110 Block vs 66 Block: Which Punch Down to Choose and How to Install

The Quick Answer

Both blocks are insulation displacement contact (IDC) terminations: you press an insulated wire into a contact slot, and a metal blade slices through the insulation to bite into the copper. The geometry, density, and signal performance are very different.

Comparing the Two Blocks

Performance side by side

When to Use a 110 Block

A 110 block (or 110-style termination on a patch panel or keystone jack) is the right choice for nearly any current installation.

• Structured cabling: Horizontal cable runs from the telecommunications room to work area outlets terminate on 110-style blocks at the patch panel.
• Voice over IP: Modern VoIP phones use Cat6 cable terminated to 110-style RJ45 keystone jacks. There is no need for a separate voice block.
• Backbone wiring: Multi-pair feeder cable between floors or buildings can terminate on 110 blocks at each end, with cross-connects to horizontal blocks.
• High-density distribution: Where rack space is limited, the higher pair density of 110 blocks fits more terminations in less space.

When to Use a 66 Block

The 66 block is now mostly a legacy product, but it still has specific roles.

• Existing telephone systems: Most pre-2000 commercial buildings still have 66 blocks in their telecom rooms. Service work on those systems requires terminating to 66 blocks.
• Analog voice in industrial environments: Some older PBX systems and intercom systems are wired exclusively to 66 blocks.
• Overhead paging: Distributed analog audio systems with multiple zones often use 66 blocks for cross-connecting amplifier outputs to speaker pairs.
• Cross-connect with bridge clips: 66 blocks support the M-block bridge clip system, which lets you connect or disconnect specific pairs without rewiring. Useful for moves, adds, and changes (MAC work) on legacy phone systems.

For new voice installations, use 110 blocks or run all voice over Cat6 to RJ45 keystones. The 66 block has no advantages over the 110 in modern designs.

Tools for Both Blocks

Punch down tool

You need a punch down tool with the appropriate blade for the block you are working on. Most quality impact-style punch down tools accept interchangeable blades. The Pro Punchdown Kit includes both 110 and 66 blades, so a single tool handles all your terminations.

Blade orientation

Each blade has a "punch and cut" side and a "punch only" side. The cut side has a small blade that trims the excess wire as the conductor seats into the IDC. Use the cut side for new terminations. Use the punch-only side when you need to add a tap (cross-connect) without trimming.

Installing a 110 Block

Step 1: Mount the block

Mount the 110 block to a backboard or to a wall. Use the screws or mounting holes specified for that block. Wall-mounted 110 blocks usually have molded standoffs that hold the block away from the surface, which leaves room for the wires to come in from behind or below.

Step 2: Lay in the cable

Bring each cable to the block and route it along the side. Strip about 1.5 inches of jacket. Lay the four pairs across the IDC slots, matching the printed color code (T568A or T568B - pick one and stay consistent across the entire job).

Step 3: Press the wires into the slots

Press each conductor into its color-coded IDC slot with your fingers. Get all eight conductors in place before reaching for the punch down tool. Keep pair twists as close to the block as possible. Untwist length should be under 0.5 inches.

Step 4: Punch down with the 110 blade

Using the punch and cut side, push the wire fully into the slot. The blade seats the conductor and trims the excess in one motion. Work through all 8 conductors of the cable.

Step 5: Install the connecting block (if applicable)

For 110-style blocks that use a separate connecting block on top, snap the connecting block into place after punching down the base wires. Then punch the patch wires into the top of the connecting block to complete the cross-connect.

Installing a 66 Block

Step 1: Mount the block

66 blocks are typically wall-mounted on a plywood telecommunications backboard. Use the included mounting screws. Some blocks come with a back box or a bracket; others mount directly with screws through tabs in the corners.

Step 2: Strip and dress the cable

Strip about 2 inches of jacket. Separate the pairs and route them to the appropriate row on the block. The 66 block has 50 rows, with each row split into a left side (4 contacts) and a right side (4 contacts). The two halves are not connected by default; a bridge clip joins them when you want to make a cross-connect.

