas used by professional electricians to size wire gauges based on various pieces of information.

    To clarify, I'm very hesitant to settle with the AWG tables because they seem overly simplified. I believe strands, temperature specifications, and conductor type (copper vs aluminum, etc) are a few of perhaps several more important factors to consider.

    Please provide assistance or explanations for proper and accurate wire sizing. Thank you!
     
  2. Brownout

    Well-Known Member

    Jan 10, 2012
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    Wire manufacutres often publish tables to aid in size selection. Check out some of the larger wire manufactures ( ie Belken ) and see if they provide a chart.
     
  3. mcgyvr

    AAC Fanatic!

    Oct 15, 2009
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    The National Electrical Code (NEC) tables 310.15/16/17 is what you should use in the US to size wire or the applicable UL standard. This IS what professional electricians must use too.
    Pretty sure there isn't a single formula but that the data is based on physical testing/temp measurements and was done YEARS ago. In general its based on certain factors and is for the most part overly cautious to prevent any fires/safety issues. But any product a company makes in the US MUST have the wiring sized per those tables.

    All that calculator is doing is looking at that table too and basing its results off that. Its not a formula. its a table lookup basically.
     
  4. GetDeviceInfo

    Senior Member

    Jun 7, 2009
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    Electricians are governed by thier national code. Anyone can purchase the code itself, which contains the formulas/data. Electricians have recieved training as to how to enterprete the code and apply it in a practical way. How you might come to know this without the training is hard to say.

    your primary concern is keeping the electrical potential safely inside the conductor. This job is assigned to the conductor insulator. The choice of insulator is critical, and for any material choosen, has limitations which must be honored. Temperature is typically top of list, but could be rivaled by mechanical / environmental concerns. An engineer will consider all the attributes that an insulator must inherit to perform it's job. The question of current in the wire only answers one simple aspect, will the resulting heat degrade the insulators performance.
     
  5. BSomer

    Member

    Dec 28, 2011
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    As others have stated the NEC covers the amperage ratings of wires with specific types of jackets/coatings according to ambient temperatures. The NEC does only go down to 14 AWG because that is the smallest wire size allowed by the NEC for branch circuits and feeders. Been using that code section for the last 16 years or so. That calculator that you linked to is supposedly using the code ratings and values so it could be correct.

    I used to have a chart at home that showed the ratings of smaller wire. I'll have to dig around a bit to find it. As Brownout pointed out, you could go to a manufacturer's web site and see if they have any datasheets/application notes/charts.

    Here are a couple of manufacturers

    Allied wire and cable - http://www.awcwire.com/
    Southwire - http://www.southwire.com/
     
  6. crutschow

    Expert

    Mar 14, 2008
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    If you are concerned about wiring in a electronic chassis then the usual concern is voltage drop due to the wire resistance, not how much current the wire can carry. Also very small wires (such as 28AWG) can be difficult to work with so a larger wire is selected, just for ease of wiring.
     
  7. mcgyvr

    AAC Fanatic!

    Oct 15, 2009
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    That all depends on what your "electronic chassis/device" really is.. I design DC Fuse Panels...I'm 100% concerned about the amount of current the wires can carry. I don't think for a second about the voltage drop in my wires. I must follow NEC/UL or else. Voltage drop/current rating are closely related anyways.
    And yes no way in hell would I use anything smaller than 22 AWG wire for ease of wiring/handling. You can practically blow on 28 AWG wire and break it. Not to mention that there are virtually no ring/fork/q.c. terminals that are UL approved for anything smaller than 22 AWG anyways.
     
  8. AutoNub

    Thread Starter Member

    Oct 14, 2011
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    I'm trying to do chassis wiring for a schematic which uses continuous operation. Some components are continuously on and some are noncontinuous. I have not received any training on wire sizing though and am not an electrician.

    We typically use #8-#28 gauge wire where I work. I'm new to this job though and am the only electrical guy here so I'm trying to teach myself using the internet (and forums) as my only resource. The engineer who sized the wires in all of the previous schematics no longer works here and unfortunately left no notes to show his work. His wire gauges unfortunately don't match up with either the AWG tables or the calculator results at the aforementioned website.

    We use PTFE E type insulation (NEMA standard HP 3-2001). Unfortunately this standard doesn't walk me through the process of choosing the correct wire gauges. It merely describes what each type within the standard adheres to in terms of construction and performance capability. It also describes standard limitations such as maximum lengths, temperatures, etc...

    I found this .pdf from a quick google search. I haven't had time to read through it yet, but if anyone here recognizes it, please tell me if it'll provide me with the answers/knowledge I seek. I'll have to spend some hours tomorrow familiarizing myself with the material. It appears to be useful at first glance...
     
    Last edited: Mar 15, 2012
  9. GetDeviceInfo

    Senior Member

    Jun 7, 2009
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    Looks like a pretty good walk through of code application. I'll add it to my library.
     
  10. crutschow

    Expert

    Mar 14, 2008
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    That's certainly true, but I would define a DC Fuse Panel as an electrical chassis, not an electronic chassis. ;) Of course, there certainly are electronic chassis that do carry high current, such as power supplies, where the current rating of the wire is important.
     
  11. crutschow

    Expert

    Mar 14, 2008
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    So what current levels do these wires carry?
     
  12. mcgyvr

    AAC Fanatic!

    Oct 15, 2009
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    The OP needs to get a copy of the NEC (its actually available on their site for free as a PDF you can view online only when you register) Then you need to find out what standards are applicable to your products (like UL,etc...) Many times the UL standard will have the requirements for the wire sizes used in your products. Then get a hold of the specs for the wire you have and go to the manufacturers site and find the current ratings for that too. With the combination of those 3 you should have all the knowledge you need. Read the NEC from page 1 to at least the 310.17 section and any other applicable sections for your products/applications.
     
