Wiring a Weidmuller WDU 35: Ferrule Crimper, Multimeter, and Voltage Drop Checklist

If you've ever had a panel pass inspection and still fail under load, you know the frustration. I've been building and repairing control panels since 2017, and I've made my share of termination mistakes. In my first year, I sent a small batch of Weidmuller WDU 35 blocks to a machine builder with ferrules that looked fine but weren't. That mistake cost about $890 in rework plus a week of delay. Since then, I keep a checklist.

This one is for terminating stranded wire into a Weidmuller WDU 35 feed-through terminal block. It's not a substitute for the datasheet or your site's electrical safety rules. It's the practical list I run through when I want the connection to survive startup.

Step 1: Confirm the block and the old 3310 reference

The WDU 35 is a 35mm² feed-through terminal block. In older Weidmuller catalogs, the same style of block is sometimes listed as 3310. I still call it by the old number because that's what the job prints used when I started.

Before you do anything, verify the printed rating on the side. The rated current of a feed-through terminal block depends on the conductor and the test conditions in IEC 60947-7-1. A 35mm² block can carry a lot, but not if you land a smaller wire and treat it as if the block rating were the only number that matters.

Step 2: Use a Weidmuller ferrule crimper that matches the ferrule

For stranded wire, I almost always use a bootlace ferrule before landing it in a WDU 35. The terminal block doesn't require it, but the ferrule keeps the strands together and prevents a stray strand from shorting to the adjacent block. In a crowded panel, that's worth the extra step.

A Weidmuller ferrule crimper is not the only tool that works. It's just the one I trust because the dies are clearly marked and the ratchet won't release until the handle has made a full crimp stroke. Trust me on this one: a cheap cutter-style tool works when it works, and then fails quietly when you're in a hurry.

Step 3: Strip, insert, crimp, and check the insertion depth

Everything I'd read about crimping said the dies do all the work. In practice, that's only true if the strands are fully inside the ferrule sleeve. If the strip length is too long, copper sticks out past the sleeve and may touch a neighboring terminal. If it's too short, the strands don't reach into the crimp zone, and you get a connection that looks fine until it starts to carry current.

The most frustrating part is that a bad crimp can look identical to a good one. You'd think the ratchet would fix that, but it can't detect a folded strand. The boring fix is to insert the wire all the way, crimp in the marked die, then pull lightly on the ferrule. It should not move. If it moves even a little, cut it off and do it again.

Step 4: How to use a multimeter on the finished WDU 35

Now comes the part where most people stop. They tighten the screw, tug the wire, and walk away. Tugging only proves the wire is mechanically held. It doesn't prove the electrical path is good.

Here's how to use a multimeter on a terminated block without fooling yourself. Set the meter to continuity. Touch one probe to the conductor you just landed and the other probe to the terminal block's other side. If you get a beep, good. Then switch to ohms and do the same thing. A good connection on a short length of wire will read close to 0.0 ohms, but meter lead resistance is real, so measure the leads first and subtract that number.

Don't stop there. Continuity and resistance tests use a tiny test current. They won't show a high-resistance connection that only appears under load. If this is a power circuit, do the next step before closing the cabinet.

Step 5: Run a voltage drop calculator before you close the cabinet

The numbers said my first bad termination was fine. I had checked continuity, the wire gauge was oversized, and the terminal block was rated way above the circuit. My gut said something was still wrong because the circuit was intermittent. It turned out the ferrule was undersized for the wire, and a few strands had never made contact. I only found it by measuring voltage drop under load.

Use a voltage drop calculator with the actual one-way cable length, the expected load current, and the resistance per unit length of the wire. The basic formula is Vd = 2 x L x I x R / 1000 for copper wire with R in ohms per 1000 feet. If the result is more than 3% for a branch circuit, you may need a larger conductor before you worry about the terminal block.

For a terminated connection inside a panel, the more direct check is to measure across the terminal while the load is energized. If you see more than a few tenths of a volt across a WDU 35 with a properly crimped ferrule, something is wrong. I learned that after a 2022 startup where a PLC input kept dropping. The continuity test passed. The loaded test found a wire sitting in the terminal block with a screw tightened over only half the strands.

Common mistakes I'm still prone to making

Use a torque driver. A screw-clamp WDU 35 has a torque specification. I used to think 'firm' was good enough. Then I made a connection that worked at the bench but loosened after a few thermal cycles. Find the number on the datasheet and set the driver.

Don't mix ferrule manufacturers without checking the strip length. A 10mm strip length from one manufacturer might be 12mm in another. The crimping dies don't care, but exposed copper and the terminal's wire guard do.

If you are replacing blocks in an existing panel and the drawing says 3310 but the spare part packaging says WDU 35, confirm the printed rating before you install. I've seen older 3310 references mixed with newer WDU 35 datasheets, and the label is the tiebreaker.

And one more thing: if you're ordering only a small handful of these blocks, a decent distributor should take you seriously. The first Weidmuller order I placed was for about $200 of bits and pieces. They could have treated it like an annoyance. They didn't. That's the supplier I still use for much bigger orders.

Bottom line: the WDU 35 is forgiving, but wire prep isn't. Match the ferrule to the wire, use a real crimping tool, test with a multimeter, and run the voltage drop calculation before you power up. It's not a long list. It just has to be done every time.