# Thread: Designing 24VAC power trunk

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## 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!

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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. 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.

4. What are you powering?

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Originally Posted by joey791
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

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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.

7. Originally Posted by supra33202
Specifically, I am trying to power 8 Honeywell Spyder LON VAV controllers
Originally Posted by supra33202
I was told the followings.
Originally Posted by supra33202
2) Each 24VAC power trunk should have maximum of 5 to 6 Honeywell Spyder controllers.

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Originally Posted by orion242
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. PM MatrixTransform. He can help

So what is the voltage at the last VAV 500ft down 18awg? Have you measured it?

10. 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!

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Originally Posted by supra33202
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.

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.

12. 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. 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!!!!

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