Splitting a 4-20mA Signal Between Two Devices Safely

A transmitter has one 4-20mA output, but the measurement often belongs in two places: a local indication point and a PLC, or two controllers with different jobs. That does not automatically mean you need a second transmitter. A current loop is a series circuit, and that fact gives you two viable ways to distribute one measurement.

The first method keeps both receivers in the original loop. The second reads the signal through a powered isolated device and creates separate output loops. The right choice depends on voltage headroom, grounding, and what must keep working when another receiver is serviced.

TL;DR: Put two compatible passive receivers in series and each sees the same 4-20mA current, but add their burdens and calculate the loop at the transmitter's maximum alarm current. Two 250 ohm receivers total 500 ohms and drop 10 V at 20 mA; use a powered isolated splitter when headroom, grounding, or independent downstream loops matter (NotebookLM, Analog Signal Standards notebook, 2026).

This guide pairs with the 4-20mA loop power-budget calculation for the supply-side check and converting 4-20mA to voltage for the burden-resistor view of an analog input. The circuit is simple. The commissioning decision is not.

Can One 4-20mA Loop Feed Two Receivers in Series?

Compatible passive receivers in series carry identical current, whereas parallel passive inputs divide it and produce incorrect readings.

Yes. Place the receivers in series in one continuous current loop, and the identical loop current passes through both devices at every operating point. If the transmitter regulates the loop at 12 mA, each series receiver carries 12 mA, rather than a fraction of it (NotebookLM, Analog Signal Standards notebook, 2026).

Parallel passive inputs do the opposite. Two equal 250 ohm burdens in parallel have an equivalent resistance of 125 ohms. With a 12 mA transmitter output, the two equal paths receive 6 mA each. Each 250 ohm burden then develops 1.5 V, not the 3 V that a 12 mA reading normally produces. Unequal burdens divide current unequally, so the resulting readings also disagree (NotebookLM, Analog Signal Standards notebook, 2026).

Citation capsule: A 4-20mA transmitter can feed two series receivers because the current is identical at every point in one series loop. At 12 mA, each series receiver carries 12 mA; by contrast, two equal 250 ohm passive inputs wired in parallel each receive 6 mA (NotebookLM, Analog Signal Standards notebook, 2026).

How Do You Check the Two-Receiver Voltage Budget?

Add every series voltage drop and compare that sum with the supply voltage at the transmitter's maximum current. The governing check is supply voltage greater than or equal to transmitter minimum voltage plus wire drop plus all receiver drops; the equivalent resistance form limits total receiver burden after the wire drop (NotebookLM, Analog Signal Standards notebook, 2026).

Write the check before tightening a terminal:

[ V_{supply} \ge V_{min} + V_{wire} + \sum V_{receiver} ]

[ \sum R_{receiver} \le \frac{V_{supply} - V_{min} - V_{wire}}{I_{max}} ]

The first equation makes the physical requirement plain. The supply must leave the transmitter its specified minimum terminal voltage after the current has crossed the wire and every burden. The second lets you solve for the maximum combined receiver resistance. Use round-trip wire resistance, because current traverses both conductors, and identify the transmitter's specified minimum voltage rather than treating the whole supply as available load voltage.

Use the transmitter's maximum over-range, fault, or alarm current for (I_{max}), not the nominal 20 mA top of the normal measuring span. The packet gives 22 mA as a standard safety over-range example and up to 24.4 mA for some manufacturer fault signaling. Voltage drop rises linearly with current, so a loop that passes 20 mA can still starve before it reaches its high alarm (NotebookLM, Analog Signal Standards notebook, 2026).

Citation capsule: Size a two-receiver loop with (V_{supply} \ge V_{min} + V_{wire} + \sum V_{receiver}) at the transmitter's maximum alarm or over-range current. Using 20 mA instead can conceal top-end roll-off, because receiver and wire voltage drops increase with current (NotebookLM, Analog Signal Standards notebook, 2026).

What Does a Two-250-Ohm Series Example Look Like?

Two 250-ohm burdens consume loop voltage together, so the final calculation must include wire loss at alarm current.

Two 250 ohm receiver burdens add to 500 ohms. They drop 10 V at 20 mA, or 5 V per receiver; that is why each familiar 250 ohm input presents a 1-5 V burden over the normal 4-20mA range. The final pass or fail decision still belongs at the alarm current, not at 20 mA (NotebookLM, Analog Signal Standards notebook, 2026).

