Primary metering and secondary metering are two different ways of metering the usage of different customers. Though the way they are installed differs, there are some similarities as well. There are also reasons why you choose primary metering over secondary metering and vice versa.
What is the Difference Between Primary Metering and Secondary Metering?
The biggest difference between primary metering and secondary metering are the voltages. In primary metering, the metering is installed on the high voltage lines. Many times when referring to primary metering we are talking about distribution lines. Some common voltages for distribution lines are 12,470v and 24 kv.
Since primary metering installs its equipment on the primary, the metering equipment is rated for this higher voltage. This means that the instrument transformers, CTs and PTs, are bigger than what you would find in a secondary metering installation. Primary metering installations are typically found in three places. They are found overhead on a pole, underground in an enclosure, and they are found in substations.
Whether on a pole, in an enclosure or in a substation, a primary metering installation uses basically the same things. Depending on the type of service, from one to three CT’s and PTs are used.
In secondary metering installations the voltage is lower than in primary installations. Secondary metering is installed on the secondary outputs of both overhead and underground transformers. It can also be installed in CT cabinets or at the riser of an overhead installation.
Depending on the utility PTs may be optional on voltages over 120v. Other utilities require PTs on anything over 240v. This is my recommendation
What are the Similarities Between Primary Metering and Secondary Metering?
Actually, they are quite similar. The secondary sides of the CTs and PTs wire the same. They are both marked with H1 H2 and X1 X2. This lets you know where to put your wires. Believe it or not, you actually use the same meter form numbers for both. Obviously this depends on the service being metered of course. If you have a single phase tap line that you want to meter with primary metering you have one CT and one PT. You can meter this with a form 3s meter. Just like if you were to have a two wire secondary service. You can use a form 3s meter.
The meter multipliers are calculated the same way. Use the CT ratio times the PT ratio and you have your multiplier.
Why Choose Primary Metering Over Secondary Metering?
This is best described with an example. Lets say that you own an apartment complex and the utilities are included with the rent. The tenants do not pay electric bills to the power company. Instead of having an hundred power bills in the mail every month you have one bill with the total usage.
The same works if you are large factory with several buildings all feeding off of the same primary. You can get one bill from just one meter. This may actually save you money as well in demand charges as well as facilities fees. Check with your utility on this though.
Why Choose Secondary Metering Over Primary Metering?
The biggest reason to choose secondary metering over primary metering is the ability to monitor the usage of each transformer. This alerts you to problems early on that can be fixed before they get too big. This can also help if you are a landlord over an apartment complex. You can make sure that everyone is paying their fair share of the electricity by having them pay their own bills.
Conclusion
Primary metering and secondary metering are both good ways to meter a customer’s service. They are two different approaches to the same problem. You can meter a large industrial customer with primary metering equipment even though they may have ten different transformers on their site. You could also meter those ten transformers with secondary meter and come out the same.