How overkill are NEC calculations?

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peter d

peter d

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Location
New England
I've been observing a lot of utility installations lately, and I can't help but notice lots of small utility wires (and many homes connected to one transformers) connected to our large wires. I know, there is the "open air" factor, but it seems like the NEC factors in massive overkill on service sizes.

Can any of the utility savvy guys shed some light on this? What are the utility guidleines for sizing outside conductors? 1/2 the NEC calculated load? 1/3? 1/4?
 
With regard to POCO's, current carrying conductor ampacity seems to be determined in accordance with NESC Rule 012C:

012. General Rules
C. For all particulars not specified in these rules, construction and maintenance should be done in accordance with accepted good practice for the given local conditions known at the time by those responsible for the construction or maintenance of the communication or supply lines and equipment.​

The way it looks to me is that the NESC only addresses conductor ampacity with regard to grounding conductors. Typically under the NESC, #2 AL is good for 200 amp, and #1/0 AL is good for 320. ​

The NEC ampacities seem to be very darned close to the old Neher-McGrath method calculations. In fact, I'm pretty sure the Neher-McGrath formula is in the NEC, but not under that name. The NESC ampacities could be based on IEEE 835, IEEE 738, or IEC 287, or just a chart developed from the utilities own historical data.​

I'm interested in learning more about this myself. The above are just little tidbits I've collected over time.​
 
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Neher-McGrath is referenced in Appendix B as the functional basis for ampacities in the tables.

As for overkill, when you realize that mechanical/civil/structural "safety factors" can range from 5 (some boiler calcs) to 25 (certain elevator calcs), we electrical types get off pretty lucky.

Utilities often operate systems to failure and then simply replace with new and/or larger equipment - it's a different eccomomic mentality than most of us operate under.
 
Some typical sizes I've observed on secondary conductors installed under the NESC:

#2 CU or 1/0 AL up to 25 KVA

#1/0 CU on 37.5 KVA

#4/0 CU or 350 AL on 50KVA

#350 CU or 500 AL on 75KVA

#750 CU on 167 to 500KVA
 
We have a moderator that designs for a utility, he is also a CMP member for Article 240 so he has an NEC background as well.

He has not been posting lately but in the past has noted that the utilities historical load data for dwelling units comes in about 50% of the NEC service calculations.

If the NEC calculations show 150 amps the utility he works for will assume 75 amps.

As far as transformer sizing don't even make an attempt to use NEC sizing techniques with utility transformers. The utility often runs them above their ratings for short periods of time.

The NEC is concerned with shutting off the electricity in case of a problem, the utility is concerned with keeping the electricity on.

And as Bob mentioned, if their system breaks they are available 24/7 to repair it for free.

Honestly......how many service drops have you seen fail from overload?:)
 
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We install wires inside of buildings. Utilities install them outside of buildings. If we have wire failures due to heat (overloading), it can get pretty catastrophic. If utilities have overheated conductors, well not so bad for the public (other than the inconvenience).

This makes me wonder though, how critical is voltage drop? Is the 3% for branch circuits overkill? The PoCo doesn't seem to be too concerned about the 2% recommendation on feeders (yea I know the NEC doesn't apply to them).
 
Lou, I see what your saying, I don't recall Charlie E ever mentioning voltage drop.

That said it seems if they where thinking about it that they would base their calculations on much lower load than our NEC calculations would have us use.

In practice it seems the power company avoids voltage drop by placing the transformers close to the high loads.
 
iwire said:
Honestly......how many service drops have you seen fail from overload?:)
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Not so many, but this one just might at some point:

monsterweatherhead.jpg
 
mdshunk said:
Not so many, but this one just might at some point:
Click to expand...

Or it might last forever.

If it fails the POCO will replace it with at least the same. ;)

Besides.....I am sure this is just temporary. ;) :grin:
 
LarryFine said:
That's equivalent to a 200a 1ph service, and we're allowed to go even smaller than that.
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I guess I was talking mostly about the tail off the pot, and not each service drop that is tapped off that tail.
 
The cabin that I'm working on right now sits in a development that is designed for 4 homes.

The homes average 2000-2500 sq ft. and are all electric....each with a 240V, 200A single phase service.

The POCO will supply all 4 homes from a central pad mounted transformer.

The transformer is 75KVA and the (underground) service lateral conductors will be #2 aluminum.

The average distance from the transformer to the houses will be approx. 200 feet.

To me, the transformer seems undersized and the voltage drop on the service conductors will (IMO) be excessive, but hey....what do I know?

If the lights go dim everytime the A/C compressor kicks on, who do you think the customer will be calling?

Luckily, my home is the first one in the development, so I won't have any transformer related problems, for now, but later on, who knows?

Steve
 
Again we are trying to think of transformers in our terms which is a mistake.

Transformer can provide much more than their rating, the utility has a great deal of first hand experience about what works and does not work.

They have no problem with a transformer being used in excess of it's rating a couple of hours a day during the dinner time hours.

If they did size transformers like we do the electric rates would have to be raised to cover the costs of larger equipment and the power lost in running over sized transformers.

As far as voltage drop most states have utility commissions that set limits on acceptable voltage drop. These commissions can force the utility to upgrade if the voltage drop is in excess of the limits.
:smile:
 
iwire said:
Again we are trying to think of transformers in our terms which is a mistake.
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Most of the modern pole-top transformers have a red "overload" light on them, which you can see from the ground. I had one customer who swore someone had rigged up a camera on the transformer in front of his house to spy on him, since he saw the red light on from time to time. :rolleyes: Many of the bigger padmounts have a 'thermotell' inside of them, which puts the needle of the thermometer at the hottest the transformer has been.


580.485501001030037901.jpg
 
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mdshunk said:
Most of the modern pole-top transformers have a red "overload" light on them, which you can see from the ground. I had one customer who swore someone had rigged up a camera on the transformer in front of his house to spy on him, since he saw the red light on from time to time. :rolleyes: Many of the bigger padmounts have a 'thermotell' inside of them, which puts the needle of the thermometer at the hottest the transformer has been.


580.485501001030037901.jpg
Click to expand...

I learned something new today....I'm happy.

steve
 
JohnJ0906 said:
What IS that, like #8?
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I am willing to bet it is more like 1 AWG.

The real question is what is the connected load?

They are not on fire. ;)

I am just kidding around here, it would be nice to know all the details of what is shown in the picture.
 
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