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voltage will be dropped in the wires (1.6 volts) leaving 22.2 volts to operate the Magnalock. This will cause a minor
reduction in holding force but in general, will be acceptable.
To calculate the wire resistance, you need to know the distance from the power supply to the Magnalock and the
gauge (thickness) of the wire. The following chart shows wire resistance per 1000 ft (305 meters):
Wire Gauge
Resistance/1,000 ft
Wire Gauge
Resistance/1,000 ft
8 Gauge
.6 Ohms
16 Gauge
4.1 Ohms
10 Gauge
1.0 Ohms
18 Gauge
6.4 Ohms
12 Gauge
1.6 Ohms
20 Gauge
10.1 Ohms
14 Gauge
2.5 Ohms
22 Gauge
16.0 Ohms
Let's look at some other sample calculations. Suppose a SAM operating at 24 volts is 1200 ft from its power supply
and we're using 20 gauge wire. First, the total length of the power wires is 2400 ft.
Remember that you combine
the wire lengths from the power supply to the lock and back to the power supply to get the total circuit wire
length.
The wire resistance than becomes 2.4 X 10.1 Ohms which is 24.2 Ohms. Adding this to the (24V) SAM
resistance of 140 Ohms yields a total resistance of 164.2 Ohms. 24.2 divided by 164.2 yields the percent drop in
the wires which is nearly 15%. This is excessive. The problem can be dealt with in 2 ways. You can utilize 16
gauge wire which would reduce the drop to a more acceptable 6% range or you can provide extra voltage at the
power supply (sometimes the wiring is pre-existing). For instance, Securitron 24 V power supplies are adjustable
from 24 to 28 volts. If you set the power supply at 28 volts, for example, you would be supplying 14% overvoltage
which would compensate for the 15% voltage drop in the wires. If the power supply is operating a number of locks,
those closer the supply will receive more voltage, but the SAM will accept up to 30% overvoltage without ill effects.
Note when you operate the SAM at 12V, it exhibits
1/4th
the resistance (35 Ohms) that it does at 24 V. This
means that wire voltage drops are 4 times more significant in a 12 volt system than in a 24 volt system.
In any job
that has wire runs long enough to be of concern, always use 24 volts.
Note also that it's common to mount 2
Magnalocks on a double door and operate them as one lock (only 2 power wires). In this case, the resistance of
the pair of locks is half the resistance of a single lock. The SAM2’s are less of a concern because they draw a lot
less current (roughly 1/3 of the SAM). The SAM-2-12 draws 125 mA which corresponds to a resistance of 96
Ohms. The SAM2-24 draws 62 mA for a resistance of 355 Ohms.
In multiple lock jobs with a single power supply, the calculation of wiring voltage drops is more difficult. So long as
you run a separate pair of power wires to each lock, the calculation is as simple as has been described above, but
if a common power wire is used in a loop structure, the locks powered by the single loop will have an increasingly
low combined resistance so that the loop wire resistance will become more significant to the point where the locks
don't receive enough voltage. To find the combined resistance of multiple locks powered by a common wire, divide
the resistance of one lock by the number of locks. For example, eight SAM’s operating at 24 volts would have a
combined resistance of 140 divided by 8 which is only 17.5 Ohms. Another method is to calculate the current in
Amps in the wire and divide that into the circuit voltage. Since each SAM operating at 24 volts draws .175 Amps,
eight would draw 1.4 Amps. Dividing this into the same 24 volt input voltage yields the same 17.5 Ohm combined
resistance.
In general, you have to be cautious about using common wires for loads in long distance situations unless you're
very confident about your ability to calculate the correct configuration. Bear in mind, however, that anytime you're
uncertain about the voltage drop in wiring, you can meter the voltage at the lock
while it's connected
and you will
be able to see if it's receiving adequate voltage. If the lock is not connected when you make this measurement, the
result will be false as the circuit will not see any lock resistance to compare to the wire resistance. You will read
the full input voltage.
PATENTS
Securitron’s Shear Aligning Magnalock is covered under U.S. patent #4,516,114 and
#6,007,119 with other US and international patents pending.