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  1. #1
    Join Date
    Nov 2010
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    11

    Designing 24VAC power trunk

    1) Is there any guide for designing 24VAC power trunk for powering up multiple 24VAC devices (24VAC VAV controllers)? Any tips?

    2) Could you confirm the followings?

    Voltage drop=2*((total VA)/24V) x Resistance of the Wire/Foot x distance in feet

    Note: Need to multiply by 2 because there are 2 conductors (24VAC+ and 24VAC-)

    In general, VAV controllers should be able to operate at 24VAC +/- 10%. Thus, the highest voltage drop should be 24 *.1= 2.4V.

    The resistance for AWG#18 copper wire is 0.006385 ohms/ft.

    Example: Find the maximum length for using AWG #18 copper wire to wire up 8 VAV controllers (10VA power consumption per VAV controller).

    distance in feet = Voltage drop / (2*((total VA)/24V) x Resistance of the Wire/Foot)

    distance in feet = 2.4 /(2*(80/24)) * 0.006385)

    distance in feet = 56 ft

    So, the max length of the AWG#18 copper wire between the 24VAC transformer and the last 24VAC controller should be 56 ft.

    Thanks!

  2. #2
    Join Date
    Apr 2014
    Location
    MN
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    49
    I don't know about your calculations, but I've ran power to VAV boxes a couple hundred feet with a 100 va transformer. I've only seen a couple instances where wire length was an issue with voltage drops. I'd stay at about 80% of the rated output of the transformer based on your load.

  3. #3
    Join Date
    Jan 2003
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    USA
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    Your math has issues. The current varies down the trunk for starters. The trunk between the last and second to last modules only has 10va, the next segment 20va, etc. You will have to calc each segment and look at that way.

    I wouldn't mess around with 18awg for that purpose, 16awg min.
    Propagating the formula. http://www.noagendashow.com/

  4. #4
    Join Date
    May 2002
    Location
    Baton Rouge, Louisiana
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    3,862
    What are you powering?
    Quote Originally Posted by MatrixTransform View Post
    very soon it is you that will be pwned

  5. #5
    Join Date
    Nov 2010
    Posts
    11
    Quote Originally Posted by joey791 View Post
    What are you powering?
    Specifically, I am trying to power 8 Honeywell Spyder LON VAV controllers (power consumption 9VA per Honeywell cutsheet) .

    And then, I saw this chart. According to the chart, for 80VA and #18AWG cable, the max cable distance is 56 ft.

    http://www.allkindscable.com/24widich.html

    And I want to know how they calculated the recommended max cable length.

    Then, I saw the following article.

    http://ecmweb.com/content/basics-cal...g-voltage-drop

    I was told the followings.

    1) Use 80VA or less on a 100 VA class II transformer
    2) Each 24VAC power trunk should have maximum of 5 to 6 Honeywell Spyder controllers.

    I believe 2) really depends on the wire type and length of the cable.

    So, I want to know the recommended max total length of a cable for powering 8 Honeywell VAV Lon Spyder controllers.

    1) #18AWG
    2) #16AWG
    3) #14AWG

  6. #6
    Join Date
    Nov 2010
    Posts
    11
    I am wondering if someone could help me out with finishing the google doc spreadsheet. Then, we all know our 24VAC power trunk design will work for sure.

    https://docs.google.com/spreadsheets...it?usp=sharing

  7. #7
    Join Date
    Jan 2003
    Location
    USA
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    Quote Originally Posted by supra33202 View Post
    Specifically, I am trying to power 8 Honeywell Spyder LON VAV controllers
    Quote Originally Posted by supra33202 View Post
    I was told the followings.
    Quote Originally Posted by supra33202 View Post
    2) Each 24VAC power trunk should have maximum of 5 to 6 Honeywell Spyder controllers.
    Seems like you answered your own question.
    Propagating the formula. http://www.noagendashow.com/

  8. #8
    Join Date
    Nov 2010
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    11
    Quote Originally Posted by orion242 View Post
    Seems like you answered your own question.
    Without understanding the calculation, I can't answer the following questions.

    1) The last VAV box is 500 ft away from the transformer, will there be an issue?
    2) With #18AWG wire, what is the max distance between the transformer and the last VAV controller?
    3) Will #16AWG wire solve the issue?
    4) Will running #14AWG wire to the first box, and running #18AWG wires to the rest of boxes work?

