Looking for an NEC-savvy critic

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nateholt

Member
Location
Toledo, OH
Looking for an expert willing to check over and critique a proposed software specification that is designed to (hopefully) help an electrician or electrical designer do the following in a logical, user-friendly, NEC-faithful way:

In AutoCAD, user launches the proposed software utility to guide in selection of conductors for a 3-phase motor load. The utility provides dialog prompts for selection of parameters like:
? Wire type standard ? AWG or MM (metric wire sizes)
? Wire type ? copper or aluminum
? Insulation temp rating
? Optional wire length (for voltage drop calculation)
? Wireway type (affects the voltage drop calculation)
? Power factor (defaults to 0.85 ? affects voltage drop calculation)
? Maximum % voltage drop allowed ? will affect the conductor sizing ? bumping up to next larger conductor size to reduce voltage drop percentage
? Continuous load (default to ?ON? for all motors)
? Fill correction de-rating
? Ambient temp de-rating
? Paralleled wire options (for larger size / larger ampacity runs)

I am a double-E but my experience has been in industrial controls design and now, for the last twenty years, more in the software engineering side of things. I?m looking for someone with current, real-world experience to verify that the ?work-flow? is logical, that it steps through the process the way an electrical designer thinks, it is faithful to NEC, and that I?m not missing anything major.

If this would appeal to anyone out there, please let me know.

Thanks!
Nate Holt.
 

nateholt

Member
Location
Toledo, OH
Voltage drop / large diameter wires / non-unity power factor

Voltage drop / large diameter wires / non-unity power factor

Another issue related to voltage drop calculation for large diameter conductors with non-unity power factors... where the k=12.9 seems to no longer remain constant. I've published a little AutoLISP utility here in the last 24 hours for comment and critique.

http://mfgcommunity.autodesk.com/blogs/blog/view/7/Voltage_Drop/

I'm posting here to broaden the audience. Critique of the logic is appreciated.

thanks,
Nate.
 

bob

Senior Member
Location
Alabama
nateholt said:
Another issue related to voltage drop calculation for large diameter conductors with non-unity power factors... where the k=12.9 seems to no longer remain constant. I've published a little AutoLISP utility here in the last 24 hours for comment and critique.

http://mfgcommunity.autodesk.com/blogs/blog/view/7/Voltage_Drop/

I'm posting here to broaden the audience. Critique of the logic is appreciated.

Wire size [? for list] =500
Select 1=PVC, 2=Alum conduit, 3=Steel conduit:3
Power factor [default=0.85]:0.85
Amp current:285
Wire length (feet):400
voltage drop=9.84, kW loss=4.9
Hmm... according to this little utility, will only have about 10 volt drop at the motor, well within spec. But this voltage at this 285 amp current, 3 phase, calculates out to almost a continuous 5 kW loss in the wiring.
Let's say today's electrical rate is 7 cents per kWh. Our wiring loss (assuming motor is always running) will be 5 x 24 x 365 x 0.07 = $3066 per year. Seems like a lot.
Nate.

Assuming you used 500 kcm AL I calculate a little different answer than you.
From Table 9 the resistance of 500 kcm AL = 0.048 ohms/1000 ft. or
0.048 x .4 = 0.0192 ohms for 400 ft.
Using the 285 amps and neglecting the pf the watts loss is
285 amps x .0192 x 1.73= 9.46 w/1000 = 0.00946 kw
0.00946 kw x 24hrs x 365 days = 82.86 kwh x .07 = $5.80 /yr.
 

nateholt

Member
Location
Toledo, OH
Hi Bob,

Thanks for taking a look at this. I need confirmation that the logic is right or, if not, guidance to make it right.

I think your 9.46 value below is actually voltage drop, not power. E = I * R * 1.732?, but P = I * I * R * 1.732? I think you need to square the current to get power?

Let me know what you think.

thanks,
Nate.

bob said:
Assuming you used 500 kcm AL I calculate a little different answer than you.
From Table 9 the resistance of 500 kcm AL = 0.048 ohms/1000 ft. or
0.048 x .4 = 0.0192 ohms for 400 ft.
Using the 285 amps and neglecting the pf the watts loss is
285 amps x .0192 x 1.73= 9.46 w/1000 = 0.00946 kw
0.00946 kw x 24hrs x 365 days = 82.86 kwh x .07 = $5.80 /yr.
 

bob

Senior Member
Location
Alabama
nateholt said:
Hi Bob,

Thanks for taking a look at this. I need confirmation that the logic is right or, if not, guidance to make it right.