The notebook packet provides a complete example: a 24 V supply, an 8 V transmitter minimum, 1,500 feet of 24 AWG wire, and a 22 mA sizing current. The round-trip length is 3,000 feet. At 26.2 ohms per 1,000 feet, the wire resistance is 78.6 ohms. Multiplying by 22 mA produces a 1.73 V wire drop (NotebookLM, Analog Signal Standards notebook, 2026).

That leaves ((24 - 8 - 1.73) / 0.022 = 648) ohms for all series receiver burdens. Two 250 ohm receivers total 500 ohms, leaving the configuration inside the calculated limit. At 22 mA, each 250 ohm receiver drops 5.5 V, so the two receivers consume 11 V and still leave the transmitter above its 8 V minimum when the stated wire drop is included.

Citation capsule: In the notebook's 24 V, 8 V-minimum, 22 mA example, 3,000 feet of 24 AWG wire has 78.6 ohms of round-trip resistance and drops 1.73 V. The remaining receiver budget is 648 ohms, so two 250 ohm burdens totaling 500 ohms pass the check (NotebookLM, Analog Signal Standards notebook, 2026).

What Can Make a Series Connection the Wrong Electrical Choice?

A series connection is wrong when its devices create more than one ground reference, use incompatible single-ended inputs, or cannot tolerate one shared uninterrupted loop. Even a series loop with enough calculated voltage can develop erroneous readings if stray ground current bypasses its intended path (NotebookLM, Analog Signal Standards notebook, 2026).

Tie the loop to earth at one point only, commonly at the supply negative terminal. Multiple earth references can form a ground loop, especially when receivers have non-isolated single-ended inputs with shared internal returns. The extra path allows circulating current to enter the measurement circuit, which can corrupt readings at every device on the shared loop.

Ground-potential differences of 1 V to 2 V are identified in the packet as common across plant sections. Differential input cards reject some common-mode voltage, but their limit is typically plus or minus 10 V to plus or minus 15 V. Above that range, the input amplifier can saturate and clamp the reading at a maximum or minimum limit. A single-ended input does not offer the same rejection (NotebookLM, Analog Signal Standards notebook, 2026).

Maintenance is the other shared consequence. Removing a panel indicator or losing power to one series receiver opens the continuous path. The other receiver then loses the loop signal too. This can be acceptable when both observations are part of one local, controlled circuit. It is a poor fit when the PLC signal must remain available while a downstream display is changed.

When mapping the terminals, confirm whether the transmitter is loop-powered or separately powered before assigning supply and return paths. Verify each device's wiring requirements rather than assuming that its supply and signal terminals share the same arrangement.

Citation capsule: Series sharing requires one intentional earth reference and compatible receiver inputs. Ground differences of 1-2 V can drive circulating current, while differential inputs typically have only a plus-or-minus 10 V to plus-or-minus 15 V common-mode range; opening one series receiver also interrupts every receiver on that loop (NotebookLM, Analog Signal Standards notebook, 2026).

How Does an Active Signal Splitter Create Independent Outputs?

A powered isolated splitter regenerates separate outputs, keeping a downstream fault or maintenance break from interrupting the primary measurement.

An active isolated splitter reads the input loop, breaks the direct ground path, and uses its external power and internal drivers to regenerate separate 4-20mA output loops. Unlike a series connection, one downstream output can have its own isolated ground and receiver burden without adding that burden directly to the original field loop (NotebookLM, Analog Signal Standards notebook, 2026).

A powered splitter applies that principle to distribution. It accepts the primary signal and drives two or more separate outputs. Because each output is physically and ground-isolated from the input and from the other output, a ground fault, noise event, or failed downstream receiver on one channel does not feed backward into the primary measurement or the other output. That separation is the real reason to add a splitter; it is not merely a more elaborate way to draw a series loop.

For the isolation mechanism, limitations, and grounding rationale in more detail, see galvanic isolation with signal isolators. A splitter still needs correct supply, output-load, and terminal checks; isolation changes the topology, not the need to read the device documentation.

Citation capsule: A powered active splitter receives one 4-20mA input and regenerates two or more isolated outputs using internal drivers. Optical couplers or transformers break the direct copper ground path, so a fault or ground loop on one output does not corrupt the primary loop or another isolated output (NotebookLM, Analog Signal Standards notebook, 2026).

When Is Series Wiring Correct, and When Is a Splitter Worth It?

Choose series wiring when both receivers are electrically compatible, one continuous loop is acceptable, and the calculated burden plus cable drop leaves the transmitter enough voltage at its maximum current. Choose an active isolated splitter when those conditions fail, especially for separate grounds, single-ended inputs, excessive loop load, or required downstream independence (NotebookLM, Analog Signal Standards notebook, 2026).

