For the inspectors out there.

[hillbilly1]

I had a helper that I had to lay off due to lack of work, call me one day, the company that he went to work for was doing a big box store, 1200 amp gear, they cut ALL of the conductors exactly the same length, then proceeded to terminate without cutting any of them. He said it was a jumbled up mess by the time they were through. :lol:

 

[kwired]

I think you are looking for 300.5(I) Exception 2.

That is it. Had to read it 3 or 4 times before it sunk in, at first I didn't think that was it.

I thought it was worded in a way that was easier to understand.

 

[texie]

I had a helper that I had to lay off due to lack of work, call me one day, the company that he went to work for was doing a big box store, 1200 amp gear, they cut ALL of the conductors exactly the same length, then proceeded to terminate without cutting any of them. He said it was a jumbled up mess by the time they were through. :lol:

This whole topic of exact lengths has been discussed here before. IMHO there are many situations where you can't get closer than 1 or 2% or you end up with what you describe or may not even be possible to do. I've never had an AHJ quibble over a couple of %. What say the EE's out there? Is it really that critical?

 

[texie]

That is it. Had to read it 3 or 4 times before it sunk in, at first I didn't think that was it.

I thought it was worded in a way that was easier to understand.

'cause it's not located in the code where you would logically think.

 

[infinity]

If I were doing parallel runs I would have my conductors measured out before they enter the conduits and then if any wire is cut from one I would cut the same amount on the other otherwise I would bend the excess inside the panel if possible.

In 25 years I have never seen anyone do this. With long runs it's impossible to get them the same length unless you want to leave a big spaghetti bowl in the enclosures.

 

[Cleveland Apprentice]

how much leeway is permitted in parallel conductors of same phase? I thought you could have up to a 12" difference in conductor lengths of parallel conductors of same phase. Most of my wiring is residential work and just curious. Thanks

 

[don_resqcapt19]

The code gives no leeway. The code rule, as written, makes the use of parallel conductors a code violation as it is not physically possible to make the conductors the same length.

From a practical matter, a couple of percent difference in length will not likely cause any real world issues. There are other things, such as the physical arrangement of the conductors, that also change the impedance of the parallel paths. The goal is to keep the current as equally divided between the parallel conductors as possible. The current will divide in inverse proportion to the lengths...the shortest length (lowest impedance) will have the most current and the longest length (highest impedance) will have the least amount of current.
If you would have a 600 amp circuit with 3 sets of 3/0s and the lengths were 10', 11' and 12', the currents would be 218.8, 198.9 and 182.3 amps. Note that in this case you would exceed the rated conductor ampacity of the 10' run.
If the lengths were 100', 102.5, and 105' the currents would be 204.9, 199.9 and 195.2 amps. Again the shortest conductor would be operating above its rated ampacity. These numbers assume that the only change in the impedance of the paths is the change in conductor length.

 

[Cleveland Apprentice]

Don, would it be better to run 4/0 instead of 3/0 in your example? this way you wouldn't have to worry as much in balancing the amperage among the 3 sets. Also, could you tell me how you did your calculations in your example just for future reference. thanks

 

[ActionDave]

If I were doing parallel runs I would have my conductors measured out before they enter the conduits and then if any wire is cut from one I would cut the same amount on the other otherwise I would bend the excess inside the panel if possible.

I was taught to do it that way

In 25 years I have never seen anyone do this. With long runs it's impossible to get them the same length unless you want to leave a big spaghetti bowl in the enclosures.

Yes, that is what it looks like when you are done.

 

[texie]

The code gives no leeway. The code rule, as written, makes the use of parallel conductors a code violation as it is not physically possible to make the conductors the same length.

From a practical matter, a couple of percent difference in length will not likely cause any real world issues. There are other things, such as the physical arrangement of the conductors, that also change the impedance of the parallel paths. The goal is to keep the current as equally divided between the parallel conductors as possible. The current will divide in inverse proportion to the lengths...the shortest length (lowest impedance) will have the most current and the longest length (highest impedance) will have the least amount of current.
If you would have a 600 amp circuit with 3 sets of 3/0s and the lengths were 10', 11' and 12', the currents would be 218.8, 198.9 and 182.3 amps. Note that in this case you would exceed the rated conductor ampacity of the 10' run.
If the lengths were 100', 102.5, and 105' the currents would be 204.9, 199.9 and 195.2 amps. Again the shortest conductor would be operating above its rated ampacity. These numbers assume that the only change in the impedance of the paths is the change in conductor length.

Although I agree in general with you, at first glance I'm not sure if your current calcs are right. Can you show the math?

 

[hillbilly1]

This whole topic of exact lengths has been discussed here before. IMHO there are many situations where you can't get closer than 1 or 2% or you end up with what you describe or may not even be possible to do. I've never had an AHJ quibble over a couple of %. What say the EE's out there? Is it really that critical?

In 25 years I have never seen anyone do this. With long runs it's impossible to get them the same length unless you want to leave a big spaghetti bowl in the enclosures.

That's what that job looked like, they took it that all phases and neutral had to be the same length, when it's only each set of parrallels.

