3 Reasons Why You Should Be Using PT’s

Why use PT’s? That is a good question and this post offers three reasons why you should be using PT’s in your transformer-rated services. First, let’s review what PT’s are. PT stands for potential transformer. Some people call them VT’s which stands for voltage transformer. The names can be used interchangeably. PT’s are used to step down the voltage in a transformer-rated service to a safer and more manageable level. If you want more information on PT’s then check out our page on CT’s and PT’s. That brings us to the three reasons why you should be using PT’s.


The biggest reason why you should be using PT’s in your transformer-rated services is safety. Safety is very critical for your metering personnel and lineman. So, how do PT’s increase safety? Remember that PT’s step down voltage from a higher voltage to a lower voltage. For instance, if you have a 3 wire 480v delta service you could use 4:1 PT’s to step the voltage down at the meter base to 120v. This is much safer for utility personnel to work with.

Anything above 240v should be using PT’s to increase the safety of metering personnel and lineman. This means that instead of allowing 480v 4 wire wye and 3 wire delta self-contained services you should consider using a CT cabinet with CT’s and PT’s no matter how small the service is. This can help prevent exposure to 480v.

Prevent Catastrophic Meter Failure By Using PT’s

Below you can see what happens when a self-contained 480v meter blows up. This particular service was not even

Form 12s 480v meter blows up

Form 12s 480v meter blows up

in use at the time. The building that this meter serves is vacant. This is an Itron Sentinel form 12s meter installed in a 480v 3 wire delta service. Things like this can and do happen. If however, this installation had been metered with instrument transformers and used a CT cabinet or even overhead metering this meter would probably have not blown up. It does need to be mentioned however that if something in the service caused this meter to blow up then most likely had PT’s been used, one of the PT’s would have blown as well.

I can hear what you are saying right now. Well, if the PT is going to blow to then what is the point? The point is that when a PT blows it is generally not a catastrophic failure like shown in the pictures. Sometimes when a PT goes bad it is even hard to tell from the outside that anything has happened. Yes, you will still have a PT to replace but the failure will not be catastrophic. Although not the case in the pictures, often times when a meter blows like this the meter base needs to be replaced as well. Also, when a meter fails like this it can create fires as well. Which brings us to our last reason why you should be using PT’s.

Reduce Customer Downtime By Using PT’s

When you have a meter failure like the one shown in the pictures what does that mean for your customers? That means that they are going to have some downtime while you make the necessary repairs. At a very minimum you are going to have to replace the meter. Many utilities now have policies that prevent their personnel from working on 480v services while they are still energized. This means that while any repairs are made power to your customers will remain off. This can be the case even in the event that you are using a transformer-rated service with PT’s. In the

Form 12s 480v meter failureevent of a meter failure on a self-contained service like the one shown above power will be turned off until a new meter can be set. If the meter base is burned up or if a fire is caused this can mean that an electrician will have to be hired by the customer and also that the fire department will come out and inspect as well. Sometimes this means that the customer will have to pass inspections before the service can be restored. This can be a few days sometimes. If however a PT fails, the power may only need to be off for 30 minutes while the bad PT is replaced and everyone can go on their merry way.


PT’s are an essential tool that can be used to increase the safety of utility personnel who work on metering installations. When used properly they can help prevent catastrophic failure of metering installations. On top of that when used, PT’s can reduce the downtime your customers experience by reducing the number of meter failures and by reducing the time it takes to make repairs when a failure does occur.

Primary Metering vs Secondary Metering

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.


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.

Meter Multipliers

Watthour meter multipliers are used whenever current transformers and/or voltage transformers are used in a metering circuit. The most often question that I am asked about watthour meter multipliers is how to calculate the watthour meter multiplier. It really is a simple thing to do and this article will explain.

Current Transformers and Potential Transformers

CT’s and PT’s are used to step down current and voltage in a metering circuit to a safe and measurable level. But this leaves us with a little bit of math to do to accurately calculate the number of KWH or kilowatt hours the customer has used.

To calculate the meter multiplier we need to know what the CT ratio and PT ratio are. The best way to explain this is with an example.

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Meter Multiplier Example 1

Let’s say for instance that you have a 120/208 three phase 4 wire wye service that is using 400/5 CTs.

How would you determine the watthour meter multiplier? You would simply do the math. 400/5 means 400 divided by 5 and if we do this math we come up with an answer of 80, this is our watthour meter multiplier. What if we had 600/5 CTs? 600 divided by 5 is 120, so our watthour meter multiplier is 120.

Now what if the service that we are metering is a 480v service and we are using PTs? For this example let’s assume we are metering a 480v three phase 4 wire wye service by using 400/5 CTs and 2.4/1 PTs. How do we come up with the watthour meter multiplier in this example? We would use the same method as before to find the CT ratio and do the math of 400 divided by 5 which equals 80 but we are not finished because in this example we also have PTs. Before we do the math let’s remember Ohms’s law which says that Watts = volts x amps. Let’s also remember what we are trying to figure out. We are trying to find the multiplier for the WATThour meter. So to find watts we need to multiply volts x amps. We already know the amps part of the equation equals 80 so what is the volts part? It is 2.4 because 2.4 divided by 1 is 2.4. So, we now multiply 80 x 2.4 and we come up with 192 which is our watthour meter multipler.

Meter Muliplier Example 2

Another example with PTs and CTs. Say you have a 480v three phase three wire service with 200/5 CTs and 4/1 PTs. What is the watthour meter multiplier? 200 divided by 5 equals 40. 4 divided by 1 equals 4. So, 40 x 4 = 160. Thus, the watthour meter multiplier would be 160.

When you divide out the CT ratio and PT ratio and multiply them together this is also known as the transformer factor.


So, there you have it.

To calculate watthour meter multipliers first you need to determine whether there are CTs and PTs or only CTs in the circuit. Next, divide out the ratios. And lastly, multiply the ratios to determine the watthour meter multiplier.