Is there any reason to oversize power cables to motors? 4

[bdn2004]

We have a project to replace a bunch of large cables feeding a significant number of 250Hp motors. The cables are being run in cable tray single file. As far as I can tell the engineering firm did the calculation right per the NEC and calculates the ampacity of the 500kmil cables after all the various temperature deratings to be around 382 Amps. Per the Code the motor leads must be sized with an ampacity of 302 x 125% = 378 Amps.

Because this is close, they say to bump it to 750kcmil. Now in my mind the 125% requirement already covered the safety factor. Is there any reason or any data anyone can provide that would justify spending this extra cash? Like will we get twice the life out of the cables?

 

[rbulsara]

1. What is the size of the existing cables? Have they served well over the years? If so, why not match it?

2. How long are the cables? Voltage drop could be a factor.

3. Compare to actual average loads as well.

4. Ask the engineering firm for the justification in writing.



Rafiq Bulsara

http://www.srengineersct.com

 

[bdn2004]

Rbulsara,

1. 1000 kcmil AL. And no, they have not served well, the insulation is tracking and causing faults.

2. About 200 - 300' feet each. Voltage drop should not be a problem.

3. Good point, will do.

4. Good point, will do.

 

[cranky108]

Tracking and causing faults? Is this a heat issue?

You said single file, how does this work? Single cable or several cables, and what order?

 

[rbulsara]

750 kcmil AL (THHN/THWN)is the minimum size for 378 amps, without any derating at 75 deg C ampacity. It is not oversized at all! I originally assumed that you are referring to cooper cables.

You may want the engineers to investigate the cause of insulation tracking and faults. Replacing them in kind (and even smaller cable), if that is the case, may result in same issues. Perhaps the type of cable used was not suitable for the application.



Rafiq Bulsara

http://www.srengineersct.com

 

[bdn2004]

The cable to be replaced is AL, 1000kcmil per phase, I'm assuming but I'll check, they are in a triplex arrangement in the existing tray. And yes there is some heat issue, these cables are located in an environment that typically stays about 100 deg F and in some places are exposed to the outside weather. The story is that the last problem was during a rainstorm when the cables got wet.

The cable is to be replaced with copper. The ampacity charts used for the derating was (I think) (no NEC here with me) 310.17 for single conductors in open air, the 90deg C column - like I say I think they did the calc right: derate for covered tray .6, derate for temp .9.

Just going from 500 to 750 copper is adding bunches of $$ to a project, not to mention the larger cable trays, and difficulty in pulling...Your calcs say your ok at 500 why go larger, how conservative do we have to be?

 

[jghrist]

What is the voltage? What kind of insulation is on the old cables? Are conditions dry?

 

[rbulsara]

500 kcmil , copper is the minimum size for 378A. Accounting for further derating and voltage drop if needed will increase the size.

Other thinking may be 1000 kcmil AL ampacity is 445A, which is close to 475A for 750 kcmil Cu. As I said, the design engineer is the only person who can answer his reasoning.



Rafiq Bulsara

http://www.srengineersct.com

 

[bdn2004]

480V is the operating voltage, 600V rated cables

and I agree 500kcmil copper min acceptable size.

 

[rbulsara]

bdn:
300 feet should not pose voltage drop issue at 480V and 302A, at all. I tend to agree with you, unless there is some other reason for derating.

Rafiq Bulsara

http://www.srengineersct.com

 

[eti]

bdn2004

Here is one factor you might consider;

The losses on the cables is I2R. Increasing the amount of copper will decrease the resistance and therefore the losses.

If you have multiple units then its quite easy to justify the extra expense of larger cables. The cost benefit of increasing the cable size can be determined by comparing the additional capital cost of the cable against the utility savings over the life of the cable. The installation cost shouldnt change as long as you dont need to fit new tray etc.

 

[mayanees]

I've always done electrical design to a minimum essential level, stemming from employment in a DuPont engineering group many years ago. That philosophy would dictate using the 500s, since they meet the criteria.

But there is one area that the NEC overlooks I believe, and that's where they allow the overloads to be increased above 120%.
Section 430.32(C)allows adjustments as high as 140% for motors with S.F. 1.15 or higher.

So if by chance these motors have service factors 1.15 or higher, it would be understandable that the cable be sized for the possibility that the overloads will grow to 140% of the fla or 302* 1.40 = 423 amps. But given that this is an existing installation, the working overload settings should be known.

