Articles 640 and 725 - Class 1 wiring

Status
Not open for further replies.
M

MikeWhitfield

Member
Okay, I've read the several posts regarding this subject, and I have two basic questions. First, the code seems pretty clear that any amplifier not labeled as Class 2 or Class 3 must be definition be considered Class 1. By Table 11(A) in Chapter 9, I see the power source's maximum nameplate rating for both Class 2 and Class 3 is 100 VA. So, why am I seeing references to 1,000W Class 2 amplifiers or 2,500W Class 3 amplifiers? I can see that a dual channel Class 2 amp bridged to mono might be 200W at most, but wouldn't that violate its Class 2 listing? And how can they get so high as 1,000W or 2,500W for a Class 2 or Class 3 power supply?

Second, it's apparent to me that the professional sound amplifcation industry is totally at odds with the NEC. They'll add the proper listing - but mention it nowhere in specs, and often have no means to ground speakers or amps which are by default Class 1. My problem is that my client (a theater chain) doesn't want the speaker wiring in conduit, his techs are going to tell him it doesn't need to be in conduit, his equipment suppliers are going to tell him it doesn't need to be in conduit, probably even the inspector is going to tell him it doesn't need to be in conduit. How are other engineers and/or contractors handling this?

Thanks for any guidance you can offer.
MW
 
They'll add the proper listing - but mention it nowhere in specs, and often have no means to ground speakers or amps which are by default Class 1.

Yup, I've written about exactly this several years ago. You still might be able to find it, Mike sent it out as a newsletter at the time.

As to your first question, forget the explanations in 725 as well as the table in Chapter 9. All Class 1, 2 and 3 equipment is designed to meet UL specs for a specific class of wiring and then is tested by them or another testing laboratory for compliance. It then must be labeled CL2, CL3 or Class 1. In the case of a power supply or simple transformer the test process is as simple as the information given in 725 or chapter 9. With something like an audio amp it is not. What you don't see in 725 or the table (which is why they need to be removed) are the UL specs that allow clever designers to use current and voltage limiting schemes that allow even large amps to require only CL3 or CL2 wiring. In short, this is one area of the NEC that has not kept up with the times.

Still, there are amps out there that require Class 1 wiring. If the inspector knows what he is doing he is going to require it. What the techs or equipment supplier says doesn't matter. I know I have worked in theaters where conduit was run from the racks to the surrounds and behind the screen. I have had THHN custom twisted up just for this purpose. Another option, particularly if you have a number of venues to do, is to have AFC make up custom MC cable. Not cheap but it can be cheaper than pipe.

As for grounding you do the best that you can. Conduit or MC should run all the way to the rack and terminate with connectors in KOs punched in the sheetmetal. The racks should then have doors. Speakers normally won't have any grounding provisions but they are usually non-metallic anyway. I have used whips of greenfield or tray cable (type TC) from a wall box to the speakers when necessary.

-Hal
 
Large Clas 2 Or 3 Amplifiers

Large Clas 2 Or 3 Amplifiers

Thanks, Hal. That answers my question about large amps. Now there's a possibility that they may be able to use twisted cable after all, except it's in a return air plenum and I don't think anyone makes #10 & #12 single conductor or twisted audio pair in a plenum-rated jacket. But I guess I need to research that before I tell my client is HAS to be in conduit.

Thanks!
MW
 
Status
Not open for further replies.
Top