  9. #9
    Join Date
    Jan 2003
    Location
    USA
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    PM MatrixTransform. He can help

    So what is the voltage at the last VAV 500ft down 18awg? Have you measured it?
    Propagating the formula. http://www.noagendashow.com/

  10. #10
    Join Date
    Mar 2005
    Location
    Pacific Time Zone
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    4,192
    The other thing is it also depends if you are daisy chaining all your power or T tapping. Daisy chaining you'll get the drop in a hurry with the added VA. What I'll do a lot of times depending on locations is make multiple home runs or run a heavier gauge for the first so many feet and then tie in all my smaller gauge runs to the heavier gauge location. We run a lot of 24V UPS powered devices with the LON so we've learned other methods as typically I've found the calcs are too conservative or are calculating it as daisy chained...JME

    *Like the PM Matrix comment!
    "How it can be considered "Open" is beyond me. Calling it "voyeur-ed" would be more accurate." pka LeroyMac, SkyIsBlue, fka Freddy-B, Mongo, IndyBlue
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  11. #11
    Join Date
    Oct 2003
    Location
    Minnesota
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    Quote Originally Posted by supra33202 View Post
    I am wondering if someone could help me out with finishing the google doc spreadsheet. Then, we all know our 24VAC power trunk design will work for sure.
    As Orion points out, you seem to have answered your own question.

    I'd personally not have gone to so much trouble myself. If one of my installers had asked I'd have simply told him to use 2 96va transformers and call it good. If for some reason it was a particularly long run, I'd have him up the cable size to 16 from 18. But we don't typically push the loading on control transformers. They're cheap. And the last thing I want or need are issues caused inadequate power supply. With the VAV controllers we use, not Honeywell, typical consumption is 5 VA with 10 VA max. And our standing rule for our installers is 6 (or less) VAV controllers on a 96VA transformer. This helps the transformers run a bit cooler, thereby lasting longer; removes most of the worry about voltage drop; and leaves some room so that if later we need to add another device needing power we may not need to add another transformer. Depends ... might add another small load. But if we're adding another unitary controller (besides a VAV controller), we'll just add another transformer. Since, as a matter of policy we put in separate transformer for each controller except in the case of the VAV controllers.

    Trying to push for as much loading on each transformer as yah can get, just doesn't make sense to me. But that's only my opinion.

    Last time I ran into such a situation on one of our jobs ... I got a warranty call. With customer complaining that controller for a fan coil unit kept going off line, and wasn't maintaining proper space temp. Went to the site, and found nothing wrong. Controller and fan coil operating okay. Customer showed me historical data, showing that there was indeed periods when it went offline and space temps wandered. Checked out controller more thoroughly, full diagnostics. Seemed okay. But just for the heck of it replaced it with a new one. I was back a week later, same complaint. Put new darn controller through ALL it's paces, everything checked okay. WTF? But historical records showed customer was right, new one was acting just like old one. Wasn't a comm issue ... comm was clear as a bell without a trace of problems. And nothing from front end banging away at controller excessively. SO .... I thought, let's drop back to basics. Traced out every connection and section of wire. And found that one of our installers had violated our rule about separate transformers. He'd doubled up on 1 transformer, using it to power two unitary controllers, with their valve actuators, relays, etc. I called him and asked why. His answer was that he'd put that in at the end of the projects, and had been short 1 transformer. He'd worked the numbers and thought it'd work ... calculated load near max but not QUITE there. Yeah, sure ... I told him NOT to do that again. Reason the trouble was only intermittent? It only showed up when each of the two fan coil systems was operating select loads at the same time. Then only the one controller would brown-out so to speak ... locking up logically. I finally figured out it was in the loop that controlled minimum on and minimum off timing for a relay. Set for 3 minutes ... it would take it hours to count down that 3 minutes after one of those brownouts. I put in an added transformer, separated power. Problem never re-occurred. Friggin hours of my time, including 2 trips to and from site, time to figure out what the heck was happening then fix it, and so forth ... over lack of using just one more $20 transformer. Sheesh. I finally suspected it was a power issue only because I'd eliminated just about every other possibility. Had seen this sort of thing before, years ago, but not for a long time because we'd adopted the policy of NOT pushing the loading on transformers. Didn't think of it right off, because it did not at first dawn on me installer might've veered from our rule.
    A site where I stash some stuff that might be interesting to some folks.
    http://cid-0554c074ec47c396.office.l...e.aspx/.Public

  12. #12
    Join Date
    Feb 2009
    Location
    Northeast
    Posts
    342
    If you are going 500' with #18 you may want to homerun the power to the transformer for every controller. This is not a bad practice for any distance. Those extra connections at each controller tend to be trouble spots. A loose connection or a reversed polarity can cause a hard to find, time eating problem that makes everyone look bad.

    Do not forget to add any actuator loads for valves and dampers getting power from the VAV controller. (Reheat, Rad, Relief?)

    I personally wish a VAV was considered something that required power just like a Fan Powered Box so the Sparky would feed 120 to all VAV. Mech and Elec Engineer would have to cooperate. I note that most Fire, Security, Networking, and Telephone devices have their power needs addressed by the Elec Engineer. Of course the Elec Engineer is responsible for those systems.

  13. #13
    Join Date
    Sep 2006
    Location
    Alaska
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    458
    Actually the most important thing to do is hide them so no one else can find them quickly. So when you get a call you can look like a genius because you know where the power supplies are located. Job security!!!!
    Law Of The Thermostat: He who has the thermostat wins!!!!!

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