wire protection

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clayton

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the senario is,

i have a transformer, 30kva, 480 pri, 240 sec,

72 amps max secondary,

im feeding 2 machines 20 amps each

the question is, if each of these machines are
internarlly fused and protected, via disconnects and fuses,

do i need to run wire that sized for the 72amps or can i use the smaller gauge wire provided from the equipment manufacturer?

if using the smaller provided wire, would i then need to provide 20amp, protection at the secondary side??

what or where does the code address this??
we are discussing this one in the shop.

[ January 18, 2005, 02:35 PM: Message edited by: clayton ]
 
Re: wire protection

clayton
Using the machine disconnects for branch circuit OCPD's is not specifically disallowed. NEC 240.10 forbids branch circuit protection according to Article 210, in 2005 it forbids it entirely. There are so many conditions required before you could use machine OCPD's as legal branch circuit OCPD's particularly because of the tansformer that I wouldn't recommend it. For example the OCPD's would have to be grouped, probably impossible.
You don't have to run 72 amps of wire, you could set 2 20 amp disconnects according to Table 450.3(B) Note 2, and 240.21(C), and run #12 wire if you wanted.
Jim T
 
Re: wire protection

the machine(s) them selves have the disconnect switch on them. not sure if this is what you mean by grouping?,

ok setting the 2 disconnects and fusing to 20amps, and running 12awg, i understand,

but, isnt that the same??thing?? as usning the ocp on the machine itself, the wire from the secondary to the disconnect again isnt like technically protected right??

im not sure i know the answer and i really appreciate your reply JT,

im going to examine the code sections u mentioned

thanks
 
Re: wire protection

quote
"but, isnt that the same??thing?? as usning the ocp on the machine itself, the wire from the secondary to the disconnect again isnt like technically protected right??

Unfortunately, it's not quite the same thing. First Table 450.3(B) requires 2 or more disconnects to be grouped at one location. I doubt that being behind equipment control panel doors will qualify, but that is up to your AHJ. Secondly, in general conductors are supposed to be protected from overcurrent at their source. You need to comply with 240.21(C) Using machine disconnets might comply in certain situations, or perhaps 240.21(C)(1) can apply. Finally, it will be forbidden in the 2005, so you should hurry if you want to try.
Jim T
 
Re: wire protection

If this is a single phase transformer, you might be able to use the primary overcurrent protection to protect the secondary. See 240.4(F) and 450.3.

When you say the two machines are 20A each, does that mean they are fused for 20A, or that they draw 20A? I ask because the wire on the secondary of the transformer would still have to meet the tap rules in 240, and also be sized for the total load of both machines. So you might have to use #8 wire.

Steve
 
Re: wire protection

heres what im thinking,

this falls under the catagory of 240.92 (b),

part viii of article 240 (2002)


tell me what you think,

im in a supervised industrial setting just to be clear.

and i wonder if someone could show the calculation of 240.92 (d) how they figure the ocpd, according to this rule, an example would be helpful thanks.

clay.
 
Re: wire protection

See the definition in 240.2 to make sure you can claim the "industrial installation".

I would also be interested in seeing an example of 240.92(D). My guess (this is only a guess), is that the time characteristic would usually be the same on the primary and secondary, and would therefore cancel out.

For example (once again I just guessing that this would hold true), a 50A breaker on the primary would protect #8 wire. 50 * 480/240 =100 amps on the secondary. So a 50A breaker on the primary would protect 100A wire on the secondary.

Steve
 
Re: wire protection

Clay, we wrote the definition of a supervised Industrial Installation to provide engineering supervision. In the process of doing that, we opened it up so that you can do almost anything you need to do for overcurrent protection. If you do not have engineering supervision, you are not permitted to use any of the provisions in Part VIII (Sections 240.90 and 240.92).

