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#11645 07/17/02 05:13 PM
Joined: Feb 2001
Posts: 308
Steve T Offline OP
Could someone give me a clear explanation of which types of insualted conductors may be used for general residential wiring? I recently had a homeowner use insualtion type MTW (not dual rated) just MTW, The rubbery stuff you see inside your power tools(if you ever take them apart to fix them).

MTW is a recognized insulation type in Table 310.13 (looking in the 93 NEC) but it is not one of the insulation types listed in the ampacity tables. 310.13 says these conductors shall be permitted for use in any of the wiring methods of chapter 3 and as specified in their respective tables.

Does this mean that an insulation type that has a special use listed can ONLY be used for that particular special use?

PFAH, for instance, particularly says "Only for leads within...", but MTW jsut says "Machine tool wiring in permitted in NFPA 79.

This does not say ONLY though.

I wouldn't allow it based on the fact that it is not listed in the ampacity tables. But why have it in the NEC if its not applicable to any of the wiring. (I don't see anything in Article 670 that mentions insulation types). How do you find MTW's ampacity if you are in the business of wiring up machine tools?

any insight on the whole use of Table 310-13 (Sorry 310.13, I'm stil using the 99 NEC) would be appreciated. Thanks.

#11646 07/20/02 06:46 AM
Joined: Dec 2000
Posts: 218
Sorry, I goofed and put the reply on the wrong post. Check the other one.

#11647 07/30/02 11:28 PM
Joined: Feb 2001
Posts: 308
Steve T Offline OP
If this is a dumb question please let me know.

#11648 07/31/02 12:07 AM
Joined: Jan 2002
Posts: 122
Nope, not a dumb question! I think you may have stumped a few people. Used to use it a lot for control work and it was always dual rated. Looking at the properties it appears that the MTW has a tougher insulation.

MTW = Flame Retardant / Moisture, Heat, and Oil resistant / Thermoplastic

THHN = Flame Retardant / Heat, resistant / Thermoplastic

A house is not a machine tool, but does the MTW exceed THHN or THWN specs?
I think it's an AHJ call it seems to be a grey area. This is the chance homeowners take when they do there own wiring.

Did a little research though, NFPA 79 Ampacities table:


[This message has been edited by wirewiz (edited 07-31-2002).]

#11649 07/31/02 12:35 AM
Joined: Feb 2001
Posts: 308
Steve T Offline OP
I don't think MTW is better than THHN because THHN is always dual rated as MTW but not vice versa. I could be wrong though. Since MTW is not listed in the NEC ampacity tables, I don't think it can be allowed using another NFPA document's ampacity table. Anyone care to elaborate on this?

And by the that a natural grey area? [Linked Image]

#11650 07/31/02 01:04 AM
Joined: Oct 2000
Posts: 2,749

See the 1999 NEC Section 310-15 (a)(1)FPN No. 2: For the allowable ampacities of Type MTW wire, see Table 11 in NFPA 79-1997, Electrical Standard for Industrial Machinery.

but .... see the 1999 NEC for the Table 610-14(a). Ampacities of Insulated Copper Conductors Used with Short-Time Rated Crane and Hoist Motors.

Detailed information for Wire and Cable can be found in the UL Marking Guide.

[This message has been edited by Joe Tedesco (edited 07-31-2002).]

Joe Tedesco, NEC Consultant
#11651 07/31/02 08:29 PM
Joined: Jan 2002
Posts: 163
Table 310.13 "Application Provisions" (for MTW) indicate this conductor is restricted to Machine Tool wiring locations as permitted in NFPA appears any other application for this conductor would be a violation of NFPA 70.

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