Okay, let's break down these calculations and see if they align with standard electrical practices. I can't provide a definitive "yes" or "no" without knowing the exact assumptions and formulas you're using, but I can point out potential issues and areas to double-check.

Key Considerations You Should Have Accounted For



Voltage Drop

This is the most likely reason for needing larger conductors, especially at longer distances. A common benchmark is to keep voltage drop under 3% for feeders.

Ampacity

The conductor must be rated to safely carry the calculated current (or be protected at the next standard size up).

Temperature Correction/Adjustment

If the cable is installed in a high ambient temperature or bundled with other cables, you need to adjust the ampacity downward.

Conduit Fill

The conduit needs to be large enough to physically accommodate the number and size of conductors.

Code Compliance

You must comply with the National Electrical Code (NEC) or your local electrical code. These codes dictate specific requirements for conductor sizing, overcurrent protection, grounding, etc. I can't give specific code citations because I don't know your local jurisdiction.

Review of Your Calculations

Let's go service by service:

1. 480V, 15kVA, Three-Phase Service



Calculated Current (18A)

15,000 VA / (480 V √3) = ~18A. This calculation is correct.

Distance (555 ft)

This is a long run, making voltage drop a very important factor.

Your Result (8 AWG 3/C)

This seems potentially undersized, especially due to the long distance. An 8 AWG conductor typically has an ampacity of around 40-55 amps (depending on insulation type and temperature ratings). It can carry 18A from an ampacity standpoint. However, the 555-foot run will almost certainly result in excessive voltage drop.

Critical Questions for the 3-phase service



Voltage Drop Calculation

What voltage drop percentage are you allowing? Show me your voltage drop calculation. What voltage drop formula did you use? (Simplified or more precise?)

Conductor Material

Are you using Copper or Aluminum?

Insulation Type

What insulation type is being used? THHN? XHHW? This affects the ampacity and temperature rating.

Installation Method

Is the cable in conduit? Direct buried? In free air?

Ambient Temperature

What is the ambient temperature where the cable will be installed?

Number of Conductors in Conduit

How many current-carrying conductors will be in the same conduit? This affects ampacity adjustment factors.

2. 480V, 15kVA, Single-Phase Service



Calculated Current (31A)

15,000 VA / 480 V = ~31.25A. This calculation is correct.

Distance (525 ft)

Again, a long run where voltage drop will be critical.

Your Result (4 AWG 2/C)

This is also likely to be affected by voltage drop. A 4 AWG conductor typically has an ampacity of around 70-85 amps (depending on insulation type and temperature ratings). It can carry 31A from an ampacity standpoint. However, the 525-foot run will likely result in excessive voltage drop.

Critical Questions for the single-phase service



Voltage Drop Calculation

Show me your voltage drop calculation. What voltage drop formula did you use? (Simplified or more precise?)

Conductor Material

Are you using Copper or Aluminum?

Insulation Type

What insulation type is being used? THHN? XHHW? This affects the ampacity and temperature rating.

Installation Method

Is the cable in conduit? Direct buried? In free air?

Ambient Temperature

What is the ambient temperature where the cable will be installed?

Number of Conductors in Conduit

How many current-carrying conductors will be in the same conduit? This affects ampacity adjustment factors.

How to Check Your Calculations (General Approach)


Ampacity Check

First, ensure your conductor size has sufficient ampacity after applying any necessary temperature correction or adjustment factors. Use the NEC ampacity tables (or your local code) based on the conductor insulation type and installation method. For example, if you have four current carrying conductors in a conduit, you need to reduce the ampacity by 80%.2.

Voltage Drop Calculation

Use a voltage drop calculator or the voltage drop formulas below. There are many online calculators available. The formulas depend on whether you're using single-phase or three-phase power:

Single-Phase Voltage Drop (Simplified)

VD = (2 K I L) / CM

Three-Phase Voltage Drop (Simplified)

VD = (√3 K I L) / CMWhere: VD = Voltage Drop (in volts) K = Conductor material constant (Copper = 12.9, Aluminum = 21.2 – these are approximations) I = Current (in amps) L = Length of the conductor (in feet) - One way distance. CM = Circular mils of the conductor (lookup based on AWG size)

More Accurate Formulas

Account for reactance and power factor for more precise calculations, especially on longer runs with inductive loads. You can usually find these formulas in electrical engineering handbooks or advanced voltage drop calculators.3.

Compare to Allowable Voltage Drop

Calculate the percentage voltage drop: (VD / Source Voltage) 100. Is this percentage within your acceptable limit (e.g., 3%)? If not, increase the conductor size and recalculate.
4.

Conduit Fill

After you have the conductor size, check the conduit fill requirements in the NEC or your local code to ensure the conduit is large enough for the conductors.

Example (Illustrative - You Must Do Your Own Calculations Based on Your Specific Conditions)

Let's hypothetically look at your single-phase example. Assume Copper conductors (K=12.9), and THHN insulation, installed in conduit, with an ambient temperature that doesn't require derating and three current-carrying conductors.
1.

Ampacity

4 AWG THHN has an ampacity of 85 amps. With derating the ampacity is 85.8=68.2.

Voltage Drop

4 AWG copper has approximately 41,740 circular mils (CM). VD = (2 12.9 31.25 525) / 41740 = ~10.19 volts Percentage Voltage Drop = (10.19 / 480) 100 = ~2.12%In this highly simplified example, 4 AWG might be acceptable. However, a slightly higher current draw, a higher ambient temperature, or needing to meet a tighter voltage drop requirement would necessitate a larger conductor.

In Summary



Voltage drop is almost certainly the determining factor here. You must accurately calculate voltage drop for both services. Gather all the details

Conductor material, insulation type, installation method, ambient temperature, number of conductors in conduit.

Use a reputable voltage drop calculator or perform the calculations manually. Consult the NEC or your local electrical code. If in doubt, consult with a licensed electrical engineer. They can perform a thorough analysis and ensure the design meets all applicable codes and safety standards.I hope this helps! Let me know if you can provide more details about your voltage drop calculations and the installation specifics. Flag for review