Current Rating and Interface Bandwidth for Conventional PA Systems

Quick answer

A "current rating" question on a conventional public-address (PA) system splits into two different calculations that are often confused. On the loudspeaker side, line current is set by the total power tapped on a constant-voltage (100 V / 70.7 V) line and follows directly from Ohm's law: I_line = P_load ÷ U_line. On the networked side, "bandwidth" is not a current at all — it is the payload rate of the digital audio link or of a control/VoIP integration, and it is rated in bit/s, not amperes. This article defines both quantities, gives the working formulas, shows a worked example, and then lists where the formulas stop being trustworthy.

1. Two parameters that share one question

1.1 Line current rating (analogue, constant-voltage)

Parameter definition: the RMS current that a loudspeaker line draws from the amplifier output at a stated line voltage and stated total load. Units: amperes (A), RMS, at the nominal line voltage (for example 100 V). It is a derived figure, not usually a nameplate rating on speakers — loudspeaker taps are marked in watts precisely because that keeps the arithmetic independent of the line voltage in use.

1.2 Interface bandwidth (digital or control)

Parameter definition: the data rate required to carry the audio and/or control streams of the system over a network or interface. Units: bit/s (kbit/s, Mbit/s, Gbit/s). For a VoIP-paging or networked-audio integration this is the codec payload rate; for control buses it is the message rate. There is no "current" here, and no Ohm's-law shortcut.

2. Formulas and variables

2.1 Loudspeaker line current

I_line = P_load ÷ U_line (RMS amperes)
P_load = Σ (tap wattage × number of loudspeakers on the circuit)
R_line = U_line² ÷ P_load (the impedance the amplifier "sees")
U_line must be the same nominal voltage the taps were chosen for (100 V taps on a 100 V line).

Because P = U·I, a 100 W load on a 100 V line draws 1 A RMS and presents 100 Ω; the same 100 W load on a 70.7 V line draws about 1.41 A and presents 50 Ω. The power is the same; the current is not. Never carry a tap figure across line voltages — a tap's wattage is defined at its own nominal voltage.

2.2 Amplifier capacity in watts, not amperes

Amplifier capacity is specified in watts of rated output, at a stated distortion limit and load, so sizing is done in watts: do not connect more tapped load than the amplifier's rated output, and in practice keep a margin so that transformer insertion loss, cable loss and tap tolerances do not push the amplifier into clipping. The size of that margin is a design decision, not a standard value — it depends on programme type, loudspeaker tap granularity and how much load can be muted or isolated during paging.

2.3 Cable loss per run

The line is a two-wire circuit carrying real current, so voltage drop matters:

Loop resistance R_loop = 2 × ρ × L ÷ A (ρ ≈ 0.0175 Ω·mm²/m for annealed copper at 20 °C; L in metres of run; A in mm²)
Fraction of voltage lost ≈ R_loop ÷ (R_loop + R_line)
Power lost ≈ I_line² × R_loop

Loss rises with the square of current for a fixed cable, which is why long 100 V runs are sensitive to cable cross-section even though the current looks small.

2.4 Digital link bandwidth

For an uncompressed PCM channel: bit rate = sample rate × bit depth × channel count. For compressed or packetised audio, use the published stream bit rate per channel, add packet overhead, and add a provisioning allowance for control, monitoring and other traffic. Both inputs — sample rate/depth and stream rate — are specification items to read from the device datasheet rather than assume.

3. Worked example (illustrative arithmetic, not a product specification)

Take a paging zone on a 100 V line with six ceiling loudspeakers tapped at 6 W and four horn loudspeakers tapped at 15 W:

P_load = (6 × 6) + (4 × 15) = 36 + 60 = 96 W
I_line = 96 ÷ 100 = 0.96 A RMS
R_line = 100² ÷ 96 ≈ 104 Ω
Cable: 40 m run of 2 × 1.5 mm² → R_loop = 2 × 0.0175 × 40 ÷ 1.5 ≈ 0.93 Ω → voltage loss ≈ 0.93 ÷ (0.93 + 104) ≈ 0.9 % → power lost ≈ 0.96² × 0.93 ≈ 0.86 W
Against an amplifier rated at 120 W, the 96 W tapped load leaves 20 % of rated output unassigned. That is a common starting point for a voice-paging zone, but it is not a rule of any standard.

