Vimpex Volt Drop Calculator
Instructions
When a number of electrical/electronic
products are connected to a cable (such as fire
alarm sounders to a sounder circuit), the
electrical current flowing in the cable combined
with the resistance of the cable causes a
reduction in voltage – a volt drop – which
means the products receive a reduced voltage
at their terminals. If the voltage is reduced too
much, the products may not operate correctly.
The Vimpex Volt Drop Calculator assists in
determining the voltage drop encountered for a
given current loading and cable length. The
voltage drop that can be tolerated is dependent
on the specifications of the equipment used.
The Volt Drop Calculator can also be used to determine the maximum number of
sounders that can be connected to a cable of a given length (resistance), to achieve a
specified volt drop. It can also be used to determine the maximum cable resistance (and
thus length) permissible for a given number of devices to achieve a specified volt drop.
Cable types directly calculated are solid copper conductor cables and denote the crosssectional area in square millimetres (e.g. 2L1.5 is two conductors of 1.5 square millimetres
each). For types of cable not shown, you will have to note the resistance readings from
the inner wheel's outer scale and calculate the cable length separately.
It may at first seem confusing to use the Volt Drop Calculator but once you become
familiar with it, it is rather straightforward. To develop familiarity, it is advisable to work
through each of the examples below to check you get the same results. This will confirm
that you are using the calculator correctly.
How it Works
The voltage drop is calculated according to Ohm’s law, which states that when a current
of 1 ampere passes through a resistance of 1 ohm, a voltage of 1 volt will appear across
the resistance. This is the well-known formula:
V=IxR
where V is the volt drop, I is the current and R is the resistance.
In the case of the volt drop calculator, the current I (which is the total current) is found by
multiplying the number of devices by the individual device current. That is to say, the
formula used is modified slightly to give:
V = n x i x R where n is the number of devices and i is the individual device current.
Note: it is assumed that all devices connected have the same current consumption.
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The Volt Drop Calculator can be used in a number of ways to determine acceptable
conditions, as shown in the examples below. The main use is to work out the volt drop
that will be produced by connecting a given number of devices to a specified cable. You
may then assess whether this volt drop can be tolerated according to equipment
specifications.
Worst Case Volt Drop Calculation (all devices connected at end of cable):
Step 1: Determine the current consumption for a single device. Set the red cursor line to
this current on the OUTER scale marked “Current per device or total Current (mA & A)”.
Example: each sounder takes 32 mA. Set the red cursor line to 32 mA.
Step 2: Turn the BLUE scale marked “Number of Devices” so that the start of the BLUE
scale (starting at the number 1) lines up with the red cursor line, making sure the red
cursor line remains set to the current chosen in Step 1 (32 mA in this example).
Step 3: Determine the number of devices to be connected. Example: 25. Turn the red
cursor line so that it rests over the number 25 on the BLUE scale marked “Worst Case”.
(You will now see that the total current, shown on the OUTER scale, reads 800 mA, this
being the total load. This should not exceed the total rating of the sounder circuit.)
Step 4: Keeping the red cursor line fixed in position, turn the INNER wheel (with the
cable length and resistance markings) so that the start of the scale (starting at 1 ohm)
lines up with the red cursor line.
Step 5: Determine the required length of cable and cable type. Example: 2L1.5 cable,
350 metres. Move the red cursor so that it is over the “Length of 2L1.5 Cable (m)” scale
at the 350 m mark. You will see that the red cursor also lies over the ohms scale at 8
ohms, this being the resistance of 350 m of 2L1.5 cable.
Step 6: Read off on the Voltage Drop (V) scale (the second outermost scale) under the
red cursor line the volt drop. In this example, it is about 6.4 V.
You need to determine whether a voltage drop of 6.4 V is acceptable or not. For example,
if the sounder’s minimum voltage is rated at 20 V and the lowest voltage applied to the
circuit is 24 V, you can only withstand a 4 V drop. So this current load and cable
resistance is not acceptable.
But this is a Worst Case calculation which assumes that all 25 of the devices are
connected at the very end of the cable, i.e. that the current drawn by each and every
devices passes along the entire length of cable. This is not likely in practice. The Volt
Drop Calculator also includes an Even Distribution scale, which assumes that the total
number of devices are spread evenly along the length of the cable. To perform an Even
Distribution calculation, proceed as follows:
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Even Distribution Volt Drop Calculation (all devices distributed evenly along cable
length):
Steps 1 and 2: same as above.
