Amp rating for bare copper wire

[carlos.miranda313]

Hi I'm installing a new feeder for a 12kv primary side transformer and I found out that the old feeder that got damage was a 4/0 bare copper with a silver plated coating on each single strand but the engineer want to replace it with just bare 4/0 copper they're all enclosed on the same box and it's only 1ft long between the isolators. What's the rating on a bare copper wire in a 12kv system? Do I have to derate the wire size because of the tension on the 12kv?

 

[kwired]

Hi I'm installing a new feeder for a 12kv primary side transformer and I found out that the old feeder that got damage was a 4/0 bare copper with a silver plated coating on each single strand but the engineer want to replace it with just bare 4/0 copper they're all enclosed on the same box and it's only 1ft long between the isolators. What's the rating on a bare copper wire in a 12kv system? Do I have to derate the wire size because of the tension on the 12kv?

I don't have a direct answer to what is the ampacity of the conductor, but will say the conductor itself likely is capable of carrying more current then the termination points can without causing damage to the conductor or the termination point.

 

[Smart $]

Under the NEC, there is no ampacity rating for bare conductors.


And I concur with what kwired posted.

 

[charlie b]

I don't have any reference materials handy, but I would wager the price of a small confectionary that the NEC does not allow the use of bare copper conductors in the type of installation you are describing.

 

[rbalex]

With regard to NEC Ampere ratings for bare conductors, see Table 310.15(B)(21)[2011 and 2014 NEC, formerly T310.21]. See the application limitations noted in the Table's title.

"Free Air" may be an issue although it doesn't necessarily mean "open"; it simply means adequate air flow and spacing between conductors. MV terminations are normally rated 90C. Table 310.15(B)(21) conductors are already limited to 80C.

Charlie b's observation is also relevant.

 

[Smart $]

With regard to NEC Ampere ratings for bare conductors, see Table 310.15(B)(21)[2011 and 2014 NEC, formerly T310.21]. See the application limitations noted in the Table's title.

...

Thanks for pointing this out. However, Tables 310.15(B)(16) thru (21) only apply to conductors rated 0 to 2000 volts [310.15(B) general statement]. I realize Table 310.15(B)(21) does not say "Rated Up to and Including 2000 Volts" like the others before it, but you have to be referred to a table by section text to be able to use it.

Regarding 310.60(B) 2001 to 35,000 volts, there is none for bare conductors in Tables 310.60(C)(67) thru (86).

 

[GoldDigger]

I don't have any reference materials handy, but I would wager the price of a small confectionary that the NEC does not allow the use of bare copper conductors in the type of installation you are describing.

It's not a bare wire. They just stripped the insulation back 6" from each end.

 

[Smart $]

FWIW...

A110, PIII Over 600 Volts, Nominal

110.36 in part states, "Bare live conductors shall comply with 490.24"... which specifies separation between conductors and grounded surfaces in Table 490.24. Has no bearing on ampacity, but seems to indicate bare conductors are permitted in certain scenarios.

On that note

(D) Insulated. Conductors, not specifically permitted else-
where in this Code to be covered or bare, shall be insulated.

 

[rbalex]

FWIW...

A110, PIII Over 600 Volts, Nominal

110.36 in part states, "Bare live conductors shall comply with 490.24"... which specifies separation between conductors and grounded surfaces in Table 490.24. Has no bearing on ampacity, but seems to indicate bare conductors are permitted in certain scenarios.

I see you've done further homework. I appreciate it.

I've done battle over this issue with a jurisdiction we're both familiar with. At the time, it essentially said I couldn't design something it didn't know how to inspect. (Overhead lines in tank farms and their current ratings in general). I had to point out the jurisdiction's own statutes said they had to either be or become familiar with the relevant construction standards.

I also think you need to re-parse the opening clause of Section 310.15(B) a bit more carefully. It effectively says that Ampere ratings may be determined from the Tables for cables that are (in noted cases) already rated 0-2000V. It doesn't necessarily say that cables in the Tables must be rated 0-2000V. As you pointed out, T310.15(B)(21) doesn't say that bare cables are rated 0-2000V in the title as the other titles do.

In any case, while there are other considerations as I mentioned supporting Charlie b's observation, there is no essential basis for differentiating Ampere ratings for bare conductors simply based on voltage alone.

 

[fmtjfw]

Huh, bare wire ampacity and voltage

Huh, bare wire ampacity and voltage

i'd be really interested to know how the voltage difference between a wire and another object affects its ampacity?
Why would the voltage difference affect the electrons flowing through the conductor?

