Twisted-Pair Cable Categories — Cat 5, 5e, 6, 6a, 7, 8

The category table

Category	Max speed	Max distance @ max speed	Frequency	Typical use
Cat 3	10 Mb	100 m	16 MHz	Legacy voice / 10BASE-T. Dead.
Cat 5	100 Mb	100 m	100 MHz	Deprecated. Any surviving Cat 5 should be re-pulled.
Cat 5e	1 Gb	100 m	100 MHz	Minimum you’d install today. Common in older buildings.
Cat 6	1 Gb / 10 Gb	100 m / 55 m	250 MHz	Common enterprise. 10 Gb only at short distance due to alien crosstalk.
Cat 6a	10 Gb	100 m	500 MHz	The current “safe” enterprise choice. Full 10G over full run.
Cat 7	10 Gb	100 m	600 MHz	Requires GG45 or TERA connector (not RJ45). Rare in the US.
Cat 7a	10 Gb (up to 40 Gb short)	100 m / short	1000 MHz	Same connector caveat.
Cat 8	25 / 40 Gb	30 m	2000 MHz	Data-center top-of-rack to server. Not a general campus cable.

The shielding letters

You’ll see labels like U/UTP, F/UTP, S/FTP. The two letters mean:

• First letter (before the slash) — overall cable shielding.
• Second letter (after the slash) — per-pair shielding.

Values:

• U = unshielded
• F = foil
• S = braided screen

So:

Label	Overall	Per pair	Meaning
U/UTP	none	none	Plain UTP. Cat 5/5e/6 standard.
F/UTP	foil	none	Foil around all four pairs. Cat 6a option.
S/UTP	braid	none	Braid around all four pairs.
U/FTP	none	foil	Foil per individual pair. Cat 6a / Cat 7.
F/FTP	foil	foil	Double-shielded. Cat 6a for high-EMI.
S/FTP	braid	foil	Braid + foil per pair. Cat 7 / Cat 7a.

Rule of thumb: higher category + high-EMI environment (factory floor, hospital MRI, PoE++) → prefer shielded. Home / typical office → UTP is fine and easier to terminate.

PoE and cable heating

At 802.3bt PoE++ (up to 90 W), 4-pair copper runs get warm. Ampacity rises. For high-power PoE:

• Use Cat 6a or Cat 6 with all four pairs.
• Prefer shielded for bundled runs (heat dissipation + crosstalk).
• Don’t bundle more than 24 PoE cables tightly — heat.
• Check that your patch cables match category rating; a Cat 5e patch on a Cat 6a link becomes the weakest hop.

Common exam / real-world mistakes

1. Assuming Cat 6 does 10 Gb over 100 m. It doesn’t — capped at 55 m. If the exam says “10 Gb over 90 m”, the answer is Cat 6a or better.
2. Confusing Cat 7 with RJ45. Cat 7 uses GG45 or TERA — NOT RJ45. Practically, most “Cat 7” installs in the US use Cat 6a electrically. Cat 8 is what you actually see terminated with RJ45 in DCs.
3. Ignoring the patch-panel and patch-cord. A rated Cat 6a horizontal run degrades to Cat 5e speed if the patch cords aren’t the same rating.
4. Overlooking bend radius. Cat 6a bent tighter than 4× cable diameter loses electrical characteristics permanently.
5. Forgetting that “shielded” needs a ground path. F/UTP with no ground bond at the patch panel does nothing.

Cheat strip

Cat 5    100 Mb  100 m   16 MHz   dead
Cat 5e   1 Gb    100 m   100 MHz  minimum today
Cat 6    1 Gb    100 m   250 MHz  or 10 Gb / 55 m only
Cat 6a   10 Gb   100 m   500 MHz  safe enterprise choice
Cat 7    10 Gb   100 m   600 MHz  GG45/TERA connector, rare US
Cat 8    25/40G  30 m   2000 MHz  DC top-of-rack

U/UTP    plain UTP                Cat 5/5e/6 default
F/UTP    foil overall             Cat 6a option
U/FTP    foil per pair            Cat 6a / Cat 7
S/FTP    braid overall + foil pair Cat 7/7a — max shielding

PoE++    prefer Cat 6a shielded. Don't over-bundle.
Bend     radius >= 4x cable diameter.