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Discussion Starter · #1 ·
What is the minimum wire size allowed to supply a 100va 120vac to 24vac step down transformer within a controls cabinet when the 120v is fused at the breaker at 15amps.

Basically we had a contractor install 18awg wire from the existing 120vac/15amp service feeding the cabinet to the requested 100va transformer with no additional fusing with 18awg wire. Would this be acceptable?
 

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Discussion Starter · #3 ·
How does a 15A OCP protect a 18AWG conductor? This is a first year apprentice question, are you an electrician?
That was the point of the question, we had an electrical contractor make additions to several HVAC control cabinets at several sites in this fashion which I would have assumed is not acceptable for several reasons which I flagged to my manager as unsafe and not to code. My background is in electrical engineering technology, but I am also a licensed Refrigeration mechanic and Licensed G1 which allows me to work on quite a bit of electrical within very large boilers and chillers. (600vac, 3phase, motor control circuits, vfd's, load and line reactors, motors, etc etc is all within my scope when related to HVACR)

So I was wondering if I missed something? Why any licensed electrician would have installed something as such.
Gadget Electronic instrument Gas Electronic engineering Electrical wiring
 

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The question isn’t “Can the 18 gage wire carry 15 amps?”, (it can).
The question should be, “Is that wire rated for the use.”
It’s a **** install IMHO.

Your pictures show the wire is from the equipment switch, to a transformer inside the cabinet.
 

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The question isn’t “Can the 18 gage wire carry 15 amps?”, (it can).
The question should be, “Is that wire rated for the use.”
It’s a **** install IMHO.

Your pictures show the wire is from the equipment switch, to a transformer inside the cabinet.
i doubt it will at 60C, or if the breaker is 40C then i am sure it wont
and the insulation may not be sufficient at 120V
 

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Discussion Starter · #6 ·
It's definitely a **** install lol
But 120vac on 18awg wire for the primary transformer I thought was also unacceptable as it is not a control circuit it's simply line power to a step down transformer? And since it's fused at 15amps #18 wire is likely undersized as well.

Not to mention there is no protection for the wire free run through the knock-out hole.

Does anyone have the definition for control circuit from the code book?

If you were to flag this what codes would you cite.

Thanks
 

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18awg wire is good for 5-10amps depending on the type of wire (table 12)
overcurrent protection on the primary of the control transformer would be sized to 125%
100/120*1.25=1amp. The wire ampacity size is also determined the same way.

If it was fused properly, then 18awg wire feeding the primary would be fine inside the cabinet.

that install looks like total ****. That looks like some trunk slammer with zero industrial experience got the job from his maintenance manager buddy and was 1/4 the price of a legitimate contractor.
 

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i doubt it will at 60C, or if the breaker is 40C then i am sure it wont
and the insulation may not be sufficient at 120V
It’s inside the cabinet so an engineer can design anything he wants to sign off on.

I’m sure the insulation on that wire isn’t rated for it’s use.
 

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18awg wire is good for 5-10amps depending on the type of wire (table 12)
overcurrent protection on the primary of the control transformer would be sized to 125%
100/120*1.25=1amp. The wire ampacity size is also determined the same way.

If it was fused properly, then 18awg wire feeding the primary would be fine inside the cabinet.

that install looks like total ****. That looks like some trunk slammer with zero industrial experience got the job from his maintenance manager buddy and was 1/4 the price of a legitimate contractor.
The highest number I’ve seen is 18 amps. But it all depends on the type of wire.

If he looked at the labeling on that shielded wire they installed. He will quickly learn that it’s the wrong wire, unless an engineer OK’ed it. (I doubt it)

It was low bid, quick close the job, type of install.
I wonder what the rest of the cabinet looks like.
 

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Discussion Starter · #10 ·
It's inside a cabinet but it's a field install by an electrician not an engineered install that has been pre approved for anything.

What is the definition for control transformer vs power transformers is it simply the VA rating or the load it's powering. As I thought 120v on 18awg is only acceptable if it's a control circuit or pre approved by engineer/design. Would not be acceptable to use 120vac on 18awg wire to power an led light bulb even if it was fused properly.

And there is no fusing until you get back to the 15amp breaker.
 

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Does anyone have the definition for control circuit from the code book?

If you were to flag this what codes would you cite.