I think your 9.46 value below is actually voltage drop, not power. E = I * R * 1.732?, but P = I * I * R * 1.732? I think you need to square the current to get power?

Let me know what you think.

thanks,
Nate.
You are correct. Tried this much too late.
 

nateholt

Member
Location
Toledo, OH
Expanded AutoLISP utility - voltage drop at power factor - CU and AL supported

Expanded AutoLISP utility - voltage drop at power factor - CU and AL supported

The AutoLISP utility has been republished. Added support for Aluminum conductors (original version was copper only). All data reflects what is published in Table 9 and all calculations follow Table 9 - "Note 2" method of determining effective impedance. Download from this blog posting:

http://mfgcommunity.autodesk.com/blogs/blog/view/7/Voltage_Drop/

Running against example above, for both copper and aluminum 500MCM conductors, 3-phase, at three different power factor values:

Copper - 500MCM, 400 feet, 285 amps, 24/7

PF -- Voltage Drop -- kW loss -- annual kWh * $0.07 wiring losses
0.85 --- 9.84 -------- 4.9 ---------- $3004
0.90 --- 9.26 -------- 4.6 ---------- $2820
1.00 --- 5.72 -------- 2.8 ---------- $1717



Aluminum - 500MCM, 400 feet, 285 amps, 24/7

PF -- Voltage Drop -- kW loss -- annual kWh * $0.07 wiring losses
0.85 -- 12.55 -------- 6.2 ----------- $3802
0.90 -- 12.13 -------- 6.0 ----------- $3679
1.00 --- 8.91 -------- 4.4 ----------- $2698



So, if the utility's logic is correct, it looks like substituting copper for aluminum in this situation (where load runs continuously 24/7), the savings amounts to $800/year if cost is seven cents per kWh.

It also looks like ignoring power factor in this calculation can cause the actual "cost" of the wire IR losses to be significantly underestimated.

Again, I ask for critique on the logic of this utility and the results above. Do these results look right?
 

mayanees

Senior Member
Location
Westminster, MD
Occupation
Electrical Engineer and Master Electrician
HP Calculation uses Voltage and Current...

HP Calculation uses Voltage and Current...

Nate,

I think that for optimal accuracy you need to better define the load characteristics.

In the case of a constant current load, I think the calculation applies.

But in the case of a motor, the voltage at the motor terminals needs to be taken into account. With CU wire, you'll have higher voltage and a resultant lower current in order to meet the HP requirements at the shaft.

With Aluminum, the voltage will be lower, but the current will be higher.

I think that with the inclusion of a current value in the calc that is based on HP requirements and the calculated voltage - the cost difference between the 2 scenarios will be significantly less.

JM
 

nateholt

Member
Location
Toledo, OH
Hi JM,

Thanks for taking a look at this. You are correct, the calculation is assuming a fixed current load whether dealing with copper or aluminum conductors. The point that is illustrated (hopefully) is that short-term savings on the wire type selected may be lost by having to pay for the extra power taxex used for additional heating of the aluminum wiring.

On your second point... if copper wiring results in higher voltage presented at the motor terminals (i.e. less voltage drop) and therefore lower current to produce "x" shaft HP... and aluminum results in lower voltage available to the motor (higher voltage drop in the wiring) and therefore higher current needed to produce the same shaft HP, wouldn't this further increase the "wire heating losses" cost difference between these two scenarios (P_wiring_loss = I * Vdrop * 1.732)?

Please let me know what you think.
thanks!
Nate.
 

shockin

Senior Member
I may be incorrect, but are you comparing appples to oranges? A load requiring 500MCM Alum would only require 350MCM copper. Shouldn't the comparison take that into consideration. Also with the copper there is smaller conduit which may produce a savings. I'm not sure if that makes a differance - you're over my head. Good Luck!
 

mayanees

Senior Member
Location
Westminster, MD
Occupation
Electrical Engineer and Master Electrician
response ... after a drink... or two :)

response ... after a drink... or two :)

Alright Nate, .. (& shockin)

Now I'm thinking a little more rational.

I missed the concept - which I now see as rationalization for an oversized (copper) conductor. And I think your logic holds true.

What we usually design to is an ampacity, like 380 amps for a CU 500 kCM feeder. To get that ampacity in Aluminum, the size goes up significantly.

