4-20 mA Instrumentation Loop Power Calculator

A two-wire 4-20 mA instrumentation loop must deliver enough voltage to operate the transmitter at every required loop current while also supplying the voltage lost in field wiring, receiver inputs, barriers, isolators, surge devices, and any other series components.

The calculation determines the voltage remaining at the transmitter at the selected low- and maximum-current conditions. The critical result is usually Transmitter voltage at maximum current, because 20 mA creates the greatest resistive voltage drop in a conventional 4-20 mA loop. That result is compared with the transmitter’s required operating or compliance voltage.

The calculation applies to DC analog signal loops used with pressure transmitters, temperature transmitters, level transmitters, valve positioners, PLC analog input cards, DCS input cards, indicators, and similar process-control equipment. It evaluates loop power availability; it does not convert a 4-20 mA signal into an engineering value such as psi, °F, flow, or percent level.

Loop Voltage and Compliance

A typical powered two-wire loop includes a DC supply, a field transmitter, one or more conductors, and a receiving input such as a 250-ohm analog input burden. Current flows through every series device in the loop.

As loop current increases from 4 mA to 20 mA, voltage drop across resistance rises proportionally:

\(\displaystyle V_{\text{drop}} = I \times R\)

The transmitter receives the remaining loop voltage:

\(\displaystyle V_{\text{transmitter}} = V_{\text{supply}} = V_{\text{resistive drop}}\)

\(\displaystyle V_{\text{barrier}} = V_{\text{other series devices}}\)

For a transmitter to operate correctly, the calculated transmitter voltage must meet or exceed its manufacturer-specified compliance requirement at the actual operating current and device configuration.

A loop can appear satisfactory at 4 mA but fail near 20 mA if the power supply is too low, the cable resistance is too high, or the receiver and inline devices consume too much voltage.

Calculation Inputs

Input	Electrical use
Minimum loop supply voltage (V)	The lowest DC supply voltage available during the stated operating condition. Use the minimum verified value rather than a nominal supply marking alone.
Low loop current (mA)	The lower loop-current check point, normally 4 mA.
Maximum loop current (mA)	The highest required current check point, normally 20 mA. This usually produces the limiting voltage-drop condition.
Transmitter minimum compliance voltage (V)	The transmitter manufacturer’s required terminal voltage at the declared loop current, load, and configuration.
Wire loop resistance (ohm)	Total resistance of both conductors in the completed loop path. If cable data is expressed as resistance per unit length, include the outbound and return conductors.
Receiver or input burden (ohm)	Resistance of the analog input card, indicator, panel meter, receiver, or other resistive receiving device.
Barrier or isolator voltage drop (V)	Manufacturer-specified voltage drop for an installed intrinsic-safety barrier, isolator, surge device, or comparable inline device.
Other series voltage drop (V)	Documented voltage consumed by additional series devices that are not represented by a resistance value.
Optional measurement resistor (ohm)	A resistor intentionally installed in series with the loop for measurement, monitoring, or another circuit function. Enter it only when it is physically part of the loop path.

Wire loop resistance is not one-way conductor resistance unless the entered value already represents the entire completed current path. A long home run to a remote transmitter has resistance in both the supply and return conductors.

Loop Power Formula

The calculator first combines all entered series resistance:

\(\displaystyle R_{\text{total}} = R_{\text{wire loop}} + R_{\text{receiver burden}} + R_{\text{measurement resistor}}\)

It then calculates the resistive loop voltage drop at each current:

\(\displaystyle V_{\text{drop at current}} = \left(\frac{I_{\text{mA}}}{1000}\right) \times R_{\text{total}}\)

The receiving-device voltage at maximum current is calculated from the receiver burden alone:

\(\displaystyle V_{\text{receiver at maximum}} = \left(\frac{I_{\text{maximum mA}}}{1000}\right) \times R_{\text{receiver burden}}\)

Available transmitter voltage at each check point is:

\(\displaystyle V_{\text{transmitter}} = V_{\text{minimum supply}} = \left(I \times R_{\text{total}}\right)\)

\(\displaystyle V_{\text{barrier or isolator}} = V_{\text{other series}}\)

The Transmitter minimum compliance voltage (V) is a comparison requirement, not an added voltage drop in the arithmetic. Compare it against the calculated transmitter voltage at the applicable current.

Calculation Example

Use the following loop data:

Input	Value
Minimum loop supply voltage	24 V
Low loop current	4 mA
Maximum loop current	20 mA
Transmitter minimum compliance voltage	12 V
Wire loop resistance	20 ohm
Receiver or input burden	250 ohm
Barrier or isolator voltage drop	0 V
Other series voltage drop	0 V
Optional measurement resistor	0 ohm

Total series resistance is:

\(\displaystyle 20\ \text{ohm} + 250\ \text{ohm} + 0\ \text{ohm} = \mathbf{270\ \text{ohm}}\)

At 4 mA:

\(\displaystyle 0.004\ \text{A} \times 270\ \text{ohm} = \mathbf{1.08\ \text{V}}\)

\(\displaystyle 24\ \text{V} - 1.08\ \text{V} = \mathbf{22.92\ \text{V}}\)

At 20 mA:

\(\displaystyle 0.020\ \text{A} \times 270\ \text{ohm} = \mathbf{5.4\ \text{V}}\)

\(\displaystyle 0.020\ \text{A} \times 250\ \text{ohm} = \mathbf{5\ \text{V}}\)

\(\displaystyle 24\ \text{V} - 5.4\ \text{V} = \mathbf{18.6\ \text{V}}\)

Result	Value
Total series resistance	270 ohm
Voltage drop at low current	1.08 V
Voltage drop at maximum current	5.4 V
Receiver voltage at maximum current	5 V
Transmitter voltage at low current	22.92 V
Transmitter voltage at maximum current	18.6 V

The transmitter has 18.6 V available at 20 mA. With a stated Transmitter minimum compliance voltage (V) of 12 V, the calculated voltage margin at the maximum-current check point is 6.6 V.

Field Verification

Use the actual minimum supply condition, not only the nameplate value of a nominal 24 VDC power supply. Supply voltage can be lower under load, during abnormal operating conditions, or at remote distribution points.

Verify the transmitter compliance requirement from its data sheet for the installed configuration. Some transmitters have different voltage requirements when fitted with a display, configured for certain output modes, connected through an approved barrier, or operating at a particular output current.

Confirm the resistance of the complete cable loop from verified conductor data, conductor length, and temperature where applicable. AWG conductor size, copper versus other conductor material, connection quality, and long field runs affect actual loop resistance. Unlike branch-circuit or feeder voltage-drop work, this calculation evaluates a low-current DC control loop rather than conductor ampacity or overcurrent protection.

Barrier, isolator, and surge-device voltage losses must come from manufacturer documentation. Do not estimate them from a device label or treat them as resistance unless the manufacturer explicitly provides a resistance-based value suitable for the required current range.

This calculation establishes available loop voltage under the entered conditions. Final installation decisions must also account for the equipment listing, manufacturer wiring requirements, intrinsic-safety design where applicable, control-system documentation, and AHJ or project requirements.