Utility Company service calculations vs NEC requirments

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hassaf

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It would seem that the service calculations section in the NEC is somehow useless to the utility company.

every single job we do, requires that we do service calculations per NEC to justify our selection of the service equipments. The AHJ, and in most cases, it is the agency granting plan approvals, require that we comply with NEC.

On the other hand, the utility company has no interest in our calculations, because they size their transformers, or their service entrance conductors according to their rules, which in most cases, has a much lower ampacity than the NEC would require.

In my opinion, the NEC should have provisions for such calculations, and the AHJ, should accept the load calculations presented by the utilities company. Furthermore, the AHJ should not require service calculations to be part of their approval if the utility company can "do their own thing" on the service side

I would agree that if there was a single building and we're trying to sub-feed it from the main service equipments, then we can use these calculations in the NEC. Other than that, why even bother with the service equipment sizing and service calculations if the utility company is going to determine the service feeder conductors by their own demand factors. Thus, we should wait to see what is the entrance conductors, or find out what the utility's company transformer, then, size the main service equipments according to that.
 
Re: Utility Company service calculations vs NEC requirments

There is a member here who can tell you how the utility does it. :)

Here is my version, the NEC wants the service to have space for expansion. I know that seems to go against 90.1(B) but we all know that the calculations come out higher than needed.

The POCO does not want to waste power running transformers larger than actually needed for the service.

If the feed they provide does end up undersized (unlikely :)
 
Re: Utility Company service calculations vs NEC requirments

Originally posted by iwire:

The POCO does not want to waste power running transformers larger than actually needed for the service.
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I think money is more accurate than power. None the less the utility company uses NESC rules, which have nothing to do with NEC minimum requirements.
 
Re: Utility Company service calculations vs NEC requirments

While I can relate to your frustration, I think most of use hve been there at one time or another in figuring service sizes. There are many projects where the only thing figured for the original bid is the building shell. Such projects typically have a service size based on what types of business will fill the various units in the building (multi-meter). The utility company typically sizes their transfomer based on what they determine the demands will be through their experience with these types of installations. The utility sees the loads on these buildings each month (demand metering)where we do not. As time goes by uses change, tennants change and loads change. I think the NEC is trying to cover a wide range of possibilities over the life of the building.
 
Re: Utility Company service calculations vs NEC requirments

I can't follow this logic at all?

What does the size of the transformer or service drop/lateral have to do with the size of the service we are required to provide to an occupancy? We are not responsible for the POCO's installation. If a building is required to have a 400-ampere service, we must install a 400-ampere service regardless of what the utility company feels is appropriate. If they undersize by design or miscalculation, that is their problem.

In many cases, serivce drop conductors can be sized much smaller than what we are required to bring to the serivce point due to the fact that the utility conductors are in free air. Also, I have never seen an undersized transformer, and in most cases, they are oversized for future known loads to be added. Especially at new construction. :confused:
 
Re: Utility Company service calculations vs NEC requirments

Originally posted by bphgravity:
I have never seen an undersized transformer, and in most cases, they are oversized for future known loads to be added. Especially at new construction. :)
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Re: Utility Company service calculations vs NEC requirments

we all know that the calculations come out higher than needed.
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I think the calculations are still based on incandescent light bulbs (and probably bulbs with the same efficency as Edison's original bulb). But when is the last time anyone ever light an entire office with incandescent?? That will never happen these days.


Steve
 
Re: Utility Company service calculations vs NEC requirments

a year ago, we sized a service for 23,000 sq.ft building. According to NEC, we calculated approximately 825Amps (@208, 3 phase) demand on the service feeder, which is about 300kVA. The utility company installed 75kVA transformer ahead of 1000amps swithcboard!!

We realize that this is the utility company problem; on the other hand, if we try to reduce the service equipment to, say 400 amps for example, so that we may be in agreement with the utility company transformer size, then the AHJ will be all over us for not complying with NEC for the load calcs.

We can argue that even if you decide to add future loads, then you would still contact utility company to let them know, and they may want to increase their service transformer if they wish. So, I don't see compeling reason to to?do load calcs according to NEC.

In addition, in the design development stages of the project, there is no load information, so we base our load calcs on watts per square footage, and we rarely go back and check with NEC for compliance even after the loads have been established. The load calculations, based on watts per sq.ft, has been working for us and it is somehow in agreement with the utility company service sizing.

I regret undermining the NEC in that aspect, but there is a time when I wish we can relax some of the NEC requirements, especially in chapter two of NEC, if it was done under the supervision of a licensed electrical engineer, or if it was done by other responsible charge, such as the utility company.
 
Re: Utility Company service calculations vs NEC requirments

I feel the code generally provides a happy medium. I much prefer the idea that half of the occupancies in existence have services larger than what would really be needed to serve the load, then half being undersized due to even more minimal code allowances or excessive demand factors. :)
 
Re: Utility Company service calculations vs NEC requirments

My experience has been that while a utility will undersize a transformer (some times drastically) it will not undersized it's service conductors. Yes, the NESC conductors are sized differently than our NEC ones but really who cares.

The utility is not worried about overloading a transformer for short periods of time (relative to it's 25 year life) but they still need to account for voltage drop and voltage regulation, two things that are not governed by the NEC at all (especially not when determining service size).

