Need Some Help Calculating Voltage Drop For A 220 Foot Run With Multiple Branches Off

I am trying to do the math as far as voltage drop. I am trying to control about 12 strikes all from the same relay (all will be locked or unlocked at the same time). The strikes are all on doors that are along a hallway that is about 220 feet long. The plan is to run one main wire down the hallway, and branch off by each door. Is an 18/2 wire a thick enough gauge for the main wire? I would have a 22/2 going for each branch?

I can use 12vdc or 24vac. Would one work better?

Thank you

The higher the voltage the less voltage drop. In order for me to calculate this I need to know the amperage rating of the strikes being used. I can almost guarantee you that 18/2 will not be enough for 12 strikes. A typical strike at 24VAC/VDC and 250mA with that distance would require 12awg wire. HES says anything lower than 21VAC/VDC is a low voltage condition which reduces the life of the strike and will also cause overheating and other issues.

24VAC/VDC @ 220 feet with a load of 250mA * 12 = 3 Amps using 12awg wire

Voltage drop: 2.10V
Voltage drop percentage: 8.75%
Voltage at the end: 21.9V

From the manufacturer of the stike I was going to be using:


or 150mA@24vdc.

I can divide the strikes into multiples groups. The only thing, is I will still have less say 4 strike at the end of a 220ft run.


If I understand things right, you want a single relay to simultaneously trip strikes on 12 different doors along a 220 foot run? So the strikes are connected in parallel?

Before considering voltage drop, do you have current worked out?

Which strikes are you using? Which relay? Which power supply?

I am open to the type of strike, but I was going to use Seco-larm SD-995C.

Yes, I would run them in Parallel.

THe relay that I am going to use is my next issue. I need to find a relay that can be triggered by another smaller relay. Power supply is also open.

To be honest, I dont need all 12 to be in parallel, I can divide it into groups of a few doors. I just want it that one "trigger" will cause all of them to activate/de-activate.

Thank you

Give this a try. Fill in all of the data (it can be obtained from the specs on your locks and power supply) then just hit the calculate key. The resulting voltage will be displayed for you.

Thank you

I would recommend an RB5 relay or as many as you need. They are flexible, durable and I believe they can take 5amps each.

So, if the runs are all parallel and spaced evenly by 20 feet (for arguments sake), the voltage drop is considerably less than a single loop.

The distance is offset partially by the drop off in current.

Assuming again that the first strike is at the relay (only because it makes the distances an even 20 ft.)

With 18AWG here are some vdrops at the different strikes.

  1. 1800ma
  2. 1650ma @ 20ft
  3. 1500ma @ 40ft = 23.23V
  4. 1350ma @ 60ft
  5. 1200ma @ 80ft
  6. 1050ma @ 100ft
  7. 900ma @ 120ft = 22.62V
  8. 750ma @ 140ft
  9. 600ma @ 160ft
  10. 450ma @ 180ft
  11. 300ma @ 200ft
  12. 150ma @ 220ft = 23.58V

Again, just showing the general effect, don't know what your actual distances between strikes are.


Don't forget to include 'inrush current' in your calculations. A strike is a motor after all. You will have to heavily suppress the relay and strike ends to avoid horrendous noise. This does not sound like something I would entertain.

This is much like calculating voltage drop for fire alarm notification circuits.

You need to understand the holding current of the strikes and the inrush current. Start by calculating the voltage drop of all devices from the relay to the door. You can use 12VDC or 24VAC for the calculations. If your starting voltage is 24v and your drop is 2 volts at the first strike, start with 22v and reduce by that strike. Each leg needs to be calculated.

My practical experience would say this isn't going to work due to holding current, inrush current.

There are programs that help calculate this.

So on the data sheet for this particular strike no inrush current is called out. It says only "current draw".

It sounds like you plan to lock and unlock a group of doors on a schedule. Besides not seeing an inrush current, I can't see if this strike is rated for "continuous duty" if that's your plan.

You should consider discussing with the manufacturer this application.

IPVM has a good series on installing strikes and some cautionary tales about cutting them into fire rated corridors and such.

The inrush current for the Seco-alarm strike is the same as the hold. It's DC-solenoid.

With that in mind, and with the addition of MOV diodes at every branch, can anyone say why they don't think the 18/2 solution will work?

I have completed the remaining vdrops on each branch below, notice how they are symmetrical values to the max. sag at 22.62 at the midway.

  1. 1800ma
  2. 1650ma @ 20ft = 23.58V
  3. 1500ma @ 40ft = 23.23V
  4. 1350ma @ 60ft = 22.97V
  5. 1200ma @ 80ft = 22.77V
  6. 1050ma @ 100ft = 22.66V
  7. 900ma @ 120ft = 22.62V
  8. 750ma @ 140ft = 22.66V
  9. 600ma @ 160ft = 22.77V
  10. 450ma @ 180ft = 22.97V
  11. 300ma @ 200ft = 23.23V
  12. 150ma @ 220ft = 23.58V

Did you start with a new wire at device #7?

Maybe something fishy in there?

I'm not sure what, since at each leg the current in the upstream wire is reduced by 150ma, and the voltage drop is straightforward enough to calculate.

Do you see anything wrong?

There's no way the voltage can go up even if you eliminate all devices after #6.

That makes sense. What do you think the voltage drops should be a the different legs?

Total Circuit Current 1.800 Wire Ohm's
Gauge Per 1000
Distance from source to 1st device 20 18 7.77
Wire Gauge for balance of circuit 18 7.77
Enter current in amps. Distance
.150 = 150 ma from Voltage
Device Device previous At Drop from Percent
Number Current device Device source Drop
Device 1 0.150 20 23.44 0.559 2.33%
Device 2 0.150 20 22.93 1.072 4.47%
Device 3 0.150 20 22.46 1.538 6.41%
Device 4 0.150 20 22.04 1.958 8.16%
Device 5 0.150 20 21.67 2.331 9.71%
Device 6 0.150 20 21.34 2.657 11.07%
Device 7 0.150 20 21.06 2.937 12.24%
Device 8 0.150 20 20.83 3.170 13.21%
Device 9 0.150 20 20.64 3.357 13.99%
Device 10 0.150 20 20.50 3.497 14.57%
Device 11 0.150 20 20.41 3.590 14.96%
Device 12 0.150 20 20.36 3.636 15.15%
END 20.36 3.636 15.15%
END 20.36 3.636 15.15%
END 20.36 3.636 15.15%
END 20.36 3.636 15.15%
END 20.36 3.636 15.15%
END 20.36 3.636 15.15%
END 20.36 3.636 15.15%
END 20.36 3.636 15.15%
Totals 1.800 240 End of Line Voltage 20.36


Was this from a tool you can share? Or do you have the formula?

One thing I don't intuitively grasp is why branching in parallel from the circuit would cause the voltage drop on later runs since it doesn't increase the resistance to the end.

Sounds like 16 gauge would be needed.

You are essentially doing a voltage drop calculation similar to fire alarm notification devices (horn/strobes) and they also run at 24VDC. I used the AFAA spreadsheet form linked here Notification Circuit Calculator. There are other variables and you have to decide what is an acceptable voltage drop, and typically 10% is the maximum.

You could calculate the first 6 devices on one pair and the balance on another pair. Make sure you input the correct starting distances.

I still haven't seen if those locks are rated for continuous duty as it appears you will be cycling them on a schedule for hours on end.

As another design consideration, if you blow a fuse you will lose all locks. You might want to run separate power runs for some to prevent this or maybe put inline fuses when you tap off the strikes.