Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!
  • Students Click Here

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Students Click Here


Primary vs. Secondary Metering

Primary vs. Secondary Metering

Primary vs. Secondary Metering

Does it really pay to go primary?

You get a better rate, but now you maintain and replace the transformers...and now have to account for transformer losses...

Unless you directly utilize medium voltage, it seems that primary doesn't make sense once you factor in transformer losses bringing everything down to 480V...

Am I missing something?
Anyone have an example/analysis...


RE: Primary vs. Secondary Metering

As with so many things that get asked here, the only answer at this point is "It depends."  First off, the rates make a tremendous difference and the rates are highly dependent on location - no idea where you are, no idea what might make sense.  Secondly, size of load can matter greatly, the impact is a function of the ...(wait for it)... rates.

Bottom line, it can be a great savings, or it can be a great waste of time and resources, you have to do an analysis based on your loads, load profile, and rates.

RE: Primary vs. Secondary Metering

I agree with David Beach, it will depend on the rates and other factors.  Just one point - transformer losses.  Transformers are about the most efficient energy conversion devices around.  Losses are extremely low when compared to just about any other system losses.


David Castor

RE: Primary vs. Secondary Metering

Yes but 1.5%-2% of a relatively continouus 1500kW load can add up to a bit of money in a year.
We could get a $0.003/kWh deduct for going primary, but my understanding is the incentive usually accounts for the transformer losses that are now on the customer, not leaving much if anything for the annual maintenance and eventual replacement of the transformers...

RE: Primary vs. Secondary Metering

You mention replacement of the transformer.  Does that mean you don't have to buy the transformer to start with?   

RE: Primary vs. Secondary Metering

Some times the savings can be a phantom.

Utilities normally are in for the long term, so they will buy higher quality equipment.

However, you can construct a substation cheeper with used, and rebuilt equipment.

That's probally how some of the primary metering starts, as the customer normally pays for a special substation (one way or another). The issue of primary metering allows the customer to construct there own substations.

However, we recently saw a customer construct there own substation because they wanted a higher reliabilty than we normally offer (they paid more than we would have).

RE: Primary vs. Secondary Metering

How many times has the utility maintained the transformer feeding you now?  Maintenance costs are not that much more on a transformer than on the rest of your electrical system - main switchgear and MCC's. But you will own it and get to replace it if it fails.  Just like your switchgear.

$0.003 x 8,000 hrs/yr x 1500 kw = $36,000 savings per year before accounting for transformer losses. Losses are about 3 kW + 22 kw load loss = 25 kw.  A good design could knock that down to 20 kw.  $/kwh = $36,000/(20kw x 8,000 h)= $.225/kwh.  If your equivalent base rate is $ 0.25/kwh or more, you might be ahead.

Many, many assumptions in this analysis: constant load, no time value of $$, no cost for metering and protection or maintenance, etc, etc.

Like the other gentlemen said, there is a lot more to look at.  Several of my clients and employers went for it and have not had large regrets. (Maybe they all went bankrupt before the transformer costs hit.)

RE: Primary vs. Secondary Metering

Power costs (and savings) are often over emphasized in this decision, and often there is not enough thought given to who will maintain the system.  Back in the 1970's it was popular for the local power company to talk some owners in to primary metering that the owners now wish they had not.  

Back in those days enforcement was a bit different, but now the AHJ is requiring a UL listed switch at point of delivery. That is a 90" tall unit, typically not vandall proof and not appropriate for unsupervised locations.  

Typical utililty pad mount transformer is not UL listed and for some duties they will run them all the way up to 220% of nameplate, that too is not allowed on an installation subject to NEC.  I am not sure if Load Break Elbows and other elements of URD construction are UL listed, typically NEC primary installations have bolted lugs with stress cones and require more switches than Load Break Elbows.  

If a facility has it's own qualified staff to work on primary that might change the analysis, but many local campgrounds with primary systems do not. When a cable fails (not unheard of) they are at a loss for whom to call. When a transformer starts leaking a little they need a major project to replace it.  

RE: Primary vs. Secondary Metering

I still stand by my previous answer that there isn't any where near enough information available; but that said, I'd generally never consider primary metering below 5MW of load without other extenuating circumstances.  Once did primary metering for about 1.5MW of load, but over 1MW of that load was over 13 miles from the metering point, secondary just wouldn't do.  But for the typical 1.5MW load I wouldn't have ever given primary metering a first thought, let alone tried to work out the economics.

RE: Primary vs. Secondary Metering

I agree with the "it depends" comments. Rates structures vary dramatically.

One other point is that, depending on the contract, you may be paying for transformer losses with secondary metering as well, as meters have the ability to include transformer loss compensation.