 

[Julius Right]

In my opinion, if the length of the feeder it is only one feet length, the maximum temperature will be in lug contact zone. In this case it is important to reduce copper to lug contact resistance. Then, a silver coating it could be necessary.
See [for instance]:
ELECTRICAL CONNECTIONS FOR POWER CIRCUITS
http://www.usbr.gov/power/data/fist/fist3_3/vol3-3.pdf
ch. 3.2.2. Silver oxide.- ?Silver oxide is easily broken down by contact pressure and forms less readily at elevated temperature. Thus it is good practice to silverplate copper contact surfaces that must be operated at elevated temperature.?

 

[mivey]

FWIW, the bare 4/0 hard-drawn copper ampacity for utility use is about 480 amps.

 

[mivey]

i'd be really interested to know how the voltage difference between a wire and another object affects its ampacity?
Why would the voltage difference affect the electrons flowing through the conductor?

It doesn't that I can think of. Higher voltage cables can have higher temperature rated insulation. For bare it is the environmental conditions (wind, solar heating, condition of the cable surface, nearby heat sources, etc.) that make a difference for a given cable type and size.

You can have other current restrictions like limiting the magnetic field strength or controlling the amount of cable sag. I don't recall voltage being a factor and I can't picture why it would be.

Maybe the code has different safety factors built in for different voltage levels?

 

[JoeStillman]

i'd be really interested to know how the voltage difference between a wire and another object affects its ampacity?
Why would the voltage difference affect the electrons flowing through the conductor?

The insulation voltage rating affects ampacity. Higher voltage insulation is thicker and is a better thermal insulator.

 

[kwired]

The insulation voltage rating affects ampacity. Higher voltage insulation is thicker and is a better thermal insulator.

I can agree with that but the discussion is about a bare conductor. In general termination temperatures will be lower then ampacity of the bare conductor being used and voltage really doesn't matter, it is current that will create heat, which is what determines ampacity. When you have current through a termination - you typically have some resistance at the point where two conductors are joined, and this is what makes the termination the weak point of the circuit especially for a short length conductor like in the OP.

 

[Smart $]

...

I also think you need to re-parse the opening clause of Section 310.15(B) a bit more carefully. It effectively says that Ampere ratings may be determined from the Tables for cables that are (in noted cases) already rated 0-2000V. It doesn't necessarily say that cables in the Tables must be rated 0-2000V. As you pointed out, T310.15(B)(21) doesn't say that bare cables are rated 0-2000V in the title as the other titles do.

In any case, while there are other considerations as I mentioned supporting Charlie b's observation, there is no essential basis for differentiating Ampere ratings for bare conductors simply based on voltage alone.

Re-parsing the opening clause of 310.15(B) with an open mind to your view did not change mine. Bare conductor is not voltage rated. But the section also infers the condition of use to be not greater than the rated voltage. In the case of a bare conductor, this limits the Table values to use at 2000 volts or less.

I agree, voltage has no bearing on the ampacity of bare conductors. Yet Code doesn't always agree completely with the physics.

 

[carlos.miranda313]

I believe it don't but at the same time we got to remember that the wire is already under tension because of the high voltage wich it could potentially increase the temperature of the wire before current its apply to it. In this scenario the transformer it's apply voltage first for atleast 4 hours before a load its connected just to warm up the windings and the dielectric oil inside scene this transformer has an unload tap changer on the primary side we like to make sure all humidity it remove before operation.

 

[rbalex]

Re-parsing the opening clause of 310.15(B) with an open mind to your view did not change mine. Bare conductor is not voltage rated. But the section also infers the condition of use to be not greater than the rated voltage. In the case of a bare conductor, this limits the Table values to use at 2000 volts or less.

I agree, voltage has no bearing on the ampacity of bare conductors. Yet Code doesn't always agree completely with the physics.

As I said, I already won this battle with a very conservative jurisdiction. But then you win some and lose some. I still think you have over-analyzed the text; nevertheless, I understand your position. Gotta love English ambiguity especially after the NEC gets wordsmithed every three years or so.

 

[rbalex]

I believe it don't but at the same time we got to remember that the wire is already under tension because of the high voltage wich it could potentially increase the temperature of the wire before current its apply to it. In this scenario the transformer it's apply voltage first for atleast 4 hours before a load its connected just to warm up the windings and the dielectric oil inside scene this transformer has an unload tap changer on the primary side we like to make sure all humidity it remove before operation.

As several have noted in various ways, "... voltage has no bearing on the ampacity of bare conductors. Yet Code doesn't always agree completely with the physics."

The definition of Ampacity in Article 100 is:

Ampacity. The maximum current, in amperes, that a conductor can carry continuously under the conditions of use without exceeding its temperature rating.

Within the context of your application, the "preheating" still has no bearing on the continuous "conditions of use."

 

[Smart $]

...

Within the context of your application, the "preheating" still has no bearing on the continuous "conditions of use."

Even outside the context, it could have bearing in the sense it is a condition of use, if significant... and I'm going to go with not.