## How do you calculate overcurrent protection of A transformer?

Using the example numbers: Iprimary = (20 x 1000)/480 = 20,000/480 = 41.6 amps. Note: If you had a 3-phase transformer, the formula would be Iprimary = KVA x 1000/(Vprimary x 1.732).

## Do transformers need overcurrent protection?

Most electrical devices require overcurrent protection and transformers are no exception. Transformer overcurrent protection is required to protect the primary windings from short circuits and overloads and the secondary windings from overloads.

**What size overcurrent protection do I need?**

The feeder overcurrent protection device must be sized not less than 125% of 184 amperes, So, overcurrent protection device size = 184 amperes x 125% = 230 amperes. According to Section 240-6(a) for “Standard Ampere Ratings of overcurrent devices”, we must select a minimum 250 ampere overcurrent protection device.

**How do you measure transformer size?**

Calculate an example as follows. A 120-volt motor has a load amperage of 5 amps. Multiply 120 volts times 5 amps this equals 600VA now lets multiply the 125 percent start factor. Take 600 times 1.25 this equals 720VA and most transformers are sized by a factor of 25VA or 50VA.

### Do you need overcurrent protection on secondary side of A transformer?

Overcurrent protection is not required on the secondary side of the transformer to protect the secondary conductors, but overcurrent protection is required for branch-circuit panelboards. This OCPD must be on the secondary side of the transformer, and typically it’s within the panelboard.

### How do you size a transformer feeder?

You must size the bonding jumper in accordance with Table 250.66, based on the total area of the largest ungrounded (hot) conductor as follows: For a 45kVA transformer: 3/0 AWG secondary conductor = 4 AWG copper bonding jumper. For a 112.5kVA transformer: 700kcmil secondary conductor = 2/0 AWG copper bonding jumper.

**How do you size a transformer?**

To determine the required transformer VA, multiply the secondary voltage by the required amperes: For example, VA=Volts x Amperes or 28V x 1.2 amperes = 33.6 VA. To determine the maximum transformer current, divide the transformer VA by transformer secondary voltage.

**What happens if an overcurrent protective device is oversized?**

Undersize your overcurrent protective device (OCPD), and normal operations might become impossible. Oversize your OCPD, and the whole place might burn down.

## How do you protect a transformer?

The protection of a transformer against the overloads is performed by a dedicated protection usually called thermal overload relay. This type of protection simulates the temperature of the transformer’s windings. The simulation is based on the measure of the current and on the thermal time constant of the transformer.