Definition

Loop current

Also known as: loop start current, line current

Loop current is the direct current that flows around the local loop when an analog device goes off-hook, powered by the line's battery feed. Its presence tells both ends the line is in use, and many analog devices sense it to detect line state.

How it works

An idle loop-start line sits at roughly −48 V DC with no current flowing. When a phone, fax, or panel goes off-hook it closes the loop, current flows (commonly in the tens of milliamps, depending on loop length and equipment), and the serving equipment interprets the closure as a request for dial tone. Devices at the premises end likewise watch the current: a drop can signal disconnect, a fault, or — for equipment like answering machines and PBX ports — the cue to hang up.

Why it matters

Analog endpoints were engineered around real loop current, and some behave badly when a replacement line feeds too little, too much, or drops it at the wrong moments. A POTS replacement's FXS ports must supply loop current within the ranges the attached devices expect — one of the compatibility details worth checking per endpoint rather than assuming.

Common use cases

Fax machines and modems sensing line state before transmitting
Alarm panels detecting line trouble via current loss
PBX and answering equipment releasing a call on a current drop

Loop current is one of those details nobody discusses until a fax machine works on the old line and fails on the new one — at which point it is the first thing worth measuring.

Measuring it when something misbehaves

Loop current becomes interesting exactly once: when an endpoint that worked on copper does not work on the replacement. At that point it is measurable, and measuring it is faster than guessing.

Read the DC current on the loop with the endpoint off-hook, using a meter capable of in-line DC measurement, and compare against the endpoint’s documented requirement. Fax machines, older modems and some alarm panels state a minimum; the failure threshold is often above what a device designed only for voice supplies.

Two related values are worth checking at the same time. On-hook voltage, which some endpoints use to detect that a line is present at all. And ring voltage, since a device may answer calls placed from it while failing to detect incoming ringing.

Where the supplied current is genuinely marginal, the remedies are ordinarily a different port configuration, a shorter or heavier inside-wire run, or a different appliance model — not a setting on the endpoint, which is usually expecting what the standard specified.

Common misconceptions

“If a line has dial tone, loop current must be correct.”

Dial tone proves the audible path; it says nothing about whether the current level and its behavior during connect and disconnect match what a particular device needs. Marginal current can produce intermittent, hard-to-diagnose failures.

Frequently asked questions

Do cellular POTS-replacement appliances provide loop current?

Yes — their FXS ports generate battery feed and loop current locally, exactly so analog devices keep working. The delivered levels and disconnect behaviors are model-specific and documented per product.