MTW for branch circuit wiring at church

[wirenut1980]

Hello all! I am working with a church troubleshooting some failed power supplies in audio mixer boards. Symptoms include the mixer board shutting down unexpectedly and eventually failing the power supply over time (weeks/months). The church has started using a borrowed voltage regulator (the type you can plug 120 V loads into) with auto tap changing. Since they have done this, there have been no problems. I opened up the subpanel with the branch circuit going to the mixer. It is a dedicated 20 amp 120 V circuit, and may have a shared neutral, I could not tell in the rats nest of wires on the last visit. I set a monitor at the outlet to record incoming voltage a couple days ago. The only thing that seemed strange to me was that the branch circuit was run with #12 AWG MTW stranded copper wire. Is this a code approved wiring method? If it is code approved, is it an acceptable wiring method from a design persepective? The distance is only about 150 feet from the panel to the outlet, but I'm not sure how long the run really is.

With load being constant, I measured 117 V at the outlet and 121 V at the subpanel, neutral-ground voltage was 1.4 V measured at the outlet. Current measured on the circuit labeled A/V equipment outlet only measured 1.4 Amps and visually resembled that of a switch mode power supply. I wonder if the standard 20 amp branch circuit breaker is not rated to receive this MTW wire?

I'll be back there in about a week to download the monitor at the outlet. Thanks in advance for your thoughts!

 

[Dennis Alwon]

I don't see how the mtw would make a difference however if this is a multiwire branch circuit and there is a periodical loose neutral then you could be putting 240V thru the system. I would think if the mixing board got 240 it would fail immediately but I am not sure what controls it has.

 

[renosteinke]

Nearly all the MTW you're going to find these days is also listed as THHN/THWN. If it has the clear nylon layer over the colored insulation, you're good to go. Even if you have an earlier version, IMO, it's more of a 'technicality' than any safety risk.

The debate of 'solid vs. stranded' is as old as the trade, and the choice is pretty much one of convenience. One thing to keep in mind, though, is to pay extra attention to the connections; stranded wire is a lot harder to connect properly to switches and receptacles. That's where the problems crop up.

 

[K8MHZ]

Nearly all the MTW you're going to find these days is also listed as THHN/THWN. If it has the clear nylon layer over the colored insulation, you're good to go. Even if you have an earlier version, IMO, it's more of a 'technicality' than any safety risk.

The debate of 'solid vs. stranded' is as old as the trade, and the choice is pretty much one of convenience. One thing to keep in mind, though, is to pay extra attention to the connections; stranded wire is a lot harder to connect properly to switches and receptacles. That's where the problems crop up.

Years ago I worked at a baby formula plant where they speced MTW because they did not want the nylon layer. The MTW we used reminded me of the wiring used in automobiles.

 

[Little Bill]

Hello all! I am working with a church troubleshooting some failed power supplies in audio mixer boards. Symptoms include the mixer board shutting down unexpectedly and eventually failing the power supply over time (weeks/months). The church has started using a borrowed voltage regulator (the type you can plug 120 V loads into) with auto tap changing. Since they have done this, there have been no problems. I opened up the subpanel with the branch circuit going to the mixer. It is a dedicated 20 amp 120 V circuit, and may have a shared neutral, I could not tell in the rats nest of wires on the last visit. I set a monitor at the outlet to record incoming voltage a couple days ago. The only thing that seemed strange to me was that the branch circuit was run with #12 AWG MTW stranded copper wire. Is this a code approved wiring method? If it is code approved, is it an acceptable wiring method from a design persepective? The distance is only about 150 feet from the panel to the outlet, but I'm not sure how long the run really is.

With load being constant, I measured 117 V at the outlet and 121 V at the subpanel, neutral-ground voltage was 1.4 V measured at the outlet. Current measured on the circuit labeled A/V equipment outlet only measured 1.4 Amps and visually resembled that of a switch mode power supply. I wonder if the standard 20 amp branch circuit breaker is not rated to receive this MTW wire?

I'll be back there in about a week to download the monitor at the outlet. Thanks in advance for your thoughts!

You may have meant it different than you wrote it, but the board won't normally fail the power supply but the power supply will fail and shut the board down. How long has the mixer been in service. I've had a few years experience with mixing consoles and 98% of the time a power supply will fail. We had 1 mixer that kept having power supplies go out. We bought a new one and had the failed one rebuilt to keep for a spare. When putting the rebuilt one back to test it, we noticed the new one that was installed just 2 weeks prior, was extremely hot. After thinking about it for a while we decided the power supply was not getting good ventilation. These are external to the mixing board and can be mounted anywhere. We moved the power supply to a spot that would receive better air flow. That's been a couple of years and we've had no more problems with it.
I don't know what kind of board you have or if it has an external power supply, but if it does, you might want to look and see if it has good air flow around it. Even if it is an internal power supply make sure the entire board has good air flow.
I mention this in case you don't find any voltage issues to the board.

 

[augie47]

Back to your original question...
as noted in Post #3, most MTW is dual rated.
MTW as a stand alone insulation is shown in Art 310 as "Machine tool wiring", but as such, it is a recognized Art 310 conductor (unlike say "URD")
Also, unlike say SJS, I see no "only" associated with the machine tool listing. For that reason I would say it was an acceptable wiring method.