Double taps - what's the difference in these?

Was looking over my pictures from today’s inspection… A house built in 1964 with a FedPacific panel. When opened, I first saw a circuit double-tapped, but when looking at the pic I thought what’s the difference between having 2 solid-strand conductors connected to the breaker, and 1 multi-strand conductor in the breaker below it? One is forbidden, and one is allowed, but isn’t it virtually the same physical connection?


Following the manufacture’s installation instructions or not.

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They’re not the same physical connection in that the terminal has been tested and listed for the single stranded conductor. It has not been listed and tested for the two solid conductors. I do see your point if you twisted a few small solid conductors together it would resemble the stranded conductor.

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That was my point… the two connections are almost identical. In fact, the multi-strand connection would seem to me to be MORE unstable than 2 solid-strand conductors pushed into the same connection… Just making an observation…

It’s kind of immaterial with a FP panel. The whole thing is a fire waiting to happen! Don’t let anybody tell you they’re okay. “it’s been that way for 40 years.” The problem with those panels that so often goes undetected is that the breakers simply will not trip. It’ll be OK till the breaker is actually needed by an overload and then the fire starts.

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Multi-stranded wiring compresses a lot better than two solid wires.

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I agree - in fact I told my client that same thing. Panels can live forever with no issues if the circuits never encounter an overload or problem. Russian Roulette!

Visualize the two single strand wires in a vise or a pair of pliers. How much of the wire is in direct contact with the vise/pliers?
Not that much.
This small surface connection is basically reducing the wire size capacity at that connection. As amperage draw is pulled through this small contact area, it heats up.

Two single strand wires, side by side; If one of the wires is loose, the amperage of both circuits must pass through the one small good connection point. Twice the heat.

As Robert pointed out, twisting the two makes things better, but still, the breaker lug was not designed for this.

Next time your at a big box store, take a look at a Sq D breaker designed for a multi-tap connection. It may be more clear.

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I think this is what @dandersen is referring to.

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I guess I wasn’t clear - I understand why double-tapping is bad… and how it can be safely done with the appropriate breakers. My point was - how is clamping a multi-strand wire into the connection absolutely safe? Isn’t the connection much like a double-tapped connection with 2 or 3 or 5 wires? When you look at my pic above, doesn’t the multi-strand connection look just like the double-tap, but just with more wires? I would think it would behoove (I like that word!) an installer to make sure that multi-strand connection is torqued down appropriately… and us to check that in an inspection.

How would you check that?

And, that would apply to all electrical connections, not just stranded wire.

Therefore I look for evidence of overheating. I do not complicate this process at the HI inspector level. Decolorization, scorching or melting or a breaker that is hot to the touch (which is a judgement call because some breakers will naturally be warm. Or conversely cold because it is not under a load). Some inspectors will break out their thermal imaging device, but that is another can o worms.

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Sort of.

In the multistrand cable case ANY of the wires that actually connect will conduct current to the cable. There’s plenty of copper mass nearby to dissipate heat.

In the case of a doubletap, EITHER wire may become poorly connected, and ALL the current has to pass into that wire.


The final answer for an inspector is “the vendor did not certify that connector for a double tap”.

Personally, I feel uncomfortable with stranded wire in a screw connection: over time the wires can shift. Depending on context: I’ve been known to run a screwdriver down a row of breakers “checking” the torque on each screw and actually tightening the loose ones. The older the panel, the more the screws turn.

Of course you should ALWAYS look for evidence of heat on the connections. Something loose leads to heat and quite often the problem will be evident long before it catches fire.

Federal Pacific stab-lok is some bad mojo. One of the service electricians I used to work with brought me some half-charred breakers from an emergency call years ago I wish I still had access to the pictures. Half of the panel was burned up, with most of the breakers burnt to a crisp and nothing but crumbling char left on the lower portion of them. Not one of them tripped he said even after a nice fire extinguisher bath.

Brian, I agree - just food for thought. i think from now on, I’ll just look at those connections, maybe a little more carefully than I have in the past.

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I became a real believer several years ago - did a 4-Point inspection for a little old lady. Her husband had passed away 10 years prior. He had a wood-working shop out back that no one had even touched since he passed. When I opened the Fed Pacific panel, I found the breaker that led to his shop sub panel was melted… At the sub panel - Square D - everything was fine. She was so scared, she had an electrician out that day to replace the Federal pacific equipment!

Say what you will about fuses… but they don’t fail.

Even the “replacement” FPE breakers are apparently defective:
https://inspectapedia.com/fpe/FPE-UBI-Replacement-Circuit-Breaker-Tests.php

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Seimans released a whitepaper regarding this issue.

Very interesting - thanks for sharing.

As I posted, it is about significant contact at the connection.
Milti-strand strand wires has more surface conductivity than single strand. If you could measure and compare the total surface contact of milti-strand wire, you would see that it is higher than single strand.

Also, multi-strand is a more efficient conductor across it’s length, thus reducing amperage draw.

You can see in your picture that you referenced, multi-strand compresses under the required torque, where single-strand does not. Smashing smaller strands together results in better connection.

Compare


max amperage wire charts between different wire types.

Some wire performs better than others, and also cost more to buy and install…

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There is no real world performance difference in solid versus stranded conductors. Both have the same ampacity for the same size. In fact over the same length solid conductors have a slightly lower resistance than stranded conductors. It is possible that based on the design of a terminal that one may be slightly better at the termination than the other but the NEC does not differentiate between the two conductor types when considering the conductor ampacity.

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