Back To Basics

By now, you should be pretty comfortable with both the function and the features of group metering. But, before you can make a product recommendation to a customer, there are a few other important concepts to consider.

Phase Balancing

First, let’s consider the topic of Phase Balancing. This only affects three-phase, incoming, single-phase outgoing tenant feed systems.

Phase balancing refers only to the metering stacks of three-phase systems. All three-phase metering stacks are factory-connected AB. This means that they pull power from the A-phase and B-phase.

When you have a system involving multiple stacks, it is critical to spread the load evenly across all three phases (A, B, and C). This can seriously impact selection of the metering stacks, as we will see in a moment.

Performing phase-balancing in the field for a metering stack is as simple as moving a jumper. Take a look at Figure 11.

Figure 11. Performing Phase-Balancing in the Field

Although it may be complex, physically performing the phase balancing procedure in the field is a simple and quick process.

Figure 12. Phase-Balancing in the Field

As mentioned earlier, it is critical to “spread the load” amongst the three-phases. This can seriously impact selection of metering stacks. But, there is a simple process for determining which metering stacks to use for any given application.

Let’s look at a few examples.

Example 1: 9 meter socket positions

If phase balancing were not an issue, it would be easy to select two metering stacks: one with four meter positions and one with five meter positions.

But with only two stacks, one of the three poles would be in use by both stacks. Recall that all stacks are factory connected as AB. Suppose you moved the jumper on the five-unit stack to AC. The result would be:

This system is out of balance. Instead, select three stacks with three meter positions each. Move one jumper to AC and another jumper to BC. The result would be:

Example 2: 16 meter socket positions

What happens when the number of meter positions isn’t divisible by three? It is not possible to perfectly balance the system. The answer is to get as close as possible.

Use two 5-position stacks and two 3-position stacks. Set one 5-position stack to AC and the other 5-position stack to BC. The result would be:

This system is not perfectly balanced, but it is as close as physically possible. The system will not suffer any adverse effects due to being slightly unbalanced.

In some areas, the utility places a limit of four meter positions on metering stacks. How can we balance a 19-position installation without using any five-position stacks?

Select one 4-position stack and five 3-position stacks. Set two 3-position stacks to AC and two 3-position stacks to BC.

The result is 13 meter sockets on A, 13 on B and 12 on C. This system is not perfectly balanced, but it is as close as physically possible.

Ring vs. Ringless

Earlier in this module, we mentioned ring-type and ringless style metering stacks. But we didn’t really get in depth regarding this distinction.

Ring-type construction allows the meter to be removed without also having to remove the individual meter cover. It uses a metal ring that physically holds the meter in place. Ring types include:

Ringless meter sockets have a drawn ridge around the cover. This ridge captures the meter and secures it in place. Thus, the cover must be removed in order to remove or install a meter.

Just like other aspects of the metering equipment, the ring specification is up to the utility company .

Bypasses

Throughout this module, we have made mention of various types of bypasses without really defining their function.

In the past, if a meter in the field is pulled out for inspection or replacement, the building would lose power. But, with the rise of computer usage, there is an increasing demand for uninterrupted power.

In response to this demand, bypass accessories have been developed. When a bypass unit is attached to a meter socket, the field technician can pull the meter without disrupting power to the building.

Although this means that the building receives unmetered power during this time, the cost of the power for so short a time is negligible.

There are three bypass types in common use today. (Figure 13.) Manual bypasses are used on ring-type metering stacks. Horn bypasses are used on ringless style metering stacks. Lever bypasses are used on ringless commercial metering and test bypasses are used on ring-type commercial metering.

Figure 13. (From Left) Manual, Horn and Lever Bypasses

Series Ratings

Metering equipment is rated by how much power it can handle. Underwriter’s Laboratories (UL) certifies these equipment ratings, called Series Ratings (or “integrated equipment ratings”).

Meter packs have a two-tier rating. For example, a 22/10 rating. This rating means that the meter pack has a 22 KAIC tenant breaker, feeding a 10 KAIC loadcenter with 10 KAIC branches.

Common meter pack ratings are 22/10, 42/10 and 100/10.

Group metering has a three-tier rating. For example, a 100/22/10 rating. This rating means that a main fused switch rated at 100 KAIC in the main service module feeds metering stacks with 22 KAIC tenant breakers. These tenant breakers in turn feed 10 KAIC loadcenters with 10 KAIC branches.

Common group metering ratings include 22/22/10, 42/42/10, 100/22/10 and 100/100/10.

Figure 14. Two-Tier (on left) and Three-Tier Series Ratings

Note that the watt/hour meter itself is not included in the short-circuit current rating. This practice is specified in UL Code 67.