doubling wire to increase amperage

So Good Morning to all

480 volts/60/ 3 phase, referencing one phase of three phase system but applies to all three. This particular system has a theoretical I of 95 amps

Being in industry for 30 some odd years my experience tells me that if you take two 6 awg (75 amps each) wires ( together equalling the cross section of a 3 awg ) that you can apply the amperage of a 3 awg (110 amps @ a T of 90C) safely to the system.

My genius mechanical engineer has nothing better to do on a Sunday than argue with me on this.

But just to error on the side of cautious arrogance, Please advise me how far off I am on my calculations.

Your time and consideration are greatly appreciated

Thank you

Per NEC 310.4, parallel conductors are only allowed for size 1/0 and larger.

g.mccormick said:

Per NEC 310.4, parallel conductors are only allowed for size 1/0 and larger.

Click to expand...

Agreed, slight correction..... it's actually 310.10 (H)

(H) Conductors in Parallel.
(1) General. Aluminum, copper-clad aluminum, or copper
conductors, for each phase, polarity, neutral, or grounded
circuit shall be permitted to be connected in parallel (electrically
joined at both ends) only in sizes 1/0 AWG and
larger where installed in accordance with 310.10(H)(2)
through (H)(6).

Click to expand...

How many current carrying conductors are in the raceway? You need to derate if there are more than 3, use
Table 310.15(B)(3)(a)

Even if the wire is rated at 90C, you cant always use that column. Most terminals on devices are only rated for 75 C so you are stuck using that column. Kind of sucks but thats the code.

See I read that article but the section before it excludes equipment building.

310.1 Scope
This article does not apply to conductors that are an integral part of cable assemblies, cords, or equipment.

I guess the real question is "what is the definition of equipment"

John Morris said:

See I read that article but the section before it excludes equipment building.

310.1 Scope
This article does not apply to conductors that are an integral part of cable assemblies, cords, or equipment.

I guess the real question is "what is the definition of equipment"

Click to expand...

When I read :

310.1 Scope. This article covers general requirements for
conductors and their type designations, insulations, markings,
mechanical strengths, ampacity ratings, and uses.
These requirements do not apply to conductors that form an
integral part of equipment, such as motors, motor controllers,
and similar equipment, or to conductors specifically
provided for elsewhere in this Code.

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I understand the code to be exempting integral parts of manufactured equipment. The NEC certainly, without a doubt covers wire that is added in the field.

I am sorry to say it but the Mech E is definitely correct that the NEC does not allowed for paralleled #6 conductors added in the field.

Are you manufacturing equipment?

What lugs are you going to land the paired #6's on? Are they rated for 2 conductors?

Saturn_Europa

We are panel building. Trying to wire in two 64 Kw heaters from the distribution block to the circuit breakers. I was going to load 3 awg but that clogged the wire way.

My argument was two 6 awg could safely carry the 76 amp load (theoretical plus S.F. is 95) per phase.

If I did the math correctly the cross sectional diameter of the 6 awg x 2 will carry 110 amps.

Maybe I did the math wrong.

Circuit breaker can have two pair, and the M.S is a 100 amp F-frame.

John,

I'm not arguing with the math. Honestly, I haven't checked it.

What I am saying is that you will have some NEC violations. Conductors from a distribution block to an OCPD can not be considered "integral" and are covered by the NEC.

Plus....most circuit breakers are only listed for 1 wire per screw terminal.

Plus......unless your breaker's terminal s are rated for 90c , you need to be using the 75 c column.

Good luck with your project!!

Saturn_Europa said:

Agreed, slight correction..... it's actually 310.10 (H)
...

Click to expand...

My code book is at home, I got reference number from quick google search.

Thank you for all valuable time and feedback

You should consider using silicone rubber motor lead wire.

http://www.awcwire.com/productspec.aspx?id=srml

Bit_Bucket_07 said:

You should consider using silicone rubber motor lead wire.

http://www.awcwire.com/productspec.aspx?id=srml

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Remember though.... that most circuit breakers and connectors are only rated 75 C.

This has caught me a few times when using SRML.

The silicone insulated wire will not get you to a different/smaller gauge (since the connections will more than likely be rated at75C you must use the 75C rating of you conductors). The silicone insulated wire may be a good idea to use for the field connection to the heater if the construction of the heater is such that the wiring could ever come incontact/close to the heater elements or be exposded to high temperatures. The heater terminal box can get very hot on some applications and silicone or other high temp wiring would be a good idea.

You could also use DLO cable

https://www.google.com/url?q=http:/...ggTMAE&usg=AFQjCNHSjSPJYHNZj1NWPFTfR0KwLVvE2w

scott.lawrence said:

You could also use DLO cable

https://www.google.com/url?q=http:/...ggTMAE&usg=AFQjCNHSjSPJYHNZj1NWPFTfR0KwLVvE2w

Click to expand...

DLO will not get you there either. (see attached)

It is a very bad misconception a lot of people have that if a conductor is rated for high amperage because it has a rating of 90 C or higher, they think they can just install it and load it up.

I deal with this out here all the time and its hard to convince a lot of these guys that you cant do this.

You can however use the higher rated ampacity for de-rating purposes. (more than 3 conductors in a raceway, conductors passing through high ambient spaces, etc) This will usually keep you from having to up size the conductor when de-rating becomes necessary for an application.

For installing as branch circuit conductors you must follow the NEC and more times than not, you will be subject to having to stay within the boundaries set forth by 75 C ampacity ratings.
The NEC does give you a break on SERVICE ENTRANCE & FEEDER conductors for residential single family homes where you can exceed this a bit. There is a table that spells out the exact allowable ampacity of each size conductor for this exception.

http://www.tfcable.com/wp-content/uploads/2013/03/DLO-90C-and-75-C-Ampacity-Chart-061711.pdf

^listen to this guy....code changed many years ago and almost always you must use 75 or 60 deg columns because EVERYTHING in a circuits path has to be considered and most terminals and lugs are rated for 75 deg only. You cannot use 90 deg column for sizing conductors...only for applying derate.