Metering

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.

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

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.