Step 3: Lay the wires across the IDC contacts

Lay each individual conductor across its target IDC contact. Unlike the 110 block, the 66 block does not have color-coded slots. You decide the order based on your wiring map. The standard practice is to put the first pair in row 1, second pair in row 2, and so on.

Step 4: Punch down with the 66 blade

Use the 66 blade in your punch down tool. Press straight down on each contact. The cut side trims the excess wire. The 66 IDC contacts grip the conductor on the left and right, holding the wire securely while making contact through the insulation.

Step 5: Add bridge clips for cross-connects

To connect the left half of a row to the right half, snap an M-block bridge clip across the gap. The bridge clip provides a removable cross-connect that you can pull out and re-seat for moves and changes.

Cross-Connecting and Bridge Clips

Both block types support cross-connection between two cable runs at the same block. The mechanism differs.

110 cross-connect

On a 110 block with a separate connecting block on top, you punch the horizontal cable into the lower contacts and the cross-connect wire into the upper contacts. The connecting block routes the signal between the two layers. To change a cross-connect, you replace the connecting block, which preserves the underlying cable terminations.

Many 110 systems are now replaced by patch panels with RJ45 ports on the front. A patch panel is essentially a 110 block with a cleaner cross-connect interface (just plug a patch cable). For organized service work, patch panels are preferred over raw 110 blocks.

66 cross-connect with bridge clips

The 66 block's row design has a left half and a right half that are not joined by default. The horizontal cable terminates on one half, the destination cable terminates on the other half, and an M-block bridge clip joins the two halves to complete the circuit.

To disconnect a circuit, pull the bridge clip. The wires stay punched down on each side; only the cross-connect is removed. This is fast for moves, adds, and changes (MAC) on legacy phone systems where lines are frequently rearranged.

Documentation matters

Both methods require labeling. Use printed labels on each block row identifying the horizontal cable destination, the desk extension, the user name, or whatever convention your system uses. Hand-written marker labels fade and become unreadable. The fifteen minutes spent labeling a freshly punched block saves hours of troubleshooting later.

Modern Alternatives

For new installations, consider whether either block is the right answer at all.

Patch panel + keystone jacks

A patch panel with RJ45 keystone jacks at each end of every horizontal run is the modern equivalent of a 110 block. Each port has a 110-style termination on the back; cross-connections happen via patch cables on the front. This is what most enterprise installations use.

VoIP over Cat6

Modern voice runs on Cat6 cable to RJ45 keystone jacks. There is no separate "voice block" in a VoIP-only installation. The same horizontal cable infrastructure carries voice and data. A 110 block in the telecommunications room handles aggregation if needed.

Existing 66 block service

If you are servicing existing 66 blocks for legacy phone lines, plan for migration. Document everything before changes and consider whether the customer is ready to move to VoIP. The cost of migrating analog phone lines to VoIP is usually less than the cumulative cost of maintaining 66 blocks long term.

Common Mistakes

Wrong blade for the block

Using a 110 blade on a 66 block (or vice versa) does not seat the conductor correctly. The wire either does not go fully into the IDC, or the cut blade lands in the wrong position and damages the contact. Always check that your blade matches the block before punching down.

Excessive untwist on the 110

The 110 block is rated up to Cat6A, but only if you preserve pair geometry. Untwisting more than 0.5 inches at the termination kills NEXT performance. Keep the twist as close to the block as possible.

Skipping the test

Both blocks should be tested after termination. Plug a test cable into the patch side and run a wire map test. The VDV MapMaster 3.0 handles 110-block-terminated cables. For full Cat6A speed certification, the Net Chaser validates 10 Gbps performance through the block.

Mixing T568A and T568B in the same block

Easy mistake when multiple installers work the same closet. Pick a standard for the entire job and document it. Inconsistent wiring on a 110 block produces "crossed pair" errors that can be hard to track down.

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