Parallel Conductors - 1/0 minimum?

[essco]

What is this 1/0 minumum wire size for parallel conductors based on? (NEC 310.4) Why can't I go smaller, as long as the conductors are identical?

Case in point is a 460V 125HP Variable Frequency Drive (VFD) rated for 164 Amps. I am using a 225A circuit breaker that also feeds other panels. The lugs on the drive don't allow a 3/0 wire, but it must be at least 3/0 for the 225A breaker.

Any thoughts?

 

[iwire]

Re: Parallel Conductors - 1/0 minimum?

Essco,

This being a branch circuit for a motor the conductors do not need to be sized to the breaker.

You may size the conductors to the full load current of the motor you will be running from this VFD.

Take a look at Table 240.4(G) Specific Conductor Applications and parts II, III, and IV of article 430.

If you post the motor size you are going to be running (you did not say if you where using the drive to the maximum rated load) we could help you out.

 

[tom baker]

Re: Parallel Conductors - 1/0 minimum?

Conductor size to a VFD is based on the input rating of the drive, not the motor HP.
See 430.2

 

[iwire]

Re: Parallel Conductors - 1/0 minimum?

Thanks for the catch Tom.

Bob

 

[bennie]

Re: Parallel Conductors - 1/0 minimum?

essco: The 1/0 minimum, for paralleling, is based on the resistance of 1/0 is not dramatically affected by a slight difference in length.

1/0 is like a median size selection.

Any size of conductors can be paralleled as long as the overcurrent device is not more than the ampacity of one conductor.

 

[bob]

Re: Parallel Conductors - 1/0 minimum?

Bennie
You comments make complete sense but I do not see where 310.4 allows it in this type installation.

[ October 09, 2003, 06:08 PM: Message edited by: bob ]

 

[Guest]

Re: Parallel Conductors - 1/0 minimum?

Originally posted by bennie:
<snip>Any size of conductors can be paralleled as long as the overcurrent device is not more than the ampacity of one conductor.

You lost me here Bennie-- but that's easy enough

Please define "one conductor". I think that will do it for me.

Thanks!

 

[bennie]

Re: Parallel Conductors - 1/0 minimum?

Check out 310.4, exception NO.2 (c).

I should have wrote, "single conductor" instead of "one conductor".

I do agree the section does not apply to this installation, when referencing the specified items.

Two or more conductors, with the ends connected constitutes one conductor. Leaves it a question as to the acceptability for equipment other than the ones specified.

[ October 07, 2003, 12:17 AM: Message edited by: bennie ]

 

[pierre]

Re: Parallel Conductors - 1/0 minimum?

Bennie

Sometimes you have a perfectly good picture in your head, but you do not give us the reader enough information to understand the point you are trying to make, it is very frustrating because we do respect what you have to say.

If you are going to take a code issue and dispute it, or try to help us understand it differently, you need to be more specific so as to 'paint' the picture for all of us to see.
Point in case 310.4

Pierre

 

[bennie]

Re: Parallel Conductors - 1/0 minimum?

Pierre: Thanks for bringing this to my attention. I have bought a new book titled "Writing Power for Dummy's"

I will work on my style.

Dummy's is referring to me, not to others.

See, I am already clarifying my writing.

[ October 07, 2003, 07:46 AM: Message edited by: bennie ]

 

[essco]

Re: Parallel Conductors - 1/0 minimum?

I suppose if anyone is guilty of not providing enough information, it is I!

Here is the details of the system:
460VAC Input
225A Circuit Breaker with lugs allowing #4 - 300MCM.

Power distribution block with line-side lugs up to 500MCM, load side lugs up to 1/0.

Load #1 - 125HP 460VAC VFD with built-in electronic overload protection
Rated Input Current: 172A
Rated Output Current: 156A
**Actual Load - 100HP motor (113FLA)
(Note - there is no additional Short-Circuit protection between the 225A CB and the VFD.)

Load #2 - Separate panel with 20A circuit breaker.
(Hence the need for the PDB)

Based on your replies above, if I size the branch conductors to the VFD for 115% of the input rating of the VFD, it would be #2/0 (Based on UL508A Table 29.1) My PDB doesn't accept 2/0 - hence my question about the 1/0 minimum for parallel conductors.

If it is sized for the actual load of the motor, it would be (113 * 1.25= 141A) #1 wire - works with my PDB.

Which is correct?

I still don't understand why I must use a minimum size of 1/0 for parallel conductors that only run about 3 feet inside an enclosure. Electrically they are the same as a single conductor. Why can't I use two #3's (120+120 = 240A)?

 

[bob]

Re: Parallel Conductors - 1/0 minimum?

Essco
430.22A exc #2 requires the conductor to be 125% of the drives rated input. Thus 172 x 1.25 = 215 a
and 4/0 conductor.
Would the tap rules apply to load#2?

 

[essco]

Re: Parallel Conductors - 1/0 minimum?

Bob,

Thanks for the tip on 430.22A Exc. 1.

Yes, I believe the tap rule would apply for Load #2, but I am not too concerned about that load. (It has its own Circuit Breaker and there is no issue with conductor size.)

My main issue has to do with parallel conductors and this limitation of 1/0 minumum. In my case, it is a problem trying to find a power distribution block that satisfies this requirement without being too huge. (We have one that will work, but it is rated for 400A!!)

It would make a lot more sense to me to use two #2 wires in parallel. The would be the same length and would be a total of maybe 4 feet, inside a single enclosure. These would fit into my power distribution block with no problem, and the "effective" ampacity would be 230A.

I saw Bennie's comment about the resistance of 1/0, as compared to smaller conductors, being a "nominal" value as not to adversely affect ampacity if the lengths are different. But 310.4 specifies that they MUST have the same length.

So... Does anyone have a solid explanation on this 1/0 minimum restriction on parallel conductors? Not that I expect the code to be changed or find some exception, but I would like to know what this rule is based on.

Thanks!