Voltage drop on 24V AC systems

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Michael Harkey

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I cannot find any language that exempts or treats differently, an AC system of 24Vrms in a health care facility, regarding voltage drop. I have a controls guy that routinely runs 24V controllers from a 24VAC 100VA transformer on long lengths of #18 wire with multiple devices on the same circuit, so that the last device on the branch sees only about 20V of the 24V if all were to operate at once (calculated). This is much more than 5% drop, and I can't find any exception for such low voltage systems to the 5% limit on voltage drop. Does anyone have thoughts on this, how the code might treat 24V and voltage drop that I just can't find?
 
I can't speak for the problems that you may or may not have on the low voltage circuit but the 5% drop is a FPN no. 4 under art. 210.19 and is not enforceable under the NEC.
 
Relays have a Pickup and Dropout voltage, say 80% & 30% of rated voltage respectively. The relay will pull in with about 19.2v and dropout at about 7.2v. That is a 20% drop to pull them in. Well over the 5% fpn design issue. Put a meter on the circuit. More than likley you will find the transfomer puts out more than the 24v.
 
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