Thanks
Definition
“The circuit of a control apparatus or system that Carrie’s the electric signals directing the performance of the controller but does not carry the main power.”

I can’t flag anything as I don’t see the whole thing.
 

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Discussion Starter · #12 ·
Definition
“The circuit of a control apparatus or system that Carrie’s the electric signals directing the performance of the controller but does not carry the main power.”

I can’t flag anything as I don’t see the whole thing.
By that definition it's not a control circuit, it is the step down transformer that powers a 24vac PC (mini dell PC) and Router. It's not a control circuit for a motor, drive, chiller, etc.
 

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It's inside a cabinet but it's a field install by an electrician not an engineered install that has been pre approved for anything.

What is the definition for control transformer vs power transformers is it simply the VA rating or the load it's powering. As I thought 120v on 18awg is only acceptable if it's a control circuit or pre approved by engineer/design. Would not be acceptable to use 120vac on 18awg wire to power an led light bulb even if it was fused properly.

And there is no fusing until you get back to the 15amp breaker.
maybe your plant spec limits wiring to 14awg, but there is no code preventing someone from using 18awg to feed this control transformer if it is properly fused.

this transformer would be considered an instrument voltage transformer and has a max fuse size of 10a on the primary. The fusing should be sized to transformer VA either way.

This is in section 26
 

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Discussion Starter · #14 ·
maybe your plant spec limits wiring to 14awg, but there is no code preventing someone from using 18awg to feed this control transformer if it is properly fused.

this transformer would be considered an instrument voltage transformer and has a max fuse size of 10a on the primary. The fusing should be sized to transformer VA either way.

This is in section 26
It's basically a commercial install, boiler room in a high rise building, not a factory.
 

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That style of transformer was quite common on combo disco/motor starter assemblies. Fed with 600V for the motor and dropped to 120V for the control circuit. The transformer always, always, always had an inline fuse on the primary side.
 

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By that definition it's not a control circuit, it is the step down transformer that powers a 24vac PC (mini dell PC) and Router. It's not a control circuit for a motor, drive, chiller, etc.
But is it on the load or line side of that switch?
That is the debarkation line.

Control and signal voltage varies by equipment. It’s a subjective term.

IMHO, it’s still a shitty install
 

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That CMG style wire is typically rated at 300V and 105° C. I think everything else has been covered here.
 

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Discussion Starter · #19 ·
But is it on the load or line side of that switch?
That is the debarkation line.

Control and signal voltage varies by equipment. It’s a subjective term.

IMHO, it’s still a shitty install
The primary 120v 15amp breaker feeds that receptical and light switch in parallel. Off of the light switch are 2 other 120vac to 24vac 40va transformers wired in parallel off of a 14awg wire all within that electrical box. Then the contractor came and added this 18awg off of the main before the switch to feed another 100va 120v to 24v transformer with no fusing, the secondary 24vac powers a mini PC and Router.

I assumed it was against code for two major reason 1) not fused down for 18awg wire.
2) since it is not a control circuit it should not have 120v on 18awg wire (this might not be the case)
3) it's just a **** install lol, and sadly they were not the cheapest contractor :rolleyes:
 

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The primary 120v 15amp breaker feeds that receptical and light switch in parallel. Off of the light switch are 2 other 120vac to 24vac 40va transformers wired in parallel off of a 14awg wire all within that electrical box. Then the contractor came and added this 18awg off of the main before the switch to feed another 100va 120v to 24v transformer with no fusing, the secondary 24vac powers a mini PC and Router.

I assumed it was against code for two major reason 1) not fused down for 18awg wire.
2) since it is not a control circuit it should not have 120v on 18awg wire (this might not be the case)
3) it's just a **** install lol, and sadly they were not the cheapest contractor :rolleyes:
Ok I understand what and why he did it now.
He cashed the check and gone.
We’ve had that same thing done here before by others. But we just go back and fix the crap like that.

It’s splitting hairs as to an electrical code issue. It’s inside the utilization equipment and as such an engineer could have signed off that “Design”.

Cheaper to replace the wire and transformer then it is to have it done by Mr Fly By Night

But keep in mind, if they, The contractor, is monitoring the system and a fuse pops, it’s their loop hole to say, not our fault the system crashed over the weekend.
 
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