MTW wire

[brother]

I found some old wiring in a building that mtw (solid copper) wire. Im having trouble finding the amacity and diameter of the wire for calculating conduit fill. We want to add in some thhn/thwn #12.

I remember some time ago it being in the NFPA 79, but I also recall it use to be in the NEC. Can you help me find it please and help in calculating fill?

 

[Sierrasparky]

I'm thinking that is just Machine tool wire, But I don't think MTW was offered as Solid.
MTW is a suffix I think.

Are you sure that is what you have?

 

[sameguy]

http://www.usawire-cable.com/pdfs/thhn_thwn.pdf
MTW, strand only, see size chart

 

[infinity]

Are you sure that the MTW isn't also rated as another insulation type like TFFN?

 

[MTW]

I believe the wire your referencing is TW or THW

 

[kwired]

I believe the wire your referencing is TW or THW

User name of MTW should know

I would guess even if it isn't, is probably similar dimension if came from time when those were what was used.

One could measure it and either calculate it's area or find a conductor in fill tables that is closest match to your measurement and use it's area mentioned in the table.

 

[sameguy]

If it is tw or thw it will be larger than mtw/thhn.
Back about 1976 I called the shop for answers on pipe runs or send me some wire, we were using thw mostly, boss drops off this " expensive " thhn wire "better not waste it". I called the shop when I got home, no pagers, no cell phones, dime call at the pay phone, told them I needed the answers on the pipe runs or bring more of the new wire because I used it all. We never bought anything but thhn for pipe work and mtw for control work after that. Tw/thw is like pulling spark plug wire compared to thhn.

 

[hbiss]

I don't believe MTW was in anything other than standard gauges, but like TFFN it could be had in 18 and 16ga. Simply take a caliper and measure the bare conductor and compare it to #14 and 12 solid if you can't eyeball it. But really you don't care about that unless you are concerned that it is the proper gauge for the circuit.

To determine fill, measure the OD of the insulation with a caliper and multiply by the number of existing runs to determine the existing fill. Then add the ODs of the new pulls and look at a conduit fill chart to see what you can do.

-Hal

 

[MTW]

User name of MTW should know

Here's a sample, still produced in 1994




Conductor Dimensions still listed in the NEC tables



By the way. MTW is not a code approved wiring material, unless it's a dual rated type.

 

[Dennis Alwon]

By the way. MTW is not a code approved wiring material, unless it's a dual rated type.

I think all the MTW is dual rated today. I believe Thwn-2 is also rated MTW.

Table 310.104(A) shows MTW as a conductor acceptable at 60C for wet location and 90C for dry location so it must be code compliant.

 

[Dennis Alwon]

Here is southwires info on Thwn-2 and MTW.. it is rated for both

https://www.southwire.com/ProductCatalog/XTEInterfaceServlet?contentKey=prodcatsheet276

 

[MTW]

I think all the MTW is dual rated today. I believe Thwn-2 is also rated MTW.

Table 310.104(A) shows MTW as a conductor acceptable at 60C for wet location and 90C for dry location so it must be code compliant.

Agreed, but Table 310.104 shows the approved use of MTW as machine tool wiring. Totally irrelevant if the wire in question has six other listings, such as the Southwire Simpull you referenced. Old style straight rated MTW, not so much.

I was thinking of the old style solid wire that the OP inquired about. .030 insulation wall thickness versus the .015 thickness of the current approved variety. Time has a way of changing things.