RE: Primary vs. Secondary Metering

I gave the load and the rate difference, but I'm not looking for a complete analysis of my situation anyway...

I was merely looking for thoughts and experiences about going primary. I have heard from more than one source that there is a "5MW rule" (At least at that level of load primary usually starts becoming more accepted).

Another consideration is the "want" to go primary even if economocally it may not be the best choice. I'm thinking maybe mission critical facilities or the like, where control and ownership may warrant a bit more $$$

RE: Primary vs. Secondary Metering

The rate difference is a lot more meaningful if the rate is also included.
It depends" is transformer size for a new installation.
I planned a small mill (that was never built due to an abrupt down turn in the economy).
With primary metering we needed a code sized transformer, about 750 KVA. With secondary metering the utility would install a 500 KVA transformer.

"Why not the best?"
Jimmy Carter

RE: Primary vs. Secondary Metering

The rate difference is my only monetary incentive to buy, maintain, and eventually replace transformers.

RE: Primary vs. Secondary Metering

I have designed many electrical building distribution systems over the years, and have only specified primary equipment twice.  The first time was for a brand new 180,000 square foot high school.  We did the calculations, and the school would have a payback for the extra equipment of about five years. The school decided it was worth the payback.  

The second scenario was for a million square foot manufacturing and warehouse facility.  The load was so large that the utility would only let the owner buy primary power.  If I remember correctly, the demand load was around 5000 kVA.   

RE: Primary vs. Secondary Metering

In reality, the transformer losses are already covered by the retail rate, along with the rest of the utility system losses, depreciation, maintenance, etc.  In the end, the customers pay for all the losses, one way or the other.

If this load can be served either at primary or secondary voltage, it just comes down to an economic analysis, as you say.


David Castor

RE: Primary vs. Secondary Metering

Another consideration is transformer spares.  If the transformer fails with secondary metering, then the utility replaces it and probably has one in stock.  With primary metering, the customer replaces it and maybe is without power for a long time.

RE: Primary vs. Secondary Metering

And is someone qualified for basic maintenance tasks like replacing a primary fuse?

I usually only consider primary for large facilities, special control and protection applications and campuses where the maintenance staff is already familiar with it.

"The engineer's first problem in any design situation is to discover what the problem really is." Unk.

RE: Primary vs. Secondary Metering

I think you made a mistake here. The utility company usually owns the meter, and metering transformers. If one of those fail the utility company should have a spare.

Just think about it, who in there right mind would trust the customer to own the cash regester (other than the post office)?

However it would be the customers requirment to own the instrument transformers for protection of there equipment. For most smaller primary metered customers, that would be the CT's. Which normally don't go bad that often.

One exception I've seen is a primary metered wind farm, which owned several sets of PT's for there internal protective uses. And strangly they did not have spares.

RE: Primary vs. Secondary Metering

I'm sure that the reference to transformers was to transformers stepping the service voltage down to utilization voltage. Of course the utiliy is going to own the metering instrument transformers.  

RE: Primary vs. Secondary Metering

True enough.
However, I have seen that a requirment for the customer to own the protection, including the breaker, as a requirment. I did have a primary metered customer wanting to use fuses, but we did not allow that as it would have required us to slow down the line protection to accomidate them.

I think they opted for circuit switchers as they were cheeper than breakers (they had two transformers, in case one failed). Strangly the transformers were not what failed and caused them a several month outage (yes we steped in and helped them out, although we probally shoulden't have).

RE: Primary vs. Secondary Metering

Re the ownership of the PTs and CTs:
Revenue class meters may not be suitable for protection and vice versa.  

"Why not the best?"
Jimmy Carter

RE: Primary vs. Secondary Metering

Another "it depends" issue is the physical configuration of the project itself.  When I have a college campus, a new hospital campus, community college campus, a 100 acre automobile racing venue, a 50 acre high school campus or a several hundred acre airport to design, the project will always be a primary metered design.  One can use secondary metering, but the distribution gets cumbersome, innefficient and expensive.  It is much cheaper and easier to deliver 500 kVA 0f power 2000' away at 12.47kV than it is at 480V.  Facilities such as these will have or will contract for maintenance services.

Just another factor that makes primary metering attractive.  In some areas local utilities will offer "added facilities" service wherein they will provide on-site distribution, maintenance and metering to multiple buildings and basically finance the primary distribution system in exchange for added charges on top of the regular energy costs the facilities used.  Oftentimes however, these costs when analyzed for the long run, will far exceed the costs for the customer to finance, own and maintain the system privately.


Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members!


Close Box

Join Eng-Tips® Today!

Join your peers on the Internet's largest technical engineering professional community.
It's easy to join and it's free.

Here's Why Members Love Eng-Tips Forums:

Register now while it's still free!

Already a member? Close this window and log in.

Join Us             Close