M-27500 wire specs

[tmo]

The PMA450 I have requires MIL-27500 wiring; the spec as explained at https://www.wiremasters.net/products/mil-spec-cable/m27500-anatomy-guide consists of the 'M27500' header, '-' as the color code, 'TG' as the wire spec, '2' as the number of conductors, 'T' as the shield type, and '14' as the outer jacket designator.

My question is if I can use a different wire spec, so instead of M27500-24TG2T14 can I use M27500-24SB2T14 or M27500-24TE2T14 which is easier to obtain here for reasonable money. A foot of M27500-24TG2T costs $3 over here as opposed to $1.35 at US Aircraft Spruce.

[carusoam]

Interesting technical question…

Let me see if Greg is around…. @Baker Avionics

Greg probably knows these kinds of wire details…

Sometimes @Marauder knows some wire detail like nobody else… but I’m not sure how much wire info he may have…

Best regards,

-a-

[PT20J]

 

On 7/28/2021 at 11:03 AM, tmo said:

The PMA450 I have requires MIL-27500 wiring; the spec as explained at https://www.wiremasters.net/products/mil-spec-cable/m27500-anatomy-guide consists of the 'M27500' header, '-' as the color code, 'TG' as the wire spec, '2' as the number of conductors, 'T' as the shield type, and '14' as the outer jacket designator.

My question is if I can use a different wire spec, so instead of M27500-24TG2T14 can I use M27500-24SB2T14 or M27500-24TE2T14 which is easier to obtain here for reasonable money. A foot of M27500-24TG2T costs $3 over here as opposed to $1.35 at US Aircraft Spruce.

Don't you just hate it when one spec. references another spec.? I looked this up as I am going to be installing a PMA450B soon, also.

According to the wiremasters link, TE signifies wire meeting SAE AS22769/16, TG is SAE AS22759/18 and SB is SAE AS22759/32. You can look the actual specs up on https://www.rdswire.com/. The conductors are all the same for 24 AWG: Tinned copper, 19/36 stranding, 26.2 ohms/1000'. The difference is the insulation. All are ETFE (a.k.a. Tefzel) and all are listed with a voltage rating of 600 volts and a temperature rating of 150C. All are described as applicable for aerospace applications. The difference is the thickness of the insulation: TE is .045" dia., TG is .037" dia., SB is .035-.039" dia. (these are the dimensions for each conductor). 

For this application, they should all be interchangeable. The TE cable would end up being larger in diameter than TE; SB would be about the same as TE.

Skip

[tmo]

Thank you. Pushing my luck, would a wire that is in ETFE covering, similar diameter, voltage and temperature parameters, but not outright M-27500, be acceptable as a "functional equivalent"? Or would that be going too far?

[Yetti]

The one thing to think about is the connectors on the 450b are sub D.   So small.   things get a little tight so the extra thickness on the insulation does matter.   But not a deal breaker.   Because you probably won't hook all the features up.

[tmo]

For me, it's just the phone/mic connections, so even less to worry about; I'm hopefully not redoing the whole loom at this time, just re-pinning it from the KMA24 harness. Existing wire should be compliant...

[PT20J]

7 hours ago, tmo said:

For me, it's just the phone/mic connections, so even less to worry about; I'm hopefully not redoing the whole loom at this time, just re-pinning it from the KMA24 harness. Existing wire should be compliant...

I just looked at the installation manual for the PMA450B and all it says is MIL-C-27500 or equivalent. They are primarily concerned with the quality of the shielding; the insulation of the individual wires is not an issue. You can use whichever is most readily available. Just be sure to use separate wires for the mic and phones and insulate the jacks from the airframe so that the only ground is at the audio panel.

Skip

[tmo]

Thank you for confirming what I hoped for when I read "or equivalent". I can order nicely shielded 2 and 3 conductor wire from a local manufacturer, for a reasonable price, so I'll go with that.

Affirm on the separate wire runs, isolating the jacks and grounding the shield at the audio panel.

