# Metering



## opes (Apr 21, 2008)

Here in Lake County Florida the AHJ is mandating all individual or group mounted utlity metering be connected to class J or T fuses per the meter label. If the fuse were installed on the line side of the meter I would say it has current limiting capabilities but when the fuse is installed on the load side of the meter it has no current limiting capabilities. 

The problem is that metering AIC ratings are not designed to handle high fault currents at present. There is little discussion in the code about utility metering. I fear we have a major problem with individually mounted metering when the current limiting device is mounted down stream on high fault current services. For example: A 500KVA transformer at 208V can produce 90,000amps at the secondaries. Utilty meter bases are not designed for this level of fault current and could result in a catastrophic failure under fault conditions. 

Any thoughts on this? 

When the electrician purchases and installs the meter base doesn't that make him responsible under NEC rules which covers fault current withstandibility?

Thanks


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## JohnJ0906 (Jan 22, 2007)

I am interested to know what your POCO has to say about it.

I know every POCO is different. Around here residential meters are owned and installed (for the most part) by them, so it is not an issue we electricians deal with.


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## opes (Apr 21, 2008)

The subject of metering has gone on deaf ears by the utility company and local building department. The meter companies would not discuss short circuit ratings and one got rather irritated that I would ask them the question about short circuit ratings.

I guess my point is this, Meter bases are rated for the most part at 10,000amps and 22,000amp withstand unless class J or T fuses are used in conjunction with the meter. The term "in conjunction with" is being applied incorrectly. The Lake County Florida building official has been forcing the contractor to install a class T or J fuse down stream of the meter regardless of what the design calls for. Of course proper engineering tells you that fuses will only protect down stream equipment from short circuit currents exceeding its rating. With the meter up stream of the overcurrent protective it is not protected. The only way of getting around the problem is specify CT metering.

We tried Ct metering which the building official approved and gave the contractor his permit then later phoned the electrical contractor and told him CT metering would not pass inspection. If wanted to pass inspection he needs to install class J or T fuses down stream of the meter and delete the CT metering requirement of the permitted design. The contractor made the change without approval from the engineer and life moved on. We would not have allowed it if we were required to certify the change, which would have opened up another can of worms.


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## Greg (Aug 1, 2007)

Who is your POCO. I'm in St. Cloud and we have never had to do this. How can the AHJ mandate this? We don't have local amendments to the code in FL. Sounds like there needs to be a coming to Jesus meeting with the POCO and AHJ. I've worked in Lake County and never had an issue like this.

This is a new one.


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## micromind (Aug 11, 2007)

If it were me, I'd ask (nicely) for a code reference. If they have one, great. Read it carefully, it might specify the requirement only on a service with a fault current rating above a certain amperage. 

The short circuit current of a 500KVA single phase transformer with a secondary voltage of 120/240 will be a bit shy of 30,000 amps. A 3 phase one will be slightly less than 20,000 amps. This assumes an infinite source (full voltage at the primary during the fault), and 7% impedance across the transformer. This is the current available at the transformer terminals, and doesn't take into account any losses in the wire from the transformer to the meter base. 

As stated above, most feed-through meter bases are rated for 10,000 amps of fault current. It's very rare that 10,000 amps is available at any meter base at 208 or 240 volts. Here's a more realistic example; suppose a single phase service is fed by a 100KVA transformer. 7% impedance is pretty typical for a transformer of this size. The short circuit current is less than 6000 amps. Suppose the service is a 120/208 wye, fed by a bank of 3-75KVa transformers. The short circuit current here is about 8900 amps. Again, this assumes an infinite source, and no losses in the secondary wire. 

I just don't get why the local AHJ is so worried about protecting meters. 

Rob


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## Podagrower (Mar 16, 2008)

Greg said:


> Who is your POCO. I'm in St. Cloud and we have never had to do this. How can the AHJ mandate this? We don't have local amendments to the code in FL. Sounds like there needs to be a coming to Jesus meeting with the POCO and AHJ. I've worked in Lake County and never had an issue like this.
> 
> This is a new one.


 
As it was explained to me. Any local amendment was required to be submitted and approved by the State (and I think written into the Florida Building Code) before it could be enforced.
That being said, sometimes it is just easier (and cheaper in the long run) to just go along with the program.


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## opes (Apr 21, 2008)

We'er not talking about a 500KVA single phase transformer. This transformer is a 500KVA three phase Y secondary with a 1.3% impedience at 208VAC. This condition means high secondary fault current.

I have sat down with the building officials about the subject and thier response. I quote, "If we enforce what your telling us no one will be allowed to have electrical service in Lake County."

Now I think this is a poor response considering the potential consequences.

I might add that single meters on services with more than 20,000Amps is rare under 400Amps but does occure.


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## JohnJ0906 (Jan 22, 2007)

opes said:


> I have sat down with the building officials about the subject and their response. I quote, "If we enforce what your telling us no one will be allowed to have electrical service in Lake County."
> 
> Now I think this is a poor response considering the potential consequences.


I'll agree this is a poor response.


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## opes (Apr 21, 2008)

My reference tis to commercial construction not residential. We're talking about a three story commercial building with multiple tenants connected to two services one from a 750KVA transformer and the other from a 500KVA transformer with 208Volt three phase secondaries. The developer added two tenenat infill projects that were more than a typical retail space. One was a 5000SQFT lounge and the other was a 4000sqft billiards hall both of which were to be designed for 100% smoking. The smoking criteria required 100% outside air HVAC systems which amounted to 60tons of AC each. Each tenant space required a separate 400Amp three phase service. The meter center was not large enough to handle the added load of two 400Amp services. The electrical service had to go directly to the utility transformer with two added services (this is another story in itself). The transformer we needed to connect to happened to be a 500KVA three phase 208VAC system (worst possible situation "very low impedience" less than 1.5%). We opted to go with CT metering to take the meter out of the potentially high fault current. Under the old rules metering on the utility side of the main switch was not a a liability becuase the meter was not part of the engineers design. Now the engineer has to specify the meter base as part of his project and sign and seal the plans. With switchable metering bases connected on systems that can deliver 40,000 or 50,000 or more amps in my opinion is an engineering liability and a safety issue should the meter base mechanism fail under fault conditions. Most likely there would never be a problem but with the jagged toothed laywers running around any item construed as non code complying or designed not using good engineering judgement is subject to all sorts of civil problems should something go wrong. Beside the civil issue an unsuspecting operator could get seriously hurt should he or she try and either close or open a meter by-pass switch under high fault conditions. These are my thoughts from an engineering perspective. Doesn't happen much but does happen from time to time on commercial buildings. I don't believe this has ever been a problem on residential construction.


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## ampman (Apr 2, 2009)

opes,i work and live here in lake co.(moved from orange) when i delt with this we would call poco(either seco or progress) and ask for available fault current and never would it be over 10,000 just lucky i dont know


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## opes (Apr 21, 2008)

Thanks for all the replies. It seems this metering issue, fault current and improper enforcement of engineering principals only appear to be a concern of mine and the meter companies refuse to talk about it. I will continue to specify CT metering with single high fault current services or metering down stream from the main with multiple tenant applications. I will not specify services with class J or T fuses up stream of a single meter as the source of protection. I guess in the long run if the authorities feel it important enough to change an installation in the field without engineering sign off or use poor engineering judgement in their decision this person or persons will suffer the consequence.


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