Bare and coverd conductors

[Son]

Hi all;

In table 310.21 NEC, with the same size, temperature, ampacities of Bare conductors are smaller than convered conductors.

But

I think that when a conductor is not covered, it will not be affected by temperature of insulation, so why is the ampacity of Bare conductor smaller ?

I don't understand why.
Please, explain to me clearly.

Tks.
Son

 

[LEO2854]

Hi all;

In table 310.21 NEC, with the same size, temperature, ampacities of Bare conductors are smaller than convered conductors.

But

I think that when a conductor is not covered, it will not be affected by temperature of insulation, so why is the ampacity of Bare conductor smaller ?

I don't understand why.
Please, explain to me clearly.

Tks.
Son

WHAT is the perpose of your question

 

[eric9822]

Never noticed that since I never deal with conductors in free air. Conductors in free air are going to dissapate heat better than conductors in a raceway and I would think uncovered conductors would dissapate heat better than covered conductors. I am sure one of the smart people on here will reply.

 

[robbietan]

Hi all;

In table 310.21 NEC, with the same size, temperature, ampacities of Bare conductors are smaller than convered conductors.

But

I think that when a conductor is not covered, it will not be affected by temperature of insulation, so why is the ampacity of Bare conductor smaller ?

I don't understand why.
Please, explain to me clearly.

Tks.
Son

however, bare conductors take in heat from their surroundings as well as heat from electricity. this could be the reason they have lower ampacity than insulated wires

 

[K8MHZ]

however, bare conductors take in heat from their surroundings as well as heat from electricity. this could be the reason they have lower ampacity than insulated wires

310.21 is only for non-insulated wires. A cover is not insulation.

I could not find a definition of a cover when it comes to non-insulated wiring.

I think we need to know just what they mean by a cover in order to try to surmise the difference in ampacity.

 

[K8MHZ]

Looking closer at the heading of the chart, I noticed there was an indication of air speed.

Could they mean the cover of an iso-phase system where air is moved through it to keep the conductors cool?

 

[nakulak]

however, bare conductors take in heat from their surroundings as well as heat from electricity. this could be the reason they have lower ampacity than insulated wires

If I understand your statement corrrectly, you are stating that a conductor with a covering dissipates heat more efficiently than a bare conductor. Could you please explain the physics behind this assertion, or your reasoning ? While on the surface this seems counterintuitive, since the table is for conductors based on a certain wind velocity, the increased surface area of the conductor with a covering might account for this difference. (that is the only logic that makes sense to me, I would be interested in an official explanation as well)

 

[iwire]

I would say it depends on the properties of the covering.

Providing more surface area could reduce the temps if the covering is a good thermal conductor.

Think of aluminum fins on a dimmer etc.

 

[charlie b]

Looking closer at the heading of the chart, I noticed there was an indication of air speed. Could they mean the cover of an iso-phase system where air is moved through it to keep the conductors cool?

I think it is just plain wind, as we commonly feel it on our faces. The conductors are in free air, and the air is moving. They are taking credit for the cooling effect of air moving across the wire.

 

[charlie b]

Providing more surface area could reduce the temps if the covering is a good thermal conductor.

I think this is it exactly. Looking through a handbook of engineering fundamentals, I see that the heat transfer rate between various types and shapes of objects tends to be proportional to the area of the heated surface. Since a larger diameter cylinder (i.e., the convering, whatever that actually is) has a higher surface area than the wire it covers, it would increase the heat transfer rate.

Another coefficent that also comes into the heat transfer equations is the thermal characteristics of the material in question. So you are also right about there being a need for the covering to be a good thermal conductor.

 

[K8MHZ]

What would be an actual example of a covered conductor in free air?

 

[LarryFine]

I could not find a definition of a cover when it comes to non-insulated wiring.

Insulation is any type recognized by the code, and a covering isn't. Someone cut-n-pasted a blurb from the NEC about this somewhere recently.

 

[LarryFine]

What would be an actual example of a covered conductor in free air?

Triplex with an insulation not NEC approved.

 

[BJ Conner]

Conductors in free air are their own support. Copper when it gets hot looses strength and begins to creep. Table 310-20 applies to cable supported from a messenger.
An historical case is the type HH cable that was used to transmit power from Bolder Dam to LA in the 1930s
IF you look down the page at this websit there is a picture of the hollow core copper cable used to cross the Mojave desert. In the summer as the cable got hotter and hotter it sagged lower and lower. The had to limit the power that could be transmitted.
The finally took it out and replaced it with ACSR. Most of the copper was used in munitions during WWII
http://www.usbr.gov/lc/hooverdam/service/memorabilia.html

 

[broadgage]

What would be an actual example of a covered conductor in free air?

Here in the UK we use "covered" conductors in free air on some 11KV overhead lines.
The covering will usually resist line voltage to earth, at least for while.
Such conductors are used to reduce the number of outages caused by wind-blown debris in exposed locations.
The plastic coating is called "covering" and not "insulation" because it provides an added factor od reliability, but is not tested or certified or approved to the standards required for it to be refered to as insulation.
The covering is green, and looks somwhat like bare copper that has aquired a green oxide coating.

Persons working near such lines are instructed to treat them as just they would bare ones.

 

[BJ Conner]

Here in the UK we use "covered" conductors in free air on some 11KV overhead lines.
The covering will usually resist line voltage to earth, at least for while.
Such conductors are used to reduce the number of outages caused by wind-blown debris in exposed locations.
The plastic coating is called "covering" and not "insulation" because it provides an added factor od reliability, but is not tested or certified or approved to the standards required for it to be refered to as insulation.
The covering is green, and looks somwhat like bare copper that has aquired a green oxide coating.

Persons working near such lines are instructed to treat them as just they would bare ones.

The same wire over here is commonly called "Tree wire" No really insulated but with a cover that lessens the posibility of trees causing shorts.

"Spacer Cable" or Hendrix cable is covered but not all of it is insulated.