A splitter moves the design boundary. The primary loop sees the splitter as its downstream load, while the powered device drives the secondary loops. That is useful where the original supply cannot carry all receiver burdens, where a higher supply would exceed a transmitter's maximum rating, or where different plant sections create unwanted ground relationships. It is also the safer topology when servicing one display must not interrupt the control input.

HART requires an additional device-specific check. The packet establishes that HART overlays a roughly 1 mA peak-to-peak AC FSK signal using 1,200 Hz and 2,200 Hz tones on the 4-20mA loop, but it does not establish how a particular splitter routes that digital overlay. If HART configuration or diagnostics must cross the distribution device, verify HART compatibility and pass-through in that device's documentation. The HART protocol guide explains the overlay and its point-to-point versus multidrop operating modes.

Citation capsule: Series wiring is appropriate only when total burden and wire resistance fit the loop voltage budget and all receivers can share one ground reference and one continuous path. An active isolated splitter is appropriate when headroom, common-mode voltage, single-ended inputs, or downstream failure independence prevent that arrangement (NotebookLM, Analog Signal Standards notebook, 2026).

Frequently Asked Questions

These questions resolve the checks that decide whether a two-receiver loop is safe: current must remain series-connected, the total burden must fit the voltage budget, and isolated outputs are needed when grounding or continuity cannot be shared between receivers (NotebookLM, Analog Signal Standards notebook, 2026).

Can one transmitter run two 4-20mA inputs?

Yes, if the passive receiver inputs are wired in series rather than parallel. In series, a 12 mA loop current passes through both inputs. Two equal 250 ohm inputs in parallel would instead split that 12 mA into 6 mA per input, creating wrong readings (NotebookLM, Analog Signal Standards notebook, 2026).

Do two 250 ohm receivers create a 1-5 V signal at both devices?

Each 250 ohm receiver develops 1 V at 4 mA and 5 V at 20 mA by (V = I \times R), so each presents the familiar 1-5 V burden range. Together they total 500 ohms and drop 10 V at 20 mA, before cable loss is counted (NotebookLM, Analog Signal Standards notebook, 2026).

Why is a 20 mA budget check not enough?

A transmitter may need to drive a high alarm above the normal range. The packet gives examples from 21.75 mA through 24.4 mA. Because burden and wire drop rise with current, a loop that holds 20 mA can still starve before it reports the higher fault signal (NotebookLM, Analog Signal Standards notebook, 2026).

Will an isolated splitter fix a ground loop?

A galvanically isolated splitter physically breaks the copper path between the input and output grounds. Optical or transformer coupling moves the measurement across the barrier, preventing circulating ground current from using the signal wiring. The packet describes continuous isolation ratings from 500 V AC to 1,500 V AC for industrial isolators (NotebookLM, Analog Signal Standards notebook, 2026).

Can I assume a splitter passes HART communication?

No. The packet explains that HART uses 1,200 Hz and 2,200 Hz FSK tones over the DC loop current, but it does not state how a specific isolator or splitter passes that bidirectional signal. Confirm HART pass-through against the exact splitter documentation before depending on it (NotebookLM, Analog Signal Standards notebook, 2026).

Citation capsule: Two receivers can share one 4-20mA transmitter only when their series burden, wire resistance, and grounding meet the loop requirements. Two 250 ohm receivers total 500 ohms and drop 10 V at 20 mA, while a HART-capable distribution path requires device-specific confirmation (NotebookLM, Analog Signal Standards notebook, 2026).

Conclusion: Choose the Topology That Preserves the Measurement

A series loop is a valid way to feed two compatible receivers. The transmitter regulates one current, so both burdens see the same 4-20mA value. The design passes only when the supply can cover transmitter minimum voltage, round-trip cable loss, and every receiver burden at the actual maximum loop current.

Use an active isolated splitter when the receivers cannot safely share that electrical life: separate grounds, single-ended returns, inadequate headroom, or a maintenance requirement for independent outputs. Start with the voltage calculation, then inspect the grounding and failure path. The correct topology follows from those checks, not from a preference for a shorter wiring diagram.

Citation capsule: A two-receiver 4-20mA design preserves the measurement when it uses either a properly sized series loop or a powered isolated splitter. Series connections share identical current but also share voltage, grounding, and continuity constraints; splitters regenerate independent loops when those constraints cannot be met (NotebookLM, Analog Signal Standards notebook, 2026).