Voltage drop calculations question

Fire Alarm Help
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Hi,

I am putting together some voltage drop calculations and I have a question about the distance of the conductor. this is a class A system so the conductor returns back to the panel after the last device, which is approx 300’ from the panel. do I include the return distance to the panel after the last device on the circuit (effectively doubling my conductor length). any industry standards that I could reference would be helpful (NFPA, NEC, IEEE)

Thanks
Russ

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Thank you to whoever moved this. I realized I posted in the wrong place but could not figure out how to delete this post and did not want to double post as that is generally frowned upon.

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There’s multiple ways to do it, when using the bulk calculation method adding the return length is standard. Try to design your circuits to loop back towards the panel instead of just going straight out like a typical class B design would.

This is why we try to avoid class A on the NAC’s, but some jurisdictions and some military bases require it.

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Thank You Chris,

The issue is that there is two circuits on each floor, so to loop one back I would just have to run the second one halfway out with no devices and then back anyways (as I get to the alarms at the far end of the building). from a physics/engineering point of view, the return wire shouldn’t matter what voltage is on it when it gets back to the panel (which is essentially what your calculating using the full length of the wire). as long as there is 16-33V at the node of the last device in line. I just need to be able to reference that somewhere.

the line is supervised with a TrueAlert (Simplex) device, so there must be some sort of signal that is sent down the NAC circuit? Im guessing adding a 24V power supply at the last device in parallel would interfere with that signal? otherwise im going to have to up the wire gauge to 14 or 12AWG to get the voltage drop to be adequate.

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The problem is if your circuit breaks two feet going out from the panel, your first device is now your last device. What is the voltage drop going to be on that device? You’ll have to calculate that entire return wire, back through all the other devices, up to the original first (now last) device.

You can also use a point to point calculation, takes a lot more time but is more accurate. The bulk loading method is quick and simple but also conservative, so if you’re really tight on runs by switching to point to point you’ll get better results.

Here’s a spreadsheet from AFAA that illustrates the difference in the two methods:
http://www.afaa.org/PDF/AZ%20AFAA%20NAC%20Circuit%20Calculator.xls

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Thanks again Chris,

this makes way more sense now. I am a little disappointed I didn’t realize this to begin with.

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