Now I need to source some good high density pins for the connectors.

Thanks again, I really appreciate it!

[0TreeLemur]

3 hours ago, PT20J said:

I just looked at the installation manual for the PMA450B and all it says is MIL-C-27500 or equivalent. They are primarily concerned with the quality of the shielding; the insulation of the individual wires is not an issue. You can use whichever is most readily available. Just be sure to use separate wires for the mic and phones and insulate the jacks from the airframe so that the only ground is at the audio panel.

Skip

Skip- let me get this straight, because I'm thinking about running wires to the back seat jacks in advance of an upgrade to the same audio panel.   For each back seat jack, run one shielded (1 or 2?) -conductor cable for the mic, and one shielded 3-conductor cable for stereo audio?

Thx,

Fred

[PT20J]

1 hour ago, 0TreeLemur said:

Skip- let me get this straight, because I'm thinking about running wires to the back seat jacks in advance of an upgrade to the same audio panel.   For each back seat jack, run one shielded (1 or 2?) -conductor cable for the mic, and one shielded 3-conductor cable for stereo audio?

Thx,

Fred

You want to keep the mic and phone circuits in separate cables to preclude possible interference since the mic signal is much lower level than the phones. The simplest way to think of it is to run a separate shielded cable for each jack (2 conductor for mics, 3 conductor for stereo headphones) all the way back to the audio panel and ground the shields at the panel end to avoid ground loops.

Skip

[0TreeLemur]

1 hour ago, PT20J said:

You want to keep the mic and phone circuits in separate cables to preclude possible interference since the mic signal is much lower level than the phones. The simplest way to think of it is to run a separate shielded cable for each jack (2 conductor for mics, 3 conductor for stereo headphones) all the way back to the audio panel and ground the shields at the panel end to avoid ground loops.

Skip

Thanks!  On first reading I was confused because the M20 is a nose dragger- then realized that you were talking about the other "ground loop"! 

[Baker Avionics]

On 7/28/2021 at 10:38 PM, carusoam said:

Interesting technical question…

Let me see if Greg is around…. @Baker Avionics

Greg probably knows these kinds of wire details…

Sometimes @Marauder knows some wire detail like nobody else… but I’m not sure how much wire info he may have…

Best regards,

-a-

I think @Mscheuer should chime in for this one, personally!  Mark, how have you been and please educate us!  Why is this the type of wire you all specified and can other types be used?

[Mscheuer]

On 7/30/2021 at 10:12 PM, 0TreeLemur said:

Skip- let me get this straight, because I'm thinking about running wires to the back seat jacks in advance of an upgrade to the same audio panel.   For each back seat jack, run one shielded (1 or 2?) -conductor cable for the mic, and one shielded 3-conductor cable for stereo audio?

Thx,

Fred

There are no 1 conductor shielded cables depicted in any of our wiring diagrams.  

It is imperative, that the installer of audio panels follow our wiring diagrams exactly.

When customers call us indicating there is a problem with their audio panel, almost 99.8% it is never the audio panel but the installation. Not everyone has the skills, tools, or the knowledge required to perform the installation. Audio panels touch every piece of the avionics suite, they are the most time consuming harness you can install.

When installed correctly, it also has the lowest cost of entry into a whole new world of avionics. Everything you hear, talk, or respond to comes from your audio panel.

When someone download our install manuals, that person has to acknowledge that there is no warranty unless installed by a PS Engineering Authorized dealer or a custom harness is built by our authorized dealer. (note: if replacing a GMA340 or GMA345, that individual simply needs a log book entry by an A & P and warranty is valid) Approach Fast-Stack system is also recognized as a great solution that we will provide warranty for our products.

We have had this policy since 1990. Why? When the audio panel has problems, the panel is the first to be blamed. It makes us look bad and the customer is unhappy. When any of our audio panels are installed per the installation manual, they just work flawlessly. 

Hope this helps.

Mark Scheuer

PS Engineering, Inc.