Question:
Why is my 5KVA step down transformer showing 130 to 140+ VAC on its output instead of 120VAC?

Product Line:
APC by Schneider Electric - 5K Stepdown Transformers

• SURT003
• SRT5KTF
• SRTL5KTF
• SRT5KRMTF
• AP9627
• AP9626
• AP9628

Scenario:
5Kva Stepdown / Isolation Stepdown transformer is outputting incorrect voltage

Answer:

Stepdown transformers has a turn base ratio and this will affect its own output VAC.

The basics of turn base ratio applied APC by Schneider Electric Stepdown transformers states that 120 Volts is equal to 1 ratio, which means 240 is equal to 2.

However, 208 would be 120 Vac on one leg and since the other leg is 120° apart, therefore the other leg would be at 88 Vac or at ratio of .73 therefore 208 is equal to 1.73. To simplify this,  we can also apply the formula of getting voltage from a 3 phase source which would be plainly √3 = 1.73.

On an AP9626 for example, since the input requirement is 208 Vac and the expected output is 120 Vac therefore the ratio for this unit is equal to 1.73:1.

Please note that all the stepdown transformers noted on this kbase is by default 1.73 : 1 (208Vac to 120Vac).

Stepdown Transformers like ap9627, ap9626, ap9628, srt5krmtf are design strictly to input 208 Vac. However, we also have our ISOLATION stepdown transformers (surt003, srt5ktf, srtl5ktf) that can be adjusted from 1.73 : 1 (208Vac to 120Vac) to 2 : 1 (240Vac to 120Vac) with the use of a Voltage Selector Switch (please see image below).

IMPORTANT NOTE : voltage adjustment needs to be done while the unit is not powered and/or not hooked to any power source.

Released for:Schneider Electric Canada