But bottom line is that we can tolerate as much as 3% voltage drop on that feeder, so we'll upsize the Aluminum until that VD is satisfied. At which point, the (I sqd R) loss is the same.

And truth-be-told Nate, I've never used aluminum conductors for motor circuits!! having grown up in industry where wire is copper. My Utility service time - wire was aluminum.

But you're pushing an oversized ampacity to limit the VD, and thus the (I squared R) losses.

I think it's a viable concept. Particularly viable since KWH rates are high - and I think the industrials are higher than the 7 cent rate you were using.

Sign me up!

JM
 

nateholt

Member
Location
Toledo, OH
Thanks JM,

I appreciate the feedback.

Yes, the idea I'm trying to move toward is just what you suggest... sizing conductors to not just meet today's minimum ampacity and maximum voltage drop requirements, but to design with the "Green" long view in mind. If I can bump up to the next larger conductor size, reduce the voltage drop and conductor losses, and still fit in the originally planned conduit, what is the potential long-term payback over the life of the installation?

Electrical rates will probably continue to rise, and this will only magnify the rightness or wrongness of today's design decisions. If electricians / designers / engineers have the information and analysis right there at their fingertips... better chance of making informed decisions.

Again, thanks!

Nate.
 

kingpb

Senior Member
Location
SE USA as far as you can go
Occupation
Engineer, Registered
I think something that needs to be considered is terminal connections. There are probably many on this forum still familiar with the fiasco created when copper was a premium (and shortage) and aluminum romex was developed. Well, we all pretty much know the outcome of that.

I can't say for sure without some research, but you may have issues at the terminals. Are the motor terminals acceptably rated for aluminum? If so, you better factor in some costs for O&M due to having to re-check and re-torque the terminal connections periodically. This could be a maintenance nightmare.

Economically, the installed cost may look acceptable and have a reasonable payback, but you have to include O&M costs as well. Usually from an Owner's perspective, if the payback is over 5-7 yrs, he's not interested, and it won't matter how "green" it is. As of now anyway, banks arn't discounting the costs to borrow based on how "Green" the design is.
 

nateholt

Member
Location
Toledo, OH
Thanks for the head's up. What is "O&M"? "Operations and maintenance"?

The software I am spec'ing is not necessarily to encourage the designer to switch from copper to aluminum but to show the user what the payback potentially could be by opting to oversize motor conductors. Let's say 4-0 copper conductors will meet the ampacity and come in just under the maximum allowed voltage drop limit. But if user bumps up to 250MCM copper, the resulting efficiency (i.e. reduced voltage drop / heating losses in the wiring) payback might cover the cost of the extra copper many times over during the life of the installation.

This may make sense only for larger motors with long wire runs that operate pretty much continuously... but having information like this could help the electrical designer / electrician / engineer make an informed design decision.

Again, thanks for the reply.

Nate.
 

mayanees

Senior Member
Location
Westminster, MD
Occupation
Electrical Engineer and Master Electrician
... what's next

... what's next

So Nate,

What's next on your to-do list to get this lisp program desirable?

Will you attempt to get the requirement into a LEEDS certification program? Clearly if you showed a payback in a reasonable time then the gov't would seize the opportunity.

My experience with industry was that if you could show a 3-year payback it was funded.

I think it has merit.

JM
 

nateholt

Member
Location
Toledo, OH
mayanees said:
So Nate,

What's next on your to-do list to get this lisp program desirable?

Will you attempt to get the requirement into a LEEDS certification program? Clearly if you showed a payback in a reasonable time then the gov't would seize the opportunity.

My experience with industry was that if you could show a 3-year payback it was funded.

I think it has merit.

JM

Funding is not an issue. I'm looking for an "NEC-savvy expert" willing to look over a couple parts of my Word doc spec. These outline a short series of AutoCAD dialogs. Hopefully they are logical, clear, and intuitive... and guide the designer / electrician / engineer through a series of choices to make an NEC-smart and "green" energy efficient smart decision. I am neither NEC-savvy nor electro-mechanical industry-current. So I want to make sure that when I turn this over to the programming staff, that what they output will be exactly what users have been waiting for: in AutoCAD, chose circuit type, motor size, incoming line voltage, ambient temp, run length, etc, etc and hit a button. AutoCAD draws the complete control circuit, pops in all the components, correctly sizes everything - all per NEC requirements.
 
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