Our problem has been the utlity designing for future emergency transformer replacement when suppling us with their SCA. In one case they gave a fault current for a transformer twice as large as the service size, they told me they could feed my 800A 208V service with upto a 500KVA transformer if they wanted to.
 
Re: Utility Company service calculations vs NEC requirments

Jim,
You are 100% right. The utilities never check or even seem to care in many cases when they upgrade a transformer. In many cases they double or triple the available fault current on the existing systems that are feed from the transformer. This is a very serious matter and needs to be addressed.

State Electrical Inspector
John Cosmo

[ June 01, 2004, 11:57 PM: Message edited by: explorer ]
 
Re: Utility Company service calculations vs NEC requirments

Jim Posted
Our problem has been the utlity designing for future emergency transformer replacement when suppling us with their SCA. In one case they gave a fault current for a transformer twice as large as the service size, they told me they could feed my 800A 208V service with up to a 500KVA transformer if they wanted to.
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I think what the utility is doing is using a 500 kva as a spare for transformers from 150 to 500 kva. If the 150 fails and there is none in stock they will use the 500 on a temp basis.

Explorer
You are correct regarding the increase in the fault current. This is an even greater problem now that the arc fault caculations are necessary. How would you solve this problem. The utility is doing its job in providing service to the customer.
I think Dereck made a comment on this subject a while back.
 
Re: Utility Company service calculations vs NEC requirments

This is an interesting subject and one where everyone has a different opinion. I will address this more later but keep in mind that the NEC is very conservative and once an installation is finished, the EC is done after the warranty period. The electric utilities will size based on any one of several factors and will generally expect a demand of about 40% of the actual service size. We size the transformation for the expected demand and take our chances. The utility regulatory commission requires us to maintain a certain level of voltage and we have to meet that forever. Additionally, if we have undersized the transformer or the load grows, we will replace the transformer . . . for free, even after years of service. We do not have a level playing field and that is the way it should be.

If we sized our transformers the way the NEC called for, we would be charging a lot more for power. Of course, if you wanted silver lined gutters, all you have to do is pay for them. :D
 
Re: Utility Company service calculations vs NEC requirments

I posted this in another forum:

You may be interested to see what I have written to train our electric utility power engineers. This is not for use outside of my industry!

"In general, any way you use to estimate load is fine if it has basis in fact. For commercial buildings, a reasonable rule is to use a demand factor of 50% for all connected load. Another rule, although somewhat shaky, is to take 80% of the switch size and multiply the resultant by 50%."

Generally speaking, the NEC is very conservative when it comes to sizing service equipment, service entrance conductors, and feeders. I do not have a problem at all with that section (the discussion is about 230.90(A)(1)Exception No. 3). However, you still have to do the calculations. It's sorta like cleaning the ice off the windshield before driving. If you clean a little hole, you can see a little bit and guess whether you are right or not. If you clean the whole thing, you can be sure. UH OH - sorry, the guys from the South won't understand the analogy. :D
 
Re: Utility Company service calculations vs NEC requirments

Originally posted by charlie:
UH OH - sorry, the guys from the South won't understand the analogy. :D
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Why would you want to put ice on the windshield for? Here in the South we just turn on the air conditioning when it gets hot. :D
 
Re: Utility Company service calculations vs NEC requirments

I worked in a church which was wired from the service head into the building with a 2000 amp capacity, but was connected to the transformer by Com-Ed with one 600 amp drop.
My complaints about low voltages and brown-outs on Sunday mornings resulted in emergency service showing up after noon, when the load had dropped, until one Sunday the lineman arrived at 11am, and burned his hand through the heavy glove by grasping the line with over 800 amps running through it. Our actual use apparently exceeded their estimated use by "a facility that size"!! Another restaurant I wired in the area had a connected load of over 1600 amps, with calculations showing a need for 1200 amps, but Com-Ed said 600 amps was fine for a restaurant that size "in their experience".
It is their equipment and their choice, I guess.
 
Re: Utility Company service calculations vs NEC requirments

We wired for a 200A 480V A/C unit and the utility hung 3 50kva cans on the pole for it. I questioned the town utility manager and he assured me they were plenty big enough. About mid August we blew a lid off one of the cans. Down time, EPA clean-up of the oil soaked in the dirt and damage to the paint jobs on a few cars in the parking lot sure cost more than the right size transformers would have. Just glad nobody got hurt or worse.
 
Re: Utility Company service calculations vs NEC requirments

We wired for a 200A 480V A/C unit and the utility hung 3 50kva cans on the pole for it.
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200 amps at 480 volts is 166 kva. The transformer bank was 150 kva. That load would not have had much effect on the 150 kva bank. You probably didn't pull 200 amps either. Something else caused that problem.

[ June 02, 2004, 11:29 PM: Message edited by: bob ]
 
Re: Utility Company service calculations vs NEC requirments

One final thought here, why is the NEC enforced on all others but the utility company? and to whom does the utility company report to for compliance? there must be a higher authority that govern utility companies electrical installations, is it OSHA?
 
Re: Utility Company service calculations vs NEC requirments

NESC and state regulatory commision.

[ June 03, 2004, 04:49 PM: Message edited by: dereckbc ]
 
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