4-20 mA loop power is the elegant trick behind the two-wire transmitter: one pair of wires delivers the device's power and carries its measurement signal at the same time. It is why so much field instrumentation needs only a single twisted pair. This guide explains how loop power works, the difference between two-wire and four-wire devices, sink versus source wiring, and what loop burden means in oil and gas.
4-20 mA Loop Power in one line: 4-20 mA loop power is a scheme where a two-wire transmitter draws its operating power from the same current loop it uses to signal. A DC supply drives the loop, and the transmitter regulates the current between 4 and 20 mA to represent its reading while living on that current.
A loop-powered transmitter has just two terminals. A DC power supply - commonly 24 V - sits in the loop and drives a current through the transmitter, its wiring, and a load resistor at the input. The transmitter does not have a separate power feed; instead it controls how much current flows, regulating the loop between 4 and 20 mA to represent its measurement. Its own electronics must run entirely on that current, which is why it must draw at least 4 mA even at the bottom of the range.
That live-zero of 4 mA is deliberate: it guarantees enough current to power the transmitter and, usefully, makes a broken wire (0 mA) distinguishable from a genuine minimum reading. The whole measurement and the device's power share one two-wire pair.
A two-wire (loop-powered) device is limited to the small power budget the 4 mA minimum allows, which suits simple transmitters. A four-wire device has separate terminals for power and signal - two wires for a dedicated supply and two for the 4-20 mA output. Four-wire is used when the instrument needs more power than a loop can provide, such as an analyzer, a display-heavy device, or one driving multiple outputs.
The trade is wiring and power: two-wire minimizes cabling and is the default for basic field transmitters, while four-wire adds a power run but removes the power-budget limit. Selecting the right type is part of instrument specification.
When wiring a loop you must match how the transmitter and the input handle current direction - sinking versus sourcing. A sourcing output pushes current out; a sinking input pulls it. Mismatched wiring means the loop will not conduct, so the device documentation defines which is which. Getting this right is a common field wiring detail.
Loop burden is the total resistance the supply must drive - the input load resistor plus wiring resistance plus any HART resistor, isolators, or barriers in the loop. The supply voltage must be high enough to push 20 mA through all of it while leaving the transmitter its minimum operating voltage; otherwise the loop saturates below full scale. A cloud SCADA such as Merobix reads the value the controller digitizes from this loop - the loop power and burden are field electrical details handled below the SCADA layer.
It draws power from the same 4-20 mA loop it signals on. A DC supply drives current through the loop, and the transmitter runs on that current while regulating it between 4 and 20 mA to represent its reading. That is why it must draw at least 4 mA even at zero.
A two-wire device draws power and signals over one pair, limited to a small power budget. A four-wire device has separate power and signal wiring, so it can use more power - suited to analyzers and complex instruments - at the cost of an extra cable run.
Loop burden is the total resistance the supply must drive: the input load resistor plus wiring, any HART resistor, isolators, and barriers. The supply voltage must push 20 mA through all of it while leaving the transmitter its minimum voltage, or the loop cannot reach full scale.
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