Repeat the same load on a 70.7 V system only if the taps are 70.7 V taps: 96 W ÷ 70.7 V ≈ 1.36 A.

4. Units and assumptions you must state

RMS vs peak: the I = P/U relation uses RMS values for a stated waveform. It is not a peak-current rating, and not a surge rating for inrush or DC offset.
Test conditions of the wattage figure: an amplifier's rated output is only comparable when the distortion limit, bandwidth, load impedance and test duration are the same. IEC 60268-3 (edition 5.0, 2018-04) defines amplifier characteristics and their measurement methods, and datasheets normally quote the condition alongside the watt figure.
Tap tolerance and losses: transformer taps carry a tolerance band and a transformer insertion loss; the tapped wattage is a nominal figure, not measured delivered acoustic power.
Programme content: a sine-wave test signal and speech/music have different crest factors, so the long-term thermal load on a line is lower than the mathematical peak for real programme material.
Supervision and emergency operation: voice-alarm systems add requirements on line supervision, fault isolation and standby supply that go beyond load arithmetic — they come from the applicable alarm-system standards (EN 54 series), not from the PA calculation itself.

5. How to read the specification

Read the load figure as the total tapped wattage per amplifier output, per zone and per circuit — three different numbers if the design is zoned.
Read the current figure as a cable-sizing and terminal-rating parameter, not as an amplifier sizing parameter.
Read the bandwidth figure as a network and codec parameter. A purely analogue 100 V system has no IP bandwidth requirement; a system with IP paging stations and a networked-audio backplane has one per station and per link.
When two datasheets give "power" without a distortion limit and load, treat the comparison as unusable rather than as approximate.

6. Where the formulas stop working

Multiple amplifier outputs: the simple division assumes one source driving one line; ratings add per output, not per chassis.
Branched lines: branch currents combine with differing cable resistance; calculate per branch and sum the loads rather than averaging currents.
Low-frequency content: transformer loading at low frequencies can differ markedly from the mid-band nominal tap figure; a 100 V line sized for paging is not automatically a full-range music line.
Supervised lines: DC supervisory current or pilot tones are not the audio current, and a line's fault-detection threshold is a separately specified parameter.
Digital links: the bit-rate formula gives payload only. It says nothing about latency, jitter, switch capacity or whether the network is provisioned for prioritised audio traffic.

7. A short worksheet

List every loudspeaker per circuit with its tap voltage and tap wattage.
Sum the wattages → P_load.
Divide by the line voltage → I_line; compute R_line if the amplifier specifies a minimum load impedance.
Compute loop resistance from cable length and cross-section → voltage drop and power loss as a percentage.
Compare P_load with the amplifier's rated output at its stated test conditions, and record the margin you chose.
For any networked element, record the published stream bit rate per channel and the control/monitoring overhead separately.

For related specification reading on load, output and test-condition issues, see the other entries in Performance & Capacity.

Source and version record

IEC 60268-3:2018, Sound system equipment – Part 3: Amplifiers, edition 5.0, 2018-04 — defines amplifier characteristics and measurement methods; consult the standard itself for the exact test conditions before comparing wattage figures. IEC Webstore
EN 54-16 and the wider EN 54 series — source for the supervision, fault-isolation and standby requirements referred to in section 4; the applicable edition is the one cited in the project specification.
Copper resistivity value used: ρ ≈ 0.0175 Ω·mm²/m at 20 °C (annealed copper), a material constant rather than a product rating.
Document date: 2026-11-12. All worked figures above are illustrative arithmetic on the stated inputs, not measurements of any product.