Quick answer: A hundred and fifty feet is the length where old Cat 5 starts to show its age. Resistance climbs steadily with distance, while crosstalk is decided almost entirely by the terminations at each end. Together they set the real ceiling on this run. On a reused reel, the honest test is a channel test with the actual patch leads attached, not a guess based on the category printed on the jacket.
Reels of Cat 5 turn up in garages and store rooms, usually left over from a job nobody remembers. A hundred and fifty feet is exactly the length people try to squeeze out of one, because it reaches the shed or the far office without a joint. It often works. It also fails in ways that look like a device fault rather than a cable fault.
The resistance curve over one hundred and fifty feet
Resistance is the honest, boring number. It rises with length, it rises faster on thinner conductors, and it rises again at every mated pair. Nothing about it is mysterious, which is why it is the first thing to check when a run behaves differently from a short bench test of the same cable.
Where the voltage goes
If the far end is a powered device, resistance shows up as heat along the cable and a lower voltage at the device. A camera that restarts when its infrared LEDs switch on is telling you the supply path is marginal, not that the network is congested.
Age and moisture in stored cable
Cable that sat in a damp shed for years may have moisture inside the jacket and corrosion on any exposed copper. A visual check of the cut end tells you a lot: green tint or dark spots mean the reel is scrap before you pull it.
Crosstalk is a termination story
Over a hundred and fifty feet the pairs have plenty of opportunity to talk to each other, yet in practice the worst coupling still happens within an inch of each plug. That is where the pairs are untwisted to reach the contacts, and that is where a rushed job shows up.
What a tester reports
A channel test reports each measurement separately, which is useful because the numbers point at different faults. High crosstalk at one end means the termination there. High insertion loss across the run means the cable itself.
Patch leads count as part of the run
The test should include the leads you will actually use. Testing a bare link and then adding a thin patch cord at each end is how a passing result becomes a marginal install.
Reading the result before you install
Four measurements cover almost every failure at this length.
| Measurement | What it tells you | Cause when poor | What to do |
|---|
| DC loop resistance | Copper condition and length | Damaged or thin conductors | Shorten the run or re-pull |
| Insertion loss | Total signal reaching the far end | Length, joints, thin gauge | Remove joints first |
| Near end crosstalk | Coupling at one termination | Untwisted pairs in a jack | Re-terminate that end |
| Return loss | Impedance consistency | Kinks, crushed jacket, bad plug | Replace the damaged section |
Practical scenarios
- Garden office link: a hundred and fifty feet is enough with a service loop at each end, and both ends land on keystones rather than field crimped plugs.
- Desk phone in a back room: the phone is powered over the same cable, so voltage headroom matters more than the speed rating on the box.
- Reused reel from a store room: test a sample length first. If one cut end looks corroded, the whole reel is a risk.
- Switch to switch uplink: this is the one place where a newer category pays for itself, because the link carries everyone else traffic.
FAQ
Is 150 feet too long for Cat 5?
It sits inside the channel allowance, so length alone is not the issue. Whether the link holds a modern gigabit rate depends on the cable condition and the quality of both terminations.
Why does the link drop only under load?
Load raises current and temperature together. A marginal joint gets warmer, resistance rises, and the error rate climbs. That pattern almost always points at a termination rather than at the middle of the cable.
Should I replace old Cat 5 instead of testing it?
If the cable is already inside a wall and the route is difficult, test it and keep it when the numbers are healthy. If it is loose in a garage, a new reel costs less than the afternoon you would spend chasing faults.
Next step
Decide with measurements rather than with the label on the jacket. The Cat 6A specification overview explains what each test measures, the shielded cable guide covers routes that need extra protection, and the Cat 6 versus Cat 6A comparison shows when a newer category is the cheaper answer.
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