General - Transformers are one of the most common components of any electrical power system. People see them all the time but don't know what they're looking at. The image below is of larger padmount (mounted on a concrete pad) transformers. These are usually placed outside of larger commercial buildings, industrial areas, and more. Smaller versions of padmount transformers are also common outside of homes in neighborhoods.
Padmount Transformers Installed at a Construction Site
Another common type of transformer is the pole-mount variety. Pole-mount transformers are commonly seen on wooden poles for distributing electricity in residential areas.
Three Single-Phase Polemount Transformers
Ratings - Transformers are often represented on electrical one-line diagrams as shown below. The most important ratings and configuration details are provided to show the system architecture:
Apparent Power Rating: Typically in kVA or MVA, the amount of apparent power that can be provided at the secondary of the transformer. Since transformers lose apparent power through their impedance and efficiency, the input limits to the transformer are actually higher than the output rating. Together with the input voltage, we can use the apparent power rating of the transformer to determine the nameplate current.
Impedance: Transformer are magnetic devices and, as such, are dominated by inductance. This inductive reactance can be modeled as a single impedance for calculation purposes. The impedance of a transformer is typically written as a percentage, referencing the per unit system.
Voltage: The voltage ratings describe what voltage goes into the primary and what comes out of the secondary.
Winding configuration: Three-phase transformers can be wired in a number of configurations, with the most common types being Wye, Delta, Wye Grounded, and Wye Impedance Grounded. The primary and secondary each have a different winding configuration and the type of winding impacts the design of the system, including fault currents, grounding, and more. The concept of winding configuration doesn't apply to single-phase transformers.
Example Transformer with Ratings
The Code - Transformers are covered in general by Article 450 of the 2020 National Electrical Code, but you'll find ampacity requirements under Article 215: Feeders. Feeders supplying transformers must be able to carry 100% of the nameplate rating of the transformer. You might be thinking "Why not 125% of the nameplate like with most loads?". That's because a transformer is always upstream of loads and should be sized to already account for continuous loading of downstream devices. If we were to size to 125% of the nameplate of the transformer we would actually be sizing to 156% of the downstream load current (or more, depending on how the load currents were calculated).
The requirements for overcurrent protection of transformers are provided in Table 450.3(A) and Table 450.3(B). Transformers may be required to have protection on their primary (where power comes in) and secondary (where power goes out). The voltage level, type of installation, and the transformer's impedance rating all impact how overcurrent protection must be applied.
Example: A 480V primary, 208V secondary three-phase transformer with Z=4% impedance and an apparent power rating of 100 kVA is installed in a public location. What size should the feeder conductors be to supply this transformer and what overcurrent protective device trip ratings are required on the primary and secondary if breakers are used? Assume there are no applicable derating factors for ambient temperature or burial depth and the conductors are fed into the transformer through a conduit.
Solution: The transformer described in this problem is the same as the one shown in the Ratings section above. Let's start by finding the transformer nameplate current. Since the transformer is three-phase, the equation for the apparent power is:
S = √(3) V I
Where:
S is the apparent power of the transformer
V is the line-line voltage rating of the transformer
I is the current passing through the transformer
This equation can be rearranged to get the current that can safely flow through this transformer:
I = S / ( √(3) V) = 100kVA / ( √(3) * 480V ) = 120.3A
Transformer feeder conductors are sized to 100% of the nameplate current. Since the conductors are fed via conduit with no other derating factors to the transformer, the applicable ampacity table for consideration is 310.16 of the 2020 National Electrical Code. Since the current is greater than 100A, 75°C terminals are applicable. Per 310.16, a 1 AWG Copper conductor can carry 130A at 75°C, so this is the minimum feeder conductor size.
Next, we have to determine the overcurrent protective device trip ratings based on the installation conditions. NEC Table 450.3(B) states that transformers may employ various protective schemes. We will use primary-only protection, and thus require a trip rating of 125% of the nameplate current:
1.25 * 120.3A =150.375A
150.375A isn't a standard size of overcurrent protection, so we can use the next standard size up per NEC 450. This would be a 175A breaker for the primary, with no secondary protection required.
This result might seem strange, but keep in mind that the overcurrent device specified above is to protect the transformer, not the feeder circuit. Because our feeder circuit was only sized to have an ampacity of 130A, the feeder circuit will not be adequately protected by the transformer primary protection device. An additional upstream overcurrent protection device at a lower rating would be required to ensure protection on a low voltage system like this.
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