Okay, let's break down this generator sizing problem and figure out the best approach. You've got a multi-tiered power distribution system and a customer who needs backup for critical 120V circuits.

Understanding the Constraints and Key Considerations 3-Phase System

The fact that you have a 3-phase system is crucial. You cannot use a 20kW 120/240V single-phase generator directly in this scenario. You must use a 3-phase generator designed for 208/120V or 480/277V, depending on where you decide to connect it.

Transformer Capacity

The 30kVA transformer is a critical limiting factor. You can only draw so much power through it.

Load Calculation

You need to estimate the total load of those (8) 120V circuits as accurately as possible. "Likely not much" isn't good enough. Get the actual amperage draw (or wattage) for each circuit. If that's impossible, make conservative assumptions (i.e., overestimate). Remember to include inrush current for any motors or devices with significant startup loads.

Code Compliance

Ensure all work complies with the NEC (National Electrical Code) and any local codes.

Analyzing Option A: Connecting to the 208/120V Panel

This is the more straightforward approach, but limited by the transformer's capacity.1. Determine Maximum Available Amperage

Your 30kVA transformer has a secondary current capacity of approximately 83.3A (30,000 VA / (208V √3)). You're already drawing 6A + 17A + 31A = 54A on that panel. That leaves you with 83.3A - 54A = roughly 29.3A available per phase to support the generator and the new loads.2.

Calculate the Load of the (8) 120V Circuits

Let's assume a worst-case scenario of 2A per 120V circuit (240VA) based on worst case scenarios of door entry controllers and cctv cameras, this would be 16A and at 120V single phase would be a total load of 1920VA 1920VA/208v/sqrt(3)= 5.33amps3.

Generator Sizing for Option A

You need to provide approximately 6A. A 5kw 208/120 generator will support these loads.4.

ATS Selection

You'll need a 3-phase, 208/120V ATS rated appropriately for the generator you choose. Make sure it's properly sized for the available fault current.5.

Pros and Cons of Option A



Pros

Relatively simple installation.

Cons

Limited by the existing transformer. If your load calculations are off, or if you need to add any significant load in the future, you'll be constrained. The transformer could be running near capacity even with the backup load.

Analyzing Option B: Connecting to the 480/277V Panel

This gives you more flexibility and capacity but requires a new transformer.1. Determine Transformer Size

You need a transformer to step down from 480/277V to 208/120V. A 20kVA transformer might just be sufficient, if your calculations are accurate and you're very confident in your load estimates. However, it's often better to size up slightly. A 30kVA transformer would give you more headroom.2.

Generator Sizing for Option B

If the transformer is going to supply the (8) 120V circuits and nothing else, you could use a smaller generator (similar to Option A). But since the customer wants a 20kW generator for future expansion, let's assume we're sizing for that. A 20kW generator at 208/120V 3-phase will draw approximately 69.4 amps per phase (20,000 VA / (208V √3)).3.

ATS Selection

You'll need a 3-phase, 480/277V ATS rated for the 150A panel. The ATS output will feed the primary of your new transformer. Make sure to coordinate the overcurrent protection (breakers/fuses) for the generator, ATS, transformer, and downstream panels.4.

Pros and Cons of Option B



Pros

More capacity for future expansion. Isolates the backup loads from the existing 208/120V panel, which might be beneficial.

Cons

More expensive due to the cost of the transformer and associated installation. More complex installation.

Recommendations1. Accurate Load Calculation is Critical

Before you make any decisions, get a precise estimate of the load of those (8) 120V circuits. Measure the actual current draw if possible. If you can't measure, use the manufacturer's specifications and add a safety factor (e.g., 25%). Include inrush current from any cameras. This is non-negotiable.2.

Consider Future Needs

The customer wants a 20kW generator for a reason. If they foresee adding more critical loads in the future, Option B is probably the better long-term solution.3.

Option A with a Small Generator is Risky

If you are pressed to do option A, make sure to calculate total amps and make sure your transformer can handle it.4.

Option B with a 30KVA transformer is my recommended option.5. Consult with a Qualified Electrical Contractor

This is not a DIY project. Involve a licensed electrical contractor with experience in generator installations. They can help with load calculations, code compliance, and ensure a safe and reliable installation.

Code Considerations NEC Article 700 (Emergency Systems) applies if these loads are legally required to be backed up during a power outage. NEC Article 701 (Legally Required Standby Systems) may apply if these loads are required for safety. NEC Article 702 (Optional Standby Systems) applies if these loads are not legally required or required for safety. Load Calculations

Proper load calculations are essential. NEC Article 220 provides the rules for calculating loads.

Overcurrent Protection

The generator, ATS, transformer, and all downstream panels must be properly protected with appropriately sized breakers or fuses.

Grounding: Proper grounding is essential for safety and to prevent equipment damage.By carefully considering these factors and consulting with a qualified electrician, you can develop a safe and reliable generator backup solution for your customer. Flag for review