Step 3: Determine the number of devices to be connected. Example: 25. Turn the red
cursor line so that it rests over the number 25 on the BLUE scale marked “Even
Distribution”. (You cannot determine the total current as done previously. You can only do
this when using the Worst Case scale.)
Steps 4 and 5: same as above.
Step 6: same as above. The volt drop now reads (for the example used) as about 3.3 V
rather than 6.4 V. This would be acceptable for devices with the specification described
above – provided the distribution of devices along the cable length was even.
If you have multiple device types with different current consumptions
In this case, you can calculate the total current consumption by hand (the Volt Drop
Calculator can assist in this) and then proceed as above (but only with a Worst Case
calculation).
Step 1: determine the number of devices of each type and their current consumptions.
Example: 20 devices at 28 mA, 14 devices at 36 mA. Determine the total current. For the
example given, this is:
(20 x 28 mA ) + (14 x 36 mA) = 560 mA + 504 mA = 1064 mA.
You can use the Volt Drop Calculator’s OUTER and BLUE Worst Case scales to do the
individual multiplications for you, but you will need to add them manually. The Volt Drop
Calculator can only multiply, not add.
Step 2: set the red cursor line to the total current on the OUTER scale (Current per device
or total Current). Example: set to 1.064 A (the scale is not fine enough for this to be seen,
set to just over 1 A).
Step 3: Proceed from Step 4 through to Step 6 in the Worst Case calculation as above.
Finding out how many sounders can be connected for a given volt drop and cable
This is really just working part of the calculation backwards. Example: each sounder takes
24 mA each. Volt drop permissible is 6 V, cable required is 420 m of 2L1.0.
Steps 1 and 2: same as above for Worst Case calculation.
Step 3: Keeping the OUTER and BLUE scales pressed together to avoid them moving,
turn the red cursor line to the required voltage drop on the second outermost scale
(Voltage Drop). Example: turn to 6 V.
Step 4: rotate the inner wheel so that the required cable type and length lines up under
the red cursor line. Example: 2L1.0 cable, 420 m.
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Step 5: move the red cursor line to the beginning of the scale on the inner wheel, at the 1
ohm point.
Step 6: observe where the red cursor line crosses the BLUE scales. Example: 16.8 on
Worst Case, 32.5 on Even Distribution.
These are the number of devices that could be connected. Numbers must be rounded
down to the nearest integer (you can’t connect a fraction of a product) so you could have
16 devices if they were all mounted at the end of the cable, or 32 if they were evenly
distributed.
Finding out what Cable Type and Length needs to be used
If you know the number of devices that need to be connected and the permissible volt
drop, you can determine what length of various cable types will be permissible. This can
be done for both a Worst Case and for an Even Distribution scenario.
Example: 25 devices taking 32 mA each. Maximum permissible volt drop is 7.5 V.
Calculate for a Worst Case condition.
Step 1, 2, 3 and 4: same as Steps 1, 2, 3 and 4 in Worst Case calculation above.
Example: 32 mA, 25 devices.
Step 5: keeping the outer and inner wheels from rotating with respect to each other, move
the red cursor line so that it lines up with the permissible volt drop on the second
outermost scale (Voltage Drop). Example: 7.5 V.
Step 6: Read off on the inner wheel scale the resistance permitted (on the outer scale of
the inner wheel – example 9.4 ohms) and the maximum length of each type of cable
permitted. In this example, they are:
2L1.0: 280 m
2L1.5: 420 m
2L2.0: 560 m
2L2.5: 700 m
Remember that these are Worst Case calculations, assuming that all devices are fitted at
the very end of the respective lengths of cable.
To do this calculation for an Even Distribution scenario, repeat the above steps but using
the Even Distribution scale in step 3.
Using the same example, this gives up to 18 ohms and the following cable lengths:
2L1.0: 520 m
2L1.5: 800 m
2L2.0: 1080 m
2L2.5: 1350 m
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Conclusion
We hope you find the Vimpex Volt Drop Calculator effective and helpful. Please let us
know if you have any queries or comments.
Vimpex Ltd
Star Lane
Great Wakering
Essex, UK SS3 0PJ
Tel: +44 (0)1702 216999
Fax: +44 (0)1702 216699
Email: sales@vimpex.co.uk
Web: www.vimpex.co.uk
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