Okay, let's break down this UPS sizing problem and address your calculations.

Understanding the Problem

You're trying to size a UPS to ride through brief power interruptions during generator testing, specifically to prevent a boiler shutdown controlled by a VFD. The core challenge is estimating the inrush current of the VFD during a power event, as this will dictate the UPS capacity.

Concerns with your calculations



kVA vs. kW

You're calculating kVA (apparent power), which is important for sizing the UPS, but for battery sizing, you need to focus on kW (real power). kVA includes reactive power (VARs) due to inductive or capacitive loads, but the battery only needs to supply the real power consumed. You'll need to estimate the power factor to convert kVA to kW.

Multiplier for Surges

While a 1.2-1.5 multiplier can be a starting point, it might not accurately reflect the VFD's inrush current. VFDs can have substantial inrush currents (sometimes 3-8x the running current) when power is restored, even for a very short interruption. A VFD manufacturer will have this information.

Feeder Breaker Size

Using the feeder breaker size is a very conservative approach. The breaker is sized for worst-case scenarios and to protect the wiring, not necessarily to match the actual load during a short interruption.

Revised Approach and Calculations

Here's a recommended approach:1. Gather VFD Specific Data (Crucial)

This is the most important step. Contact the VFD manufacturer and ask for the following:

Inrush Current

Ask for the peak inrush current of the VFD during a power restoration, expressed as a multiple of the full load current (FLA). This is essential. A safe bet to ask is a locked-rotor amperage, so 6x FLA.

Inrush Duration

How long does this inrush current last? Typically, it's very short (milliseconds to a few cycles), but you need to know.

Operating Power Factor

The power factor of the VFD when the boiler is running at its normal load. This is needed to convert kVA to kW.

Full Load Amperage (FLA)

Confirm the full load amperage from the VFD's nameplate or documentation. The 198.3A in the boiler spec is a good starting point, but confirm it.

Operating Voltage

Confirm the 480V operating voltage of the VFD.2.

Calculate Inrush kVA

`Inrush Current (Amps) = FLA Inrush Multiplier` `Inrush kVA = (Voltage Inrush Current 1.73) / 1000` `Inrush kVA = (480 (198.3 6) 1.73) / 1000 = 988.00kVA` This number is very large. It is important to have specific data to reduce this kVA.3.

Calculate Inrush kW

`Inrush kW = Inrush kVA Power Factor` (Use the VFD's operating power factor) Using a Power Factor of 0.9, `Inrush kW = 988.00 0.9 = 889.20kW`4.

Calculate Run Time kW (Assuming Generator Start-up)

Boiler spec states minimum circuit ampacity is 198.3a. `Operating kVA = (Voltage Current 1.73) / 1000` `Operating kVA = (480 198.3 1.73) / 1000 = 164.67kVA` `Operating kW = Operating kVA Power Factor` (Use the VFD's operating power factor) Using a Power Factor of 0.9, `Operating kW = 164.67 0.9 = 148.20kW`5.

UPS kVA/kW Sizing



UPS kVA Rating

The UPS should be rated at least as high as the Inrush kVA. It must be able to handle the peak inrush current. Consider derating the UPS based on environmental factors (temperature, altitude).

UPS kW Rating

Should be rated at least as high as the Operating kW.

UPS should be 988kVA / 148.20kW.6. Battery Sizing (Most Accurate Method)



Select your Battery Voltage

Standard UPS battery voltages are 48VDC, 120VDC, 240VDC, etc. This will depend on the UPS model.

Convert Watts to Watt-Hours

For 15 seconds of backup, use the Operating kW value. Convert it to Watts. Watts = kW 1000 Watts = 148.20 1000 Watts = 148,200 Watts

Calculate Watt-hours for your runtime (15 seconds)

(This converts Watts to Watt-seconds, then to Watt-hours) `Watt-seconds = Watts Seconds` `Watt-seconds = 148,200 15` `Watt-seconds = 2,223,000 ws` `Watt-hours = Watt-seconds / 3600` (3600 seconds in an hour) `Watt-hours = 2,223,000 / 3600 = 617.5wh`

Account for Battery Depth of Discharge

Batteries should not be fully discharged. Typically, a depth of discharge (DoD) of 80% is used (meaning you can only use 80% of the battery's capacity). `Adjusted Watt-hours = Watt-hours / DoD` `Adjusted Watt-hours = 617.5 / 0.8 = 771.88wh`

Calculate Amp-Hours

`Amp-hours = Watt-hours / Battery Voltage` `Amp-hours = 771.88 / 240 = 3.22ah` (using 240 VDC)

This is the minimum capacity in Amp-hours that your battery bank needs to provide at your chosen battery voltage. The higher the Voltage, the less the Amp Hour rating. Battery Selection

Select batteries based on: Voltage: Must match the UPS's DC voltage requirement. Amp-Hour Capacity: Must meet or exceed your calculated Amp-hour requirement. Type: VRLA (Valve-Regulated Lead-Acid) are common in UPS systems, but Lithium-Ion is becoming more prevalent. Lifespan: Consider the expected lifespan of the batteries.

Important Considerations UPS Efficiency

UPS systems have efficiency losses. Factor this into your calculations (typically 85-95% efficiency).

UPS Overload Capacity

UPS systems typically have a short-term overload capacity. Check the specifications.

ATS Transfer Time

Your assumption of 1-2 seconds for ATS transfer is important. Confirm this with the ATS manufacturer.

Future Load Growth

Consider potential future increases in the boiler's load.

Environmental Conditions

Temperature affects battery performance. Consult battery specifications for temperature derating factors.

VFD Harmonic Distortion

VFDs can generate harmonic currents that can affect UPS performance. Check the UPS's ability to handle harmonic loads.

Where to Find Information in the Code NEC Article 430

Motors, Motor Circuits, and Controllers (VFDs are motor controllers). This article covers motor sizing, overload protection, and disconnecting means.

NEC Article 700

Emergency Systems. If the boiler is critical for life safety or other essential functions, this article applies. It covers emergency power sources and transfer switches.

NEC Article 708

Critical Operations Power Systems (COPS). If the boiler is essential for critical operations (data centers, hospitals, etc.), this article may apply. It has stricter requirements than Article 700.

NEC Article 620

Electric Vehicle Charging System

Manufacturer's Specifications

The most important source of information for the VFD and the UPS.

Summary1. Crucially, get the inrush current data from the VFD manufacturer.2. Calculate the inrush kVA and kW.
3. Calculate the runtime kW.4. Size the UPS based on the inrush kVA and the runtime kW.5. Calculate the battery size based on the runtime kW*, desired backup time, battery voltage, and depth of discharge.6. Consult the NEC articles mentioned above for code compliance.
7. Factor in UPS efficiency and overload capacity.

Important Disclaimer: I am an AI chatbot and cannot provide professional engineering advice. Consult with a qualified electrical engineer for your specific application. Flag for review