Mike Dolt Enterprises
Mike Dolt Enterprises
User: Frank6172, Location: Maryland
Asked on: Monday, May 12, 2025 at 9:44 am EDT
Answered on: Tuesday, May 13, 2025 at 8:24 pm EDT
Question: Hi all,The site got 208V, 1200A utility service and there's an existing 208/120V to 480V step-up transformer to feed some 480V machine. Also there's a 480V, 400A AC disconnect on high voltage side of the transformer. We want to avoid 208V inverters as much as possible in this project so can we do the interconnection on the 480V disconnect of the transformer?The transformer nameplate is below and our intended inverter is 480V, 120kW.
Okay, let's break down the feasibility of connecting your 480V, 120kW inverter to the existing 480V disconnect fed by the step-up transformer, considering the provided information and safety standards.
Here's a breakdown of the considerations and my assessment
Information Summary
Utility Service
208V, 1200A
Existing Transformer
208/120V to 480V Step-Up (Need nameplate details to make a final decision)
Existing 480V Disconnect
480V, 400A (feeds 480V side of the transformer)
Proposed Inverter
480V, 120kW
Issues and Considerations
Transformer Capacity
Crucially important
You must determine the kVA rating of the existing step-up transformer. The nameplate information is essential for this. You'll need to look for the kVA rating on the transformer nameplate. Without knowing the kVA rating, it's impossible to determine if the transformer can handle the additional load of the 120kW inverter in addition to its existing load.
Calculating Inverter Load
A 120kW inverter at 480V will draw approximately:
Assuming a power factor (PF) of 1.0 (best case, inverters often have a PF closer to 0.9-0.95):I = kW 1000 / (√3 V PF)
If the power factor is less than 1.0, the current will be higher. For example, at PF=0.95:I = 120 1000 / (1.732 480 1.0) = 144.3 Amps (approximately)
2. I = 120 1000 / (1.732 480 0.95) = 151.9 Amps (approximately)
Disconnect Capacity
The existing 480V disconnect is rated for 400A. You need to calculate the existing load on that disconnect before adding the inverter.
Rule of Thumb
Generally, you shouldn't load a disconnect (or breaker) continuously above 80% of its rating (NEC requirement for continuous loads). So, the existing load plus the inverter load should ideally be less than 400A 0.8 = 320A. This is a problem if the existing load on the transformer is already high.3.
Reverse Power Flow
This is a critical consideration. If the inverter is generating more power than the 480V load is consuming, power will flow back through the step-up transformer to the 208V service.
Is the Transformer Rated for Backfeed? Many (but not all) transformers are bi-directional. You must confirm with the transformer manufacturer if your transformer is designed to handle reverse power flow. If it's not, you risk damaging the transformer. Look for language like "suitable for backfeed" or "bi-directional operation" in the transformer documentation. Utility Interconnection Agreement
Your utility will absolutely need to approve this interconnection. They will likely have specific requirements for protection (relaying) and metering to prevent issues on their grid. They will need to see the design and verify the transformer's suitability for backfeed.
Anti-Islanding Protection
Inverters that are interconnected to the grid must have anti-islanding protection. This means the inverter must automatically shut down if it detects a loss of utility power. This is a safety requirement to prevent the inverter from energizing a portion of the grid that is supposed to be de-energized.4.
Overcurrent Protection
You will almost certainly need additional overcurrent protection (breakers/fuses) for the inverter circuit. The 400A disconnect likely protects the transformer and the downstream loads that it was originally designed for. You'll need to coordinate the overcurrent protection devices so they properly protect both the inverter and the transformer. Consideration should be given on installing properly sized overcurrent protection device at the inverter output.5.
Harmonics
Inverters can introduce harmonic distortion into the power system. This could potentially affect the existing 480V equipment fed by the transformer. Good quality inverters minimize harmonics, but it's something to be aware of. May need a harmonic study done to analyze the impacts if the AHJ requires.6.
Voltage Rise
When the inverter is exporting power, it can cause a voltage rise on the 480V bus. This needs to be analyzed to ensure the voltage stays within acceptable limits for the equipment connected to the 480V system.7.
NEC (National Electrical Code) Compliance
This installation must comply with all applicable NEC articles, including Article 690 (Solar Photovoltaic (PV) Systems), Article 705 (Interconnected Electric Power Production Sources), and any other relevant articles. A licensed electrician and engineer are crucial for ensuring compliance.
General Considerations
Coordinate with Utility
The most important step is to contact your utility immediately. They will have specific interconnection requirements, including protection schemes, metering requirements, and transformer backfeed approval processes.
Engage a Qualified Electrical Engineer
You need a professional electrical engineer experienced in grid-tied renewable energy systems to perform a thorough system study, including load calculations, short-circuit analysis, protection coordination, and voltage rise analysis. They will design the system to meet all safety and code requirements.
Licensed Electrician
A licensed electrician with experience in commercial power systems and renewable energy installations is essential for the physical installation and wiring.
Recommendation
Gather the Transformer Nameplate Details
This is the first and most critical step. You need the kVA rating, impedance (%), voltage ratings, and any information about suitability for backfeed.2.
Determine Existing 480V Load
Accurately measure or calculate the existing load on the 480V disconnect under typical operating conditions.3.
Contact the Utility
Discuss your project with the utility and understand their interconnection requirements.4.
Engage an Electrical Engineer
Hire a qualified electrical engineer to perform the necessary studies and design the system.
Conclusion:
Connecting the inverter to the existing 480V disconnect might be possible, but it's highly dependent on the transformer's capacity, its suitability for backfeed, the existing load on the 480V system, and the utility's interconnection requirements. A thorough engineering analysis and coordination with the utility are absolutely essential before proceeding. Do not attempt this without professional guidance. Flag for review