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

Is there any reason to oversize power cables to motors?

Is there any reason to oversize power cables to motors?

(OP)
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?

RE: Is there any reason to oversize power cables to motors?

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

RE: Is there any reason to oversize power cables to motors?

(OP)
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.
 

RE: Is there any reason to oversize power cables to motors?

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?

RE: Is there any reason to oversize power cables to motors?

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

RE: Is there any reason to oversize power cables to motors?

(OP)
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?





 

RE: Is there any reason to oversize power cables to motors?

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

RE: Is there any reason to oversize power cables to motors?

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

RE: Is there any reason to oversize power cables to motors?

(OP)
480V is the operating voltage, 600V rated cables

and I agree 500kcmil copper min acceptable size.

 

RE: Is there any reason to oversize power cables to motors?

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

RE: Is there any reason to oversize power cables to motors?

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.

RE: Is there any reason to oversize power cables to motors?

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

RE: Is there any reason to oversize power cables to motors?

(OP)
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.  

 

RE: Is there any reason to oversize power cables to motors?

"minimum essential engineering philosophy"

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

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

RE: Is there any reason to oversize power cables to motors?

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.

RE: Is there any reason to oversize power cables to motors?

Quote:

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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RE: Is there any reason to oversize power cables to motors?

(OP)
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?

RE: Is there any reason to oversize power cables to motors?

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

RE: Is there any reason to oversize power cables to motors?

I have to quote my post:

Quote:

"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.

RE: Is there any reason to oversize power cables to motors?

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

Quote (NEC 110.10):


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.
 

=====================================
Eng-tips forums: The best place on the web for engineering discussions.

RE: Is there any reason to oversize power cables to motors?

Smaller longer cable would reduce the fault current at the load end.  If the load end equipment was not rated for the full fault current, this could conceivable help, but I would not consider it a good design.  A lower fault current could also increase clearing time and increase arc flash incident energy.
 

RE: Is there any reason to oversize power cables to motors?

The short circuit does not always occur at the end of the cable. Thats why NEC requires the interrupting device to be able to interrupt the available current at the interrupting device, not at at some distant remote end.

RE: Is there any reason to oversize power cables to motors?

(OP)
Thanks for the interesting discussion. It makes me wonder if decisions like this shouldn't have to go through an engineer instead of selecting it from a table in the NEC.

It reminds me of this somewhat toothless Board I sat on a few years ago in a fairly large city to make recommendations for the city's electrical code above or below the NEC. Whenever we had a meeting about once every 3 months the IBEW was right there demanding to be heard for something else to be done strictly by licensed electricians. Maybe at some point they wrestled this away from engineers.
 

RE: Is there any reason to oversize power cables to motors?

I vote burnt2x a LPS for bringing up an important aspect that was not mentioned in the early posts.  (I certainly didn't think of it when I read this thread.)

My comments applied to the cable damage scenario he described. (not to breaker interrupting capacity).

I agree with you your comment bdn2004 – I can't find any table in the NEC that will help you meet I^2*t requirements for the cable.   But there's a lot I don't know.

Again I would like to hear rbulsura explain what he meant by this:

Quote (rbulsura):

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

=====================================
Eng-tips forums: The best place on the web for engineering discussions.

RE: Is there any reason to oversize power cables to motors?

electricpete:

One can check this with a software all they want. Fuse/breakers sized for overcurrent protection per NEC would always turn out to be adequate for the thermal damage curve of the cables. The ampacity tables of NEC are not without any basis nor are the limits on OCPD rating there in based on conductor ampacity.

NEC does not preclude proper engineering however, as you already quoted per NEC 110, however. So when in doubt, check it.

NEC is for minimum safety code against fire and personnel hazard, not an assurance of complete engineering or meeting design objectives.

Rafiq Bulsara
http://www.srengineersct.com

RE: Is there any reason to oversize power cables to motors?