The intent is for you to go to your engineer and he will design your protection scheme for you. You are not permitted to do that type of work without have it checked by the supervising engineer. :D
 
Re: wire protection

thanks charlie,

yeah im in an industrial setting, i work for a large earthmoving machinery maker.

however at times our engineering dept. is ummmm

i have a question, maybe you could answer charlie,

as an electrician am i not qualified to make decisions on an istallation??

if not how can electrical contractors make decisions??

and if not what would be the purpose of my college and apprenticeship??

do i have to have an engineering degree to choose the conductor size? or the fuses? where can an electrician make a decision? im just wondering

im still realatively young in this field (25 yrs old) and just dont have all the experience behind my belt yet

thanks you guys. :)
 
Re: wire protection

I think its a bad practice to allow this kind of thing in the first place. Its better to just have one set of readily available rules (that would be the code book) that everyone has available rather than something thats in some engineers head.

Stuff in this kind of environment changes constantly, and what the engineer determined may have been quite adequate for the intial condition, may well not be suitable down the road.

Maybe this kind of thing should be required to have a big bright neon orange sign posted on it that "this is a non-standard installation - no changes allowed without engineer approval".

Otherwise, how would anyone know that the thing is different than what you would expect?
 
Re: wire protection

Originally posted by charlie:
Clay, we wrote the definition of a supervised Industrial Installation to provide engineering supervision. In the process of doing that, we opened it up so that you can do almost anything you need to do for overcurrent protection. If you do not have engineering supervision, you are not permitted to use any of the provisions in Part VIII (Sections 240.90 and 240.92).
Click to expand...
Just curious. What section of the code actually says this? I looked in the definitions section and did not find a definition for "supervised" or "supervision" of anything.

I did not find a definition of "engineer" either.

Does that mean the guy that drives a train can decide such things?
 
Re: wire protection

good point petersonra.

i work specifically in test and development.

we make changes practically hourly to things,

allll of our installations are non stantard,
ok not all but a majority,

put it in one day take it out the next.

and its all supervised by engineers, but they are not all electrically biased engineers

but i like it in that it gives one alot of experience in many areas, which often leads to many discussions.

[ January 20, 2005, 10:03 AM: Message edited by: clayton ]
 
Re: wire protection

charlie posted January 19, 2005 07:44 PM

we opened it up so that you can do almost anything you need to do for overcurrent protection.
Click to expand...
This looks like the Wild West from where I'm sitting.

As I holster my Kleins, my sidekick says, "OK Pardner. . .The Engineer's ridin' in. Listen to 'em, he's the Law."

Respectfully, to the PEs in this conversation, though, where's the check & balance?
 
Re: wire protection

Take a look at 240.92(B)(1)(3) and 240.92(B)(2)(4). In an industrial setting with engineering supervision, there should be written procedures for things that are "different" or "engineered". For everything else, follow the rules in the Code.

As far as the definitions of words, grab a dictionary. I think the words you are looking for are in there and that was our intent.

Clay, you most certainly have the ability to make decisions but there are limits to what you can do with the knowledge you have. An engineer can protect a circuit without using either fuses or circuit breakers, you can't do that without engineering supervision.

Al, we have a wide latitude in the electric utility business but we are not blowing things up all the time. The same applies to the large industrials, they get more latitude because of what they have the capability to do. By the way, most of the time, you can't successfully sue your employer.
 
Re: wire protection

i have one more question.
but first thanks charlie for your responses, makes sense, and i appreciate your time,

ok the question ive been wondering about is this,

we have no statewide requirement for the nec it is all up to the municipalities,

i work in a plant that is in a county not under any jurisdiction,

how or even does, the nec apply, ?? i guess who would be requiring it? what would enforce it? does this come under an osha rule??

anyway thanks again you guys.

clay.
 
Re: wire protection

Originally posted by clayton:
how or even does, the nec apply, ?? i guess who would be requiring it? what would enforce it?
Click to expand...
As I understand it, no one.

If there is a fire, and an insurance claim is made, the insurance "inspector" will use the NEC as a basis for his/her investigation. If the wiring falls short of the NEC, you're in trouble. :)
 
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