Mike Dolt Enterprises
Mike Dolt Enterprises
User: mikev86, Location: Easton, MD
Asked on: Sunday, April 6, 2025 at 5:45 pm EDT
Answered on: Sunday, April 6, 2025 at 5:49 pm EDT
Question: Hello,I am looking for some guidance in sizing conductors for a piece of equipment coming from overseas. Main switch for the equipment is rated at 400A at 400V. (integral control panel with fuses).There is a 480V Square D QED style switchboard that feeds a terminal box on a piece of bus duct (800A breaker in the MSB). Terminal box will go to a zig-zag transformer with a primary voltage of 480V delta and a secondary voltage of 400V zigzag + neutral (see attached photo). I am being asked to help size the terminal box, conductors from terminal box to transformer, and conductors on the secondary side of the transformer to the new equipment control panel.Terminal box to transformer will be ~15' and transformer to equipment will be ~5'.My thoughts below:350A terminal box.(2) sets of 2/0AWG CU phase conductors + 3AWG CU grounding conductor to the transformer in 2" conduits (assuming worst case since we do not have MCA on equipment).Customer supplied 280kVA transformer.(2) sets of 3/0AWG CU phase conductors, (1) 3/0AWG CU neutral, + (1) 2AWG CU grounding conductor in 2" conduits.Any input and direction would be greatly appreciated as I have not worked with this equipment before.Thank you,Mike
Okay, Mike, let's break this down and review your assumptions and calculations. It sounds like you have a good start, but there are a few areas where we need to tighten things up and ensure NEC compliance (assuming you're in the US).1. Understanding the Load and Requirements 400A at 400V
This is your equipment's maximum demand. This is not necessarily what you'll use for calculations, but it sets an upper limit.
No MCA (Minimum Circuit Ampacity)
This is problematic. Without an MCA from the manufacturer, you must assume the equipment will draw 400A continuously. This will significantly impact your conductor sizing. You absolutely need to try to get the MCA. If you can't, you're forced to design for the worst case.
Zig-Zag Transformer
A zig-zag transformer is specifically for creating a neutral point in a delta system. It does not change the kVA rating of the system. It will also introduce some additional losses, so keep this in mind.
Customer-Supplied 280kVA Transformer
Is this transformer properly sized? We need to verify this.
2. Transformer Sizing and Primary Current Calculations Transformer kVA Calculation (Secondary)
Your equipment is 400A at 400V, 3-phase. Let's calculate the required kVA:
Amps sqrt(3)) / 1000kVA = (400V 400A 1.732) / 1000kVA = 277.12 kVA kVA = (Volts
Conclusion
A 280 kVA transformer is barely sufficient if the 400A is indeed continuous. If it is continuous, you will likely overload the transformer and it will be highly inefficient, which is a safety hazard. I would try and get the MCA on the equipment. I would also make sure that there are overtemperature windings on the transformer and a means to monitor and shut down the equipment in case of an issue.
Transformer Primary Current (480V Delta)
Let's calculate the transformer's primary full-load current:I_primary = (kVA 1000) / (Volts sqrt(3))I_primary = (280 kVA 1000) / (480V 1.732)I_primary = 337 Amps
3. Conductor Sizing (Primary Side - Terminal Box to Transformer) Continuous Load Consideration
If the equipment operates at 400A continuously, you must multiply the primary current by 125% (NEC 215.3).I_design = 337A 1.25 = 421.25 Amps
Your suggestion of (2) sets of 2/0 AWG CU Each 2/0 AWG copper conductor (THHN, assumed for conduit) is rated for 175 amps in the 75-degree column of NEC Table 310.16. Two conductors in parallel gives you 350A, which is not sufficient for a calculated load of 421.25 Amps. Suggested primary circuit conductor sizing
Recommend using two sets of 4/0 copper conductors which are rated for 230A (460A when paralleled).
Grounding
Per NEC 250.122, the Equipment Grounding Conductor (EGC) size is based on the size of the overcurrent device protecting the circuit. Your 800A breaker in the switchboard requires a minimum of a 1/0 AWG copper EGC. However, you must also consider voltage drop. It's better to upsize the EGC to match the parallel conductors.
