LowVoltCalc
Tools

Cable run length calculator

Will this run make it, and if not, how many switches or extenders go in the middle?

–

Limit for this medium
–
Segments
–
Each segment
–
Permanent link
Most cord allowed
–

ft
Units
ft

How this works

Each medium has a longest supported segment. The calculator divides your run by it: a run that fits in one segment passes, and a longer one needs a device in between for each extra segment: a switch for Ethernet and PoE, an extender or repeater for the rest.

segments = run length ÷ limit, rounded up devices in between = segments − 1

The 100 m Ethernet channel

TIA-568 defines the channel as at most 100 m (328 ft): a permanent link of up to 90 m (the horizontal cable in the wall) plus up to 10 m of patch cords at both ends. Cords lose more signal per metre than solid cable, so thinner cords shorten what is allowed. TIA-568.0-E (2020) gives the formula:

cord length ≤ (102 − permanent link) ÷ D D = 1.0 (22/23 AWG), 1.2 (24 AWG), 1.5 (26 AWG), 1.95 (28 AWG, 15 m at most)

PoE is specified over the same channel, so a PoE run has the same 100 m limit; the power budget is on the PoE calculator.

Faster Ethernet

2.5GBASE-T runs 100 m on Cat5e and 5GBASE-T 100 m on Cat6 (IEEE 802.3bz). 10GBASE-T needs Cat6A for 100 m; on unshielded Cat6 it is limited to 55 m, and to 37 m where alien crosstalk is severe (TIA TSB-155). Past 37 m on Cat6, the calculator asks for a field test.

HDBaseT

HDBaseT's reach depends on the version and on the extender class. The full specification reaches 100 m; Class B ("Lite") extenders, which are cheaper and common, reach less. Check the extender's own datasheet, which may be shorter still.

What this does not account for

  • Cable that is not to category, such as copper-clad aluminium, or a channel that fails certification. A field test is the only proof of a channel.
  • Voltage drop on long powered runs. For 12/24 V power, use the voltage drop calculator.
  • Vendor extenders and "long-range" PoE modes beyond the standards.
  • Wiegand and RS-485 limits depend on the reader, controller and cable; the figures here are the manufacturers' and Texas Instruments' published guidance.

Common questions

Is the Ethernet limit 100 m or 90 m?
Both: 100 m for the whole channel, of which at most 90 m is the permanent link. With the usual 10 m of cords, the cable in the wall can be 90 m.
Does PoE shorten the distance?
No. PoE is specified over the full 100 m channel. It does lose some power in the cable, which the PoE budget calculator counts.
How far can RS-485 go?
About 1,200 m at low data rates. At higher rates, bit rate × length should stay under 108, so 1 Mbit/s reaches about 100 m.

Sources

  1. ANSI/TIA-568.0-E (2020), 5.10.2.2 and formula (3): channel, permanent link and cord derating.
  2. IEEE 802.3bz-2016 as described by Fluke Networks; Ethernet Alliance, 10GBASE-T white paper (2007), Table 3, and TIA TSB-155.
  3. HDBaseT Alliance, HDBaseT technology; ATEN VE901 (HDBaseT Class B) datasheet.
  4. Texas Instruments, SLLA070D, RS-422 and RS-485 Standards Overview and System Configurations (2010).
  5. Wiegand: OSSI and DSX published cable specifications.

Every source used on this site, and how each number is used.