As we all know, the capacity selection of AC power supply equipment is very important.choosing the right AC power supply is very important for production, manufacturing, testing and R&D！

excessive capacity selection will increase unnecessary property investment; too small capacity selection will affect the normal production, testing and R&D process and even lead to unusable.

therefore, mastering and understanding the calculation of alarm current will clearly know how to choose the capacity of AC power supply equipment. Suitable power supply equipment will make people feel like a tiger!

AC power supply is divided into single-phase power supply and three-phase power supply according to the different output power.therefore, the calculation method of alarm current can also be divided into two kinds: single-phase power supply and three-phase power supply.
the following are introduced separately:

One：Single-phase power supply
Output 0-300V: divided into low and high gear.the alarm current of low-voltage gear is 120 V, and that of high-voltage gear is 240V. capacity (VA)/gear voltage (V)=gear alarm current (A).

Take 5KVA power supply as an example: 5000VA (capacity)/120V (low grade)≈42A; 5000VA/240V (high grade)≈21A.so we get the alarm current: the output of 5KVA power supply is in the range of 0 -150V, the maximum alarm (limit) current is 42A; the output of 5KVA power supply is in the range of 0 -300V, and the maximum alarm (limit) current is 21A.

Output 0-600V, low-grade 240V, high-grade 480V, the same method to get the alarm current of 21/10.5A.
The simple method is that we only need to calculate the low-grade or high-grade alarm current. Another kind of alarm current can be obtained immediately according to the low-grade alarm current = 2 times the high-grade alarm current

Two: Three-phase power supply
The alarm current is a single-phase alarm current, so the three-phase power supply needs capacity/3 to get the single-phase capacity, and then according to the calculation method of the alarm current of the single-phase power supply, the alarm current of the three-phase power supply can be obtained.

Take three-phase 15KVA power supply as an example: 15KVA/3 = 5KVA, 5000VA/120V≈42A (low-grade alarm current), 42A/2 ≈21A (high-grade alarm current).therefore, the alarm current of three-phase 15KVA power supply is 42/21A.

It is not difficult to find that the calculation of alarm current focuses on the unit of capacity is VA, the unit of voltage is V, and the unit of alarm current is A. Therefore, if the known unit of capacity is KVA, the unit of KVA should be converted to VA when calculating the alarm current. 1KVA is 1000VA.