Conduit Sizing (Primary)
You'll have 6 current-carrying conductors (2 sets of 3 phase conductors) + 1 EGC. NEC Chapter 9, Table 4 allows for a maximum fill of 40% for 3 or more conductors. Consulting Chapter 9, Table 5 and using THHN conductors: 4/0 AWG = 0.3725 in2 1/0 AWG = 0.2036 in2 Total Conductor Area: (6 0.3725) + 0.2036 = 2.44 in2 A 2" conduit (Chapter 9, Table 4) has an allowable fill area of 3.356 in2, which is sufficient. It is recommended to use 2.5" conduit to pull the conductors easier.
4. Conductor Sizing (Secondary Side - Transformer to Equipment) Continuous Load Consideration
Again, if the 400A is continuous, you must multiply the equipment's current by 125% (NEC 215.3).
1.25 = 500 Amps I_design = 400A
Your suggestion of (2) sets of 3/0 AWG CU
Each 3/0 AWG copper conductor (THHN, assumed for conduit) is rated for 200 amps in the 75-degree column of NEC Table 310.16. Two conductors in parallel gives you 400A, which is not sufficient for a calculated load of 500 Amps.
Suggested secondary circuit conductor sizing
You'll need to use two sets of 350 kcmil copper conductors which are rated for 310A (620A when paralleled). This is significantly larger than your initial estimate, and it's due to the continuous load requirement.
Neutral Conductor Sizing
The neutral conductor must be sized to carry the maximum unbalanced load. Since this is industrial equipment, it's likely that the unbalanced load will be minimal. However, with the presence of harmonic currents from electronic components, it is wise to size the neutral conductor to match the phase conductors. Therefore, use two sets of 350 kcmil copper conductor (same as phase conductors).
Grounding
Per NEC 250.122, the Equipment Grounding Conductor (EGC) size is based on the size of the overcurrent device protecting the circuit (the 400A main switch in the equipment). A 2 AWG copper EGC is sufficient (minimum is 6 AWG for a 400A breaker), but again, consider upsizing to match the parallel conductors for voltage drop considerations. I would make this the same size as the current carrying conductors (350 kcmil) to handle any potential fault currents.
Conduit Sizing (Secondary)
You'll have 8 current-carrying conductors (2 sets of 3 phase conductors + 2 neutral conductors) + 1 EGC. Using the same method as before: 350 kcmil = 0.5374 in2 Total Conductor Area: (8 0.5374) + 0.5374 = 4.83 in2 You'll need at least a 3" conduit (allowable fill = 5.059 in2). A 3.5" would be easier to pull.
5. Terminal Box Sizing Primary and Secondary
You need to ensure the terminal box has sufficient space for all conductors, connectors, and required bending space as per NEC 314.28. Consider the dimensions of the lugs you'll be using. You should also ensure the lug ratings (ampacity and voltage) are sufficient for this application.
Recommendations and Cautions
GET THE MCA! This is the most important thing. It will significantly impact your conductor sizing and cost. Contact the equipment manufacturer and insist on the MCA (Minimum Circuit Ampacity). Transformer Sizing
Re-evaluate the transformer if the 400A is continuous. Consider a larger transformer (e.g., 400 kVA or larger).
Overcurrent Protection
The 800A breaker in the main switchboard seems very large for the transformer primary current. Make sure this is appropriately sized for the transformer's primary current. This breaker exists to protect the conductors feeding the transformer, not the load side of the transformer. The transformer also needs to be properly protected. You may need to consider adding a separate overcurrent device, such as a fused disconnect, on the primary side of the transformer.
Voltage Drop Calculation
Calculate voltage drop for both the primary and secondary circuits. Make sure the voltage drop is within acceptable limits (typically 3% for power circuits). This may require upsizing conductors further.
Harmonic Considerations
Be aware of potential harmonic currents, especially on the neutral. The neutral current could be significantly higher than the calculated unbalanced load. Consider using a K-rated transformer and an oversized neutral conductor.
Local Codes and Regulations
Always consult and comply with all applicable local codes and regulations.
Professional Engineer Review
This is a complex installation. It is highly recommended* that a qualified Professional Engineer review and approve your design to ensure compliance with all applicable codes and standards.
In summary, your initial conductor sizing is likely too small, especially on the secondary side if the equipment draws 400A continuously. Transformer sizing should be re-evaluated. Get the MCA from the manufacturer, and consult with a Professional Engineer.
Good luck! Flag for review