To address the last post suggesting that the cable losses would decrease enough to provide a payback, that's easy enough to calculate using the cable resistance tables. But the payback period needs to be allot less than the life of the cable, which in my experience is not enough to provide justification.
John M

 

[bdn2004]

Mayanees,

Great answer. I also used to work at DuPont and know the minimum essential engineering philosophy. At one plant I remember they had 6-circuit panelboards all over the plant - minimum essential means what it says I was told.

I'll check the service factor and overloads.

 

[ScottyUK]

"minimum essential engineering philosophy"

No spare capacity, no future-proofing. Engineering by accountants.


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If we learn from our mistakes I'm getting a great education!

 

[burnt2x]

I maybe late chiming-in but here's my take on the matter.
I believe in what others were saying about voltage drop, voltage dip during starting, and steady-state cable operating temperature(current-carrying capacity)as minimum requirements in sizing supply cables.
But I did experience a cable failure, a meltdown to be exact, when the I²t aspect (short circuit withstand)was left out during calculations! This happened when the cable length was short and the motor terminals accidentally got short-circuited while in operation. I reviewed the cals and cable size passed the cable operating temp calculations, passed the VD calcs and the allowable voltage dip during starting but failed on the short-circuit withstand! During that incident, it should have been better if the cable was smaller and longer as opposed to choosing a short and bigger cable.
When we think we got it all figured out, think again. Maybe some other factors were not considered, and that also includes how deep the pockets of the company are.

 

[electricpete]

During that incident, it should have been better if the cable was smaller and longer as opposed to choosing a short and bigger cable.

Can you explain how smaller cable would have helped?


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[bdn2004]

I agree, how could a smaller cable have helped? But on the larger point, I don't think the average engineering firm doing work for most plants is considering the short circuit withstand rating of cables to motors, particularly if they don't, and most don't, have access to the protective relaying and short circuit software. Isn't this in part what the 125% requirement of the NEC is for?

Is there a rule of thumb method or other quick way to calculate this, that is the short circuit withstand rating of a cable?

 

[rbulsara]

It is well known (to those who consider short circuit ratings of the cables) that overcurrent protection provided per NEC, especially for LV installation, is more than adequate to protect against thermal damage to cables during a short circuit.

No, there is no quick way or rule of thumb for this. This is among one of the many advantages of using NEC guidelines to assure minimum safety. Many NEC guideline are intended to minimize burden/expense of heavy engineering or calculations on small users/owners for "run of the mill" applications. History has also proven them right, one of the basis of evolution of NFPA standards, including NEC.

Rafiq Bulsara

http://www.srengineersct.com

 

[burnt2x]

I have to quote my post:

"smaller and longer as opposed to choosing a short and bigger cable"

I know you understand what that specific conditions means; you get a lower SC current for the same load at the farthest end of the cable!
The case seems similar to the OP - replacing old cables and they just chose cables without checking everything.

 

[electricpete]

I tend to think that cable impedance is usually a small player in the source impedance (is it?). But even if it is a big factor, I can’t see how providing a smaller cable would ever help improve fault performance under any circumstance

Cable I^2*t limit ~ 1/A^2 where A is area. (Area shows up twice – once in the resistance and once in the heat capacity which is proportional to area)
Fault current < 1/A.... i.e. fault current would be proportional to A only if there were no other series impedances and no cable self-inductance... in reality the reduction in fault current would not be as much as 1/A.

Only if we pick the unrealistic case of no impedance other than the cable, then we could break even (not improve) by making a smaller cable since current would goes as 1/A, I^2 goes as 1/A^2, and this matches the 1/A^2 change in I^2*t limit. (I’m assuming t is not a variable that would change in this comparison... but depends on the protecton devices). For any realistic case where there is impedance other than cable resistance, you wouldn’t break even, you would get worse.

If your saying the cable length was the relevant factor, I can understand that better.

Going to the NEC question. I am by no means a code guy. From reading through it I see there is a whole lot of discussion of ampacity, but very little discussion of I^2*t. In fact the only I^2*t discussion I saw was a very generaly motherhood discussion

Circuit Impedance and Other Characteristics
The overcurrent protective devices, the total impedance, the component short-circuit current ratings, and other characteristics of the circuit to be protected shall be selected and coordinated to permit the circuit-protective devices used to clear a fault to do so without extensive damage to the electrical components of the circuit

Is there something else in the NEC that ensures cables will be protected for their I^2*t limit? I didn’t see it.


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