  13. AutoNub

    Thread Starter Member

    Oct 14, 2011
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    The entire NEC book in .pdf format is available for free? I registered here but haven't found the free NEC book yet.

    In retrospect I should have been more clear about the sort of electrical work I do here. I work on industrial products which consist of various types of meters, PLCs, transformers, lamps, power supplies, etc... The currents in a single product can range anywhere from about 500mA to 15A, so it isn't unusual to see a wide range of wire gauging used in a single product. I'm afraid I can't be much more specific than that, but it ought to be enough information to point me to the right section(s). Which examples or practice problems from the originally mentioned pdf would be most suitable for my application? I'm not sizing wires for raceways or feeders, but do the same standards apply? I didn't see a section specific to chassis wiring. I'm thinking page 15 is where we get into the most relevent material though.
     
    Last edited: Feb 15, 2012
  14. mcgyvr

    AAC Fanatic!

    Oct 15, 2009
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    Log in..Click the big "Codes & Standards" button, find NFPA 70 in the list and click that link, then towards the bottom there is a section that says "View the document online (read only)" click that..
     
  15. AutoNub

    Thread Starter Member

    Oct 14, 2011
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    My new NEC 2011 book arrived today. I found an equation on page 70-153 for calculating conductor ampacities. The book simply provides the equation with a brief definition of each variable therein. However, I'm a little unsure about which values to use.

    Lets say we are using PTFE E type silver coated wire with a maximum conductor temperature of 200 degrees Celsius. However, the corresponding standard (NEMA Standard HP 3-2001) material for this wire type lists DC resistance in ohms/1000ft at 20 degrees Celsius. Which value for Tc do we use, 200 or 20? I believe I need to use 200 C because that is the maximum tolerance, but then how do I find the DC resistance at this temperature? If the resistance were higher I could use a multimeter, but with short hookup wires the resistances should be so small the multimeter won't measure them. Without knowing the voltage drop or conductance at this temperature, I'm uncertain how to calculate the resistance. I'm also not sure about the simplest method for finding Rca or the effective thermal resistance between the conductor and surrounding ambient (room temperature). Am I correct that Rca is the inverse of the heat transfer coefficient? I'm also unsure how to find delta TD.

    How would you find Rdc, Rca', Yc and delta TD?


    Here's the equation:

    Neher-McGrath Equation


    Thanks!
     
  16. mcgyvr

    AAC Fanatic!

    Oct 15, 2009
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    Autonub,
    I'd suggest looking at other products your company has build in the past. It will likely come down to we use "X" AWG wire for signals and "Y" AWG wire for power. Don't over complicate it. I'd bet that you might oversize certain wires solely to increase the usage of a certain size.. Like you might just use 22 AWG for all signal wiring across the board so that you only need to purchase 1 wire size (in multiple colors)
    The smartest move you could make is to ask the people on the floor who assembly the products as they will probably know just as much as your engineer who left you in the cold. Let them tell you what they use and where/why. (They will only laugh at you in Spanish for a couple weeks.) :) no seriously do it.


    And technically you use the terminal temperature rating (what the wire is attached to) and not the wire rating in your calculations. ie. if you attach it to a circuit breaker with a 75 deg C terminal rating then your wire/ampacity calculations must be based on 75 deg C and not 200 deg C. but it doesn't matter as that formula does not apply really and you can't use it because you don't have all the variables and you won't for that.
     
  17. BSomer

    Member

    Dec 28, 2011
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    I still think that going to the manufacturer of your specific wire is the best route. They will have performed these and other tests on the wire to derive some sort of specifications. I imagine they will have the numbers you are looking for in a datasheet.
     
  18. AutoNub

    Thread Starter Member

    Oct 14, 2011
    44
    1
    I actually contacted both Anixter and Belden last week. Belden didn't respond. Anixter responded and seemed eager to help, but unfortunately their engineer is out all week on vacation. The sales representative I spoke to said the information I require can only be provided by their engineer.

    This does sound like good advice except for two problems. First, if something were to happen and a fire or injury due to a short were to occur, it'd be my freedom and career on the line as I would be held accountable. Second, it is a mystery how wires were sized in the past as there is not only no notes on the subject but also little consistency. However, my supervisor agrees with me that it'd be better to establish a procedure for calculating appropriate wires specific to our products utilizing our PTFE type E (600v), SCC (silver-coated copper; 200 degrees Celsius) wires. We have a wide range of stock so supplies aren't an issue.

    Coincidentally, we are actually having a minor problem with a product currently under development and the suspect is a high voltage drop across wire(s) due to incorrect wire gauging (not my work). So the bottom line is if I'm going to be held accountable for wire sizing I want to be certain it'll work without any problems. Since our products are typically in enclosures and in very hot environments and primarily used in military applications, things just have to be safe without exception. Also, it was suggested that once I develop a procedure for properly calculating and documenting wire sizes, I should revise past product schematics based on the new procedure.

    Hopefully when the Anixter engineer returns from vacation he will be able to provide me with some helpful information. Thanks for the replies!
     
  19. mcgyvr

    AAC Fanatic!

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    Here you go.. Then use NEC 310.17 (90 deg C column) for anything larger than 14 AWG. I'd use this as a starting point and then derate using the formulas at the bottom of the NEC tables for higher ambient. Or cut it down by a certain percentage based on running through raceways vs free air,etc..
    http://www.tempco.com/Catalog/Section 15-pdf/PTFE Lead Wire.pdf
     
  20. gerty

    AAC Fanatic!

    Aug 30, 2007
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    How high is this voltage drop?
    What size wire is this drop occuring on?
    At what current does this drop occur?
    How long is the wire with the drop?
     
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