Uses and ampacity of MTW

Status
Not open for further replies.
Twoskinsoneman said:
If the type of wire is not found in Table 310.16 how can you find the ampacity? Can you just match up the operating temp?
Click to expand...

I guess I misread 310.13. I read it to mean any wire in table 310.13 (a) thru (e) was allowed as a chapter 3 method...
But I guess it can be chapter 3 AS SPECIFIED in the respective table...
This seems odd to be because "310.1 Scope.
This article covers general requirements for conductors and their type designations, insulations, markings, mechanical strengths, ampacity ratings, and uses. These requirements do not apply to conductors that form an integral part of equipment, such as motors, motor controllers, and similar equipment, or to conductors specifically provided for elsewhere in this Code."
Seems to imply that this article doesn't concern "machine tool wiring" such as MTW
 
NFPA 79 states that 310.16 and 310.17 should be used for ampacity of MTW. I would assume you would just use the corresponding temperature rating of the MTW.
 
Lots of times MTW is part of a dual listing THWN/MYW...
 
I would take another look too. When I used to order MTW, I had to make sure the person taking the order knew exactly what MTW is. If I failed to remind them, I would get THHN/THWN.
The only difference I see is that THHN and THWN have a thermoplastic coating where true MTW does not. It is very soft and workable. I mainly used it in control panels.
 
I would be careful about saying 'true' MTW, because clearly the dual rated stuff still meets all of the requirements that go into the MTW rating. Wire manufactured with the same insulation and outer covering as the dual rated stuff, but only tested to be MTW would still be considered MTW. It does get annoying when desired features are not part of the specification.

Another difference that I've noted between the 'MTW' that I've purchased, and the 'THHN/THWN/MTW' is that the 'MTW' had finer stranding. Also MTW that I purchase from McMaster-Carr is frequently but not always _tinned_.

-Jon
 
All of the #12 my supply house stocks is marked like this: E51583 S (UL) AWG 12 CU TYPE MTW OR THWN OR THHN OR GASOLINE AND OIL RESISTANT II OR AWM 600 VOLTS VW-1 ---c (UL) T90 NYLON ORTWN75 FT1. Is it "true" MTW ?
 
amptech said:
All of the #12 my supply house stocks is marked like this: E51583 S (UL) AWG 12 CU TYPE MTW OR THWN OR THHN OR GASOLINE AND OIL RESISTANT II OR AWM 600 VOLTS VW-1 ---c (UL) T90 NYLON ORTWN75 FT1. Is it "true" MTW ?
Click to expand...
Not is the sense that you are using the word "true".

There is MTW which is single listed (although it is often also AWM) and then there is MTW/THHW/THHN/AWM which is multi/quad-listed.

"True MTW" does not have a nylon sheath and is a thermoform type of insulation (it is easy to bend without kinking) whereas quadlisted MTW/THHN is a thermoset insulation (it will bend very sharply and then remember it was bent). "True MTW" insulation is thicker than the quad listed THHN, and it has finer conductor stranding.

These are all design issues, all the code cares about is if it has the correct rating.
 
Hello all, it seems like this problem should have its own section in the code.. and if it does why is it so hard to find? My question refers to wire sizes 16 AWG or less, and in this particular situation it is like Twoskinsoneman is talking about MTW, but anything 16 AWG or less I cannot find ampacities for.

"E51583 S (UL) AWG 16 CU TYPE MTW 600 VOLTS VW-1/LL90458 CSA TYPE TEW OR TYPE AWM-I A OIL RESISTANT FT1 105C" is the labeling on my wire. It seems that there should be a table right before 310.16 for wire sizes 16-22..Anyone have information on the allowable ampacity and/or where I could find this information in the NEC 2008 book? Would be greatly appreciated.

Leo
 
Intended use of mtw wires

Intended use of mtw wires

Internal cable connection to equipment.
Please see attached file for reference.
 
benaround, using the 16 AWG to wire in a second Emergency Stop switch on an existing CNC milling machine.

Thank you topgone, this is to stop a 24VDC control circuit drawing 6.3A max so the 16AWG will do. I figured it was more than enough but working in an industrial setting its good to have the facts. I am not familiar with the NFPA 79, is this a cut directly from a page in the NFPA 79 reference book? Also, do you know if this information exists in the NEC 2008 NFPA 70 reference book?

Leo
 
mdakxotaz said:
benaround, using the 16 AWG to wire in a second Emergency Stop switch on an existing CNC milling machine.

Thank you topgone, this is to stop a 24VDC control circuit drawing 6.3A max so the 16AWG will do. I figured it was more than enough but working in an industrial setting its good to have the facts. I am not familiar with the NFPA 79, is this a cut directly from a page in the NFPA 79 reference book? Also, do you know if this information exists in the NEC 2008 NFPA 70 reference book?

Leo
Click to expand...

The attachment says "compliant" to NFPA 79, the electrical standard for industrial machinery. Chapter 12 of NFPA 79 spells out in detail the types of wire and cable permitted in industrial machinery. In general, Listed wire types are permitted, but Recognized types are not. These are a few of the many Listed wire and cable types that are permitted:
  • MTW,
  • THWN
  • RHW-2
  • THHN
  • RHH
  • SJOW
  • SEOW
  • SJEOW
  • STOW
  • MI
  • TC
  • PLTC
  • ITC
  • CM
  • CL2
  • XHHW
  • THW
  • RHW XHHW-2
  • SOOW
  • WDW
Please note that electrical wire can have multiple ratings. Wire manufactured with multiple ratings must comply with all the requirements of all ratings. Just make sure, wires selected and installed in such a manner that they comply with all other Code requirements?including those for:
  1. conductor stranding,
  2. conductor size (ampacity),
  3. voltage rating,
  4. temperature rating,
  5. moisture resistance and
  6. flame-retardant properties.
 
Status
Not open for further replies.
Top