# Calculating voltage drop in class A fire systems

#### electrofelon

##### Senior Member
Am I correct in assuming that when calculating voltage drop for notification appliances in a class A fire system, I use both the "supply" and "return" conductors for the resistance or in other words half the loop length assuming even distribution of devices?

#### fmtjfw

##### Senior Member
As a first approach I take would be to sum all the loads and calculate the drop as though they were at at the farthest end of the run. If the answer is an acceptable voltage drop, then you are done.

Then I would take your approach and see if the voltage drop was clearly below the maximum allowed. If it was near the max, I might just up the wire by a size (assuming conduit installation). The cost of the wire is swamped by the cost of the devices.

To correctly calculate the voltage drop you need to calculate the drop to each load in the circuit. Take the first load, calculate the drop using total current. Then use the voltage at this point and the current for all but the first for the next drop. Rinse, Repeat, Rinse.

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#### electrofelon

##### Senior Member
As a first approach I take would be to sum all the loads and calculate the drop as though they were at at the farthest end of the run. If the answer is an acceptable voltage drop, then you are done.

Then I would take your approach and see if the voltage drop was clearly below the maximum allowed. If it was near the max, I might just up the wire by a size (assuming conduit installation). The cost of the wire is swamped by the cost of the devices.

To correctly calculate the voltage drop you need to calculate the drop to each load in the circuit. Take the first load, calculate the drop using total current. Then use the voltage at this point and the current for all but the first for the next drop. Rinse, Repeat, Rinse.
But that method would result in a vd of about twice the actual. Also if the devices are powered by both ends of the loop as I asked in the op then your method results in 4 times the vd. No small difference.

#### iwire

##### Moderator
Staff member
But that method would result in a vd of about twice the actual. Also if the devices are powered by both ends of the loop as I asked in the op then your method results in 4 times the vd. No small difference.
The reason we go class A is to make sure the system will still work with an open wire.

That being the case it is possible that all the devices would have to be supplied from one direction. I would base my VD on that worst case.

#### electrofelon

##### Senior Member
The reason we go class A is to make sure the system will still work with an open wire.

That being the case it is possible that all the devices would have to be supplied from one direction. I would base my VD on that worst case.
Of course. Got it thanks

##### Senior Member
If you have a total lenght of "X" to the last device, the conductor length for a Class "B" circuit is 2X, and for a Class "A" circuit is 4X. This is clear in Siemens' wiring guide for their XLS system. I've seen the consequences in action. We wired an area Class "A" and the strobes died out about halway around the room. When I lifted the return leg on the circuit, basically converting it to Class "B", everything fired just fine. We had to keep the circuit as Class "A" due to contract specs, so we had to run additional wire and break the circuit in half.

#### electrofelon

##### Senior Member
If you have a total lenght of "X" to the last device, the conductor length for a Class "B" circuit is 2X, and for a Class "A" circuit is 4X. This is clear in Siemens' wiring guide for their XLS system. I've seen the consequences in action. We wired an area Class "A" and the strobes died out about halway around the room. When I lifted the return leg on the circuit, basically converting it to Class "B", everything fired just fine. We had to keep the circuit as Class "A" due to contract specs, so we had to run additional wire and break the circuit in half.
I dont buy that. Lets expand/clarify with your example and say the last device is X distance from the FACP and the return (class A) is also X. I dont see any possible way the conductor length would be 4X. I admit I do not know the specific power scheme for a class A loop: if both supply and return terminals are common or isolated supplies but either way how could we have 4X and not X for conductor length?