...and to reinforce what rafiq is stating, when you plot the time-current-characteristic of an overcurrent device, say a 100 amp circuit breaker, and include on the drawing the thermal damage (withstand) curve for the #3 CU cable that's connected to it, the situation is always such that the cable is completely protected up until it runs out to the 1000 second part of the curve. At that point the breaker protects the overload capacity of the cable and it's much closer to the melting curve for the cable.
That's to say that when you protect a cable with the NEC approved protection, there is always adequate protection for short-time withstand type faults.
As alluded to in his response - that's not always the case with medium voltage cables, particularly since NEC has much less stringent protection requirements over 600 volts.
John M

RE: Is there any reason to oversize power cables to motors?

Thanks Rafiq and John.   I appreciate your comments. I have no doubt I can learn a lot about NEC and protection from you guys.

What about short circuits when a high fault current level is available?  There is a finite delay for the instantaneous element to sense the short circuit and the breaker to open the circuit.  During that time the cable is exposed to the fault current.  I don't think any of the tables consider the available fault current as an input.   

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RE: Is there any reason to oversize power cables to motors?

Pete,
Here's a Time-Current-Characteristic curve showing a 4000 amp main on the secondary of a 3000 kVA transformer.
The instantaneous is off, which is often done to improve selectability with downstream distribution breakers.
But you can see the 4000 amp damage curve is far to the right of the maximum available fault current that the cable would see - denoted with a flag marking SQD NW, which is about 55kA.
You can see that the cable/4000A bus is totally protected - even with the Instantaneous off.
John M

RE: Is there any reason to oversize power cables to motors?

Thanks John.  A large bus will have a large withstand which will be difficult to challenge without an unrealistic high fault current.   How about a smaller conductor like your previous example with 3AWG cable.

Attached is I^2*t curve for 3AWG copper cable (26 mm^2 cross section) showing time to rise from 75C to 100C.  The equation of the resulting curve is I^2*t = 7.5E6 Amp^2*sec.

The withstand time for various current levels is:
I    t
10000    0.074919467
20000    0.018729867
30000    0.008324385
50000    0.002996779

If we have 50,000A fault current, we need to interrupt in 0.003 sec – doesn't sound likely.
If we have 30,000A available, we need to interrupt in 0.0083 sec which is still less than half a cycle and probably not realistic.   I read somewhere one half cycle is the time used by ICEA for rating cable short circuit.  I would think it depends on the interrupting device and some surely will not be that fast.  Let me know if I am looking at it wrong.

=====================================
Eng-tips forums: The best place on the web for engineering discussions.

RE: Is there any reason to oversize power cables to motors?

Correction in bold

Quote:

Attached is I^2*t curve for 3AWG copper cable (26 mm^2 cross section) showing time to rise from 75C to 150C

=====================================
Eng-tips forums: The best place on the web for engineering discussions.

RE: Is there any reason to oversize power cables to motors?

.. hard to resist this topic line, but limited on my availability.
You're certainly looking at it correctly.
I've attached another hastily assembled TCC that shows a #2 cable with 100 and 110 amp SQ D breakers.
Note that the breaker with adjustable instantaneous protects - but only at the Low Inst setting. The thermal-mag breaker with no adjustments leaves the cable vulnerable in the short-time/inst region.
This pretty much says that it needs to be looked at on a case-by-case basis, which is a good thing because it keeps us engineers working!
But relaize that this is a very special case where a 100 amp cable is terminated at a point where there's 55kA of fault current, which usually wouldn't be the natural order of things. At a switchgear bus with that level of fault current the breakers are usually on the order of 400 amps or larger, and an 400 amp cable would tolerate that level of instantaneous fault current.
John M

RE: Is there any reason to oversize power cables to motors?

But you only need to protect a conductor against damage from through faults, and smaller conductors tend to limit through fault currents. If the fault happens in the conductor it is damaged any way, no matter how fast you interrupt the fault. The catch 22 is that as fault current goes down the clearing time goes up.  

