Question

Question: I was told by engineer that NEC 2017 Article 220.5 is about turning kw to kva. It requires turning kw to kva. Can anyone here explain to me how this is so? I dont see it in the code itself

Answer 1

The engineer is partially correct, but their statement is misleading and incomplete. NEC 220.5 is related to kW and kVA, but it doesn't mandate converting kW to kVA in every situation. It deals with using volt-amperes (VA) or kilovolt-amperes (kVA) as the basis for calculations rather than watts (W) or kilowatts (kW) when dealing with loads where power factor is significant.

Here's a breakdown: What 220.5 Actually Says

220.5(A) Volt-Amperes

This section emphasizes that for load calculations, volt-amperes (VA) must be used as the basis for calculations for things like branch circuits, feeders, and service conductors. This is because VA directly reflects the apparent power the system must be able to deliver, regardless of the power factor.

220.5(B) Kilovolt-Amperes

Kilovolt-amperes (kVA) is simply VA expressed in thousands. So, 1 kVA = 1000 VA. The same principle applies – use kVA for larger loads.

220.5(C) Power Factor

This is the key. The code states that appliances and utilization equipment for which volt-amperes (VA) are not marked, and for which watts (W) are marked, can have their VA calculated by dividing the wattage by the power factor. In other words:

VA = W / Power Factor

However, if the power factor is not known, a power factor of 1.0 must be used, effectively making VA = W in those cases.

Why VA/kVA Instead of Watts/kW? Power Factor

Most electrical loads aren't purely resistive. They have reactive components (capacitive or inductive). Reactive power (measured in VARs) doesn't do real work (like heating or running a motor), but it still circulates in the system, requiring the electrical infrastructure to be sized to handle it.

kW vs. kVA

kW (Kilowatts)

Measures real power – the power that performs useful work.

kVA (Kilovolt-Amperes)

Measures apparent power – the total power flowing in the circuit, including both real and reactive power. The relationship is:
kVA = kW / Power Factor
Therefore
kW = kVA Power Factor

Example

A motor might draw 10 kW of real power (kW). But because of its inductive nature, it might have a power factor of 0.8. This means the apparent power (kVA) is:
kVA = 10 kW / 0.8 = 12.5 kVA
The wiring, overcurrent protection, and transformers need to be sized for the 12.5 kVA, not just the 10 kW.

When is kW to kVA NOT Required? Purely Resistive Loads

If a load is purely resistive (like a resistive heater or incandescent light bulb), the power factor is essentially 1.0. In these cases, kW = kVA, and no conversion is needed.

Loads Already Specified in VA/kVA

If equipment is rated in VA or kVA (as many appliances and larger equipment are), you simply use that value directly for your calculations.

In Summary

NEC 220.5 prioritizes the use of VA/kVA for load calculations to account for power factor. It doesn't always require converting kW to kVA. The conversion is needed when the load is rated in watts (kW) and has a significant power factor (less than 1.0). If a load is purely resistive (power factor of 1.0) or already rated in VA/kVA, no conversion is necessary. The goal is to accurately size the electrical system to handle the total apparent power demand, including both real and reactive power.

The engineer's statement is an oversimplification. It's more accurate to say that NEC 220.5 requires the use of volt-amperes (VA) as the basis for load calculations and provides guidance on how to determine VA when only watts (W) are given, especially when power factor is a consideration.To get a more specific answer, you'd need to ask the engineer why they made that statement in the context of your particular project. There might be specific equipment being used that necessitates the kW to kVA conversion. Always clarify the context and reasoning with the engineer. Flag for review

Answer 2

The engineer is partially correct, but their statement is misleading and incomplete. NEC 220.5 is related to kW and kVA, but it doesn't mandate converting kW to kVA in every situation. It deals with using volt-amperes (VA) or kilovolt-amperes (kVA) as the basis for calculations rather than watts (W) or kilowatts (kW) when dealing with loads where power factor is significant.

Here's a breakdown: What 220.5 Actually Says

220.5(A) Volt-Amperes

This section emphasizes that for load calculations, volt-amperes (VA) must be used as the basis for calculations for things like branch circuits, feeders, and service conductors. This is because VA directly reflects the apparent power the system must be able to deliver, regardless of the power factor.

220.5(B) Kilovolt-Amperes

Kilovolt-amperes (kVA) is simply VA expressed in thousands. So, 1 kVA = 1000 VA. The same principle applies – use kVA for larger loads.

220.5(C) Power Factor

This is the key. The code states that appliances and utilization equipment for which volt-amperes (VA) are not marked, and for which watts (W) are marked, can have their VA calculated by dividing the wattage by the power factor. In other words:

Topic: Kw to kva