 

[don_resqcapt19]

Don, would it be better to run 4/0 instead of 3/0 in your example? this way you wouldn't have to worry as much in balancing the amperage among the 3 sets. Also, could you tell me how you did your calculations in your example just for future reference. thanks

I chose 3/0 to make a point.
The calc is in a spreadsheet and is a lot like solving parallel resistances. For the lengths of 100, 102.5 and 105' and a total current of 600 amps the current in the 100' run is found as follows.
600/(((1/100) + (1/102.5) + (1/105)))1/100

 

[hurk27]

You have the right code section - I was going to add that you can have all phase A in one conduit , B in another, .... IF it is underground in non metallic raceway, but am not finding that - did that allowance disappear?

Also remember 300.20(B) if the non-metallic conduits land in a ferromagnetic enclosure don't forget to cut the slots between the conduit entries.

As for the other questions, I agree with Don, it doesn't take much to have a higher current on the shorter conductor, I usually cut all the same length then I cut and land the shortest conductor using this cut off piece to measure how much to cut the other ones, this way they stay very close.

Would I loose sleep over one conductor a few inch's longer? in most cases not, but if the conductors were sized close to the load then maybe.

In most cases if the conductor has the 125% head room that many runs would have, it would most likely never cause a problem, other factors are the terminations as the double barrel lugs would dissipate more heat knowing the fact that most all failures is at the termination not mid span.

 

[hurk27]

How many of you have ever verified that paralleled conductors were the same length?

To tell you the truth I really haven't worried about it for years, until yesterday when a young electrician had all of the neutrals and grounds in one conduit and all of the phase conductors in the other.

When I told him that they were also supposed to be the same length, I got that deer in the headlights look.

Also keep in mind the requirments of 250.122(F) and 250.102(C)(2) if it applies.

 

[texie]

I chose 3/0 to make a point.
The calc is in a spreadsheet and is a lot like solving parallel resistances. For the lengths of 100, 102.5 and 105' and a total current of 600 amps the current in the 100' run is found as follows.
600/(((1/100) + (1/102.5) + (1/105)))1/100

Help me here...your formula is saying that if you increase the length of 1 conductor 2.5% you decrease the current by 2.5%. Yes that increases the DC resistance by 2.5% of the conductor, but that is a very small percentage of the total resistance of the load.

 

[don_resqcapt19]

Help me here...your formula is saying that if you increase the length of 1 conductor 2.5% you decrease the current by 2.5%. Yes that increases the DC resistance by 2.5% of the conductor, but that is a very small percentage of the total resistance of the load.

Yes it is a small percentage of the total circuit resistance, but the circuit is a current divider and the differences between the impedance of each of the parallel paths change the amount of current. The formula just uses feet of wire in place of the actual resistance because the resistance is directly proportional to the length.

You can do the calculation using voltage drop based on the resistance of the wire from the tables in Chapter 9, but that takes a lot more work. With the votlage drop method, you find the total resistance of all of the paths in parallel and use that resistance to find the voltage drop of the parallel circuit. Since the paths are parallel the voltage drop for each of the paths are the same. You then take the calculated voltage drop and the resistance of each of the single paths to find the current on that path.

 

[kwired]

'cause it's not located in the code where you would logically think.

Help me here...your formula is saying that if you increase the length of 1 conductor 2.5% you decrease the current by 2.5%. Yes that increases the DC resistance by 2.5% of the conductor, but that is a very small percentage of the total resistance of the load.

I didn't check his math but when a small change in resistance results in an inverse proportion change in current it doesn't take much resistance change to make a big current change.

I have clamped meter around individual conductors many times on parallel conductor installations and there is almost always a difference in current - probably because conductors are not same length in most cases. As I said earlier a longer set should have more tolerance as the percentage of resistance change will not be as high as it will be for a short set.

A 1000 foot parallel that has 5 feet of difference in length will have lower percent difference in resistance of each conductor than a 10 foot parallel with 6 inches difference in length.

It is basic parallel resistor calculations. Find resistance of conductors, plug in voltage and current and calculate how much of the current passes through each resistor.

 

[hillbilly1]

I was doing some load checks for the customer's engineers from India, and since I didn't have a amp clamp that would reach around 6-500 kcmil paralleled conductors per phase, so I did them Individually and totaled up the sum. All but one had readings within 1 amp of the others, but it was nearly 50 amps lower than the others, and just on that one phase. The engineers were baffled as well on what I found. Notified the POCO to check their end, because it was probably a loose connection in the transformer, because everything checked good temp and connection wise inside. Never did hear back from them, so they must have found the problem.

 

[dicklaxt]

I think if the truth be known that was written to keep the conduit runs close together in routing and not one on opposite sides a support rack.

dick

 

[don_resqcapt19]

...It is basic parallel resistor calculations. Find resistance of conductors, plug in voltage and current and calculate how much of the current passes through each resistor.

The method I use actually does that, but uses wire length and not the actual resistance and without actually calculating the voltage drop. The resistance is directly proportional to the resistance so the results are the same without using the actual resistance.