RE: Is there any reason to oversize power cables to motors?

Thanks for the heads-up, David.

RE: Is there any reason to oversize power cables to motors?

If the cable damage itself is not the cause of the fault, cable itself seldom gets damaged ( beyond repairs) during a through fault.

Most cable and insulation damage (and fire) does not occur because of high fault current, but rather during the low fault current events such as arcing faults. Low fault currents do not clear the OCPD, building up localized heat, melting insulation, ionizing air and resulting in a much greater fault and damage. Another common cause for cable insulation damage is loose connections, followed probably by misapplication of cable insulation types.

Rafiq Bulsara
http://www.srengineersct.com

RE: Is there any reason to oversize power cables to motors?

To David's point, absolutely, and thanks for that clarification.
If that switchgear had 50 kA of available fault current, and 25 feet of #2 AWG cable is terminated on a 100 amp breaker, the fault current is reduced to about 30 kA at the termination of the cable.

John M

RE: Is there any reason to oversize power cables to motors?

Most low voltage cable circuits are self protecting from damage to short circuits at the end of the run.  The impedance of the cable reduces the short circuit level to where the typical MCB clearing time protects the cable.

I use a spreadsheet calculation that looks at the source short circuit MVA and calculates the length of cable needed to self protect the circuit from damage.

My spreadsheet says a #4 copper wire fed from a 100 KA bus by a 100 A MCB needs 35 feet of conductor length to be self- protected. At 50 kA the length drops to 27 feet.

A #2 awg fed by a 225A frame MCB needs 38 feet at 100 kA and 25 feet at 50 kA.

As long as the cable length is more than this critical length, a bolted fault at the motor, the panel or equipment fed by the wires will not permanently damage the wires.
 

RE: Is there any reason to oversize power cables to motors?

And the breaker at the switchgear location supplying the #2 must be capable of interrupting 50 kA per NEC 110.9 and not just 30 kA line end value.

Davidbeach,

If you protect based on the cable end fault, you will be replacing more cable than if you protect for the close-in fault, in the event of a mid-cable fault. Or do you always replace the entire cable and avoid splicing?

RE: Is there any reason to oversize power cables to motors?

If we consider the specific short-circuit withstand of a #2 cable, is it good allowing 50kA on it? ICEA says #2 can safely be allowed to be subjected with around 36kA (assuming thermoplastic insulation)for 1 cycle (0.017 sec).
Am I missing something here?
 

RE: Is there any reason to oversize power cables to motors?

When I was doing that type of calculation in my designs, the first point at which a splice might be possible was beyond the critical length (to use rcwilson's terms).  Now working at much higher voltages the fault currents are generally low enough and the clearing fast enough that I'm not much worried about cable damage.

RE: Is there any reason to oversize power cables to motors?

(OP)
Getting back to my original question...I just talked to the engineer in charge of the area and he told me that these motors are started under a heavy load and the starting currents are over typically over 1000 amps for as long as 15 seconds. For one, is this really atypical? Would the NEC Tables still be applicable?

Can anyone refer me to an example problem where this is worked out so that I can get all the required information and make this a more scientific analysis? Thanks.

RE: Is there any reason to oversize power cables to motors?

bdn:
No that is not atypical. NEC tables are still applicable. To verify cable thermal withstand capacity, you need to compare the cable damage curves with the breaker/fues TCC. Similar to those posted by mayanees. NEC cable sizing is minimum requirement, if you have special needs you need to address those without violating NEC. This assumes that NEC is applicable in your jurisdiction.

Rafiq Bulsara
http://www.srengineersct.com

RE: Is there any reason to oversize power cables to motors?

Also prolonged starting (acceleration) time may justify upsizing the cable to minimize voltage drop at the motor terminals to provide adequate torque during starting period. Motor torque varies in proportion to square of the voltage.

Rafiq Bulsara
http://www.srengineersct.com

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