When you run cables underground, buried directly in the earth without conduit, you have to size them properly so they:
- Carry enough current without overheating.
- Stay safe and code-compliant under all conditions (soil, temperature, depth, etc.).
How We Size Cables for 3-Phase Service (Direct Burial)
Step-by-Step:
- Determine the load
- This is the total amperage the service will carry.
- For example: 100A 3-phase service.
- Use CEC (Assorted Table D’s in Appendix D)
- These tables are in the Canadian Electrical Code and give you the ampacity (current-carrying capacity) of different cable sizes.
- Consider soil temperature
- Ampacity tables assume a certain ground temperature (usually 50-60°C).
- If your soil is hotter, you may need to de-rate the ampacity (lower it).
- Factor in length (voltage drop)
- Long underground runs may cause a voltage drop, meaning the voltage at the far end is lower.
- Use voltage drop calculations (Table D3) (using cable resistance and reactance) to check that voltage stays within acceptable limits (usually 3–5%).
- Choose the smallest conductor that meets all these conditions
- Use the ampacity table to find the smallest size that can safely carry the load, stay cool enough, and meet voltage drop rules.
Why All This Matters
- Underground cables can’t cool off as easily as cables in open air.
- The soil traps heat, especially if dry or compacted.
- Overheated cables = fire risk, insulation breakdown, early failure. Fun fact: Fire is bad.
- Getting the size right ensures safety, long life, and efficient power delivery.
Simple Analogy:
Think of a direct burial cable like a garden hose buried in the ground. If it’s too narrow and buried deep under heavy dirt, the water (current) can’t flow properly, and pressure (heat) builds up. A wider hose (larger cable) solves the problem and keeps things safe.
Tables D8-D11 have both Diagrams AND Tables. All of these tables are for Voltages UNDER 5000V.
(If you want more than 5000V, you have to go to Table(s) D17+)
They’re surprisingly easy to understand. Which is a very weird thing to say about the Code book.
Each option has a “Detail”. You can see how many conductors and the configuration, and the spacing info in each detail. You then go to the Table, and find the relevant Detail.
The C of Q Exam is going to tell you exactly which Diagram and “Detail” to go to (don’t worry, you don’t have to guess!)
So what do we do once we get there?
Well, it depends on what type of configuration we need to deal with.
What do we do if we have 1 cable per phase?
- The service is rated 200A, so each phase conductor must be able to safely carry at least 200A.
- We size our conductor for 200A
What do we do if we have 2 cables per phase?
- We need 200A total per phase, but we are using 2 conductors per phase, so each conductor will carry half the current.
- 200 A / 2 = 100A per conductor
What do we do if we have 3 cables per phase?
- 200 A / 3 = 66.7A per conductor
You get the point.
BUT (because there’s always a “but” with code….)
You have to pay attention very closely to what the question is asking/how the question is phrased.
Sometimes Multiply, and Sometimes Divide:
It all depends on what the question is asking:
WHEN TO MULTIPLY:
You multiply when the question gives you the amperage per conductor and you need to find the total required ampacity.
Example (like earlier):
“The service is 100A, and we’re using 3 cables per phase. What’s the total ampacity required from all cables?”
- Since you're using 3 parallel cables, and the load is 100A per phase, you multiply:
- 100A×3=300A total ampacity needed from the cable group
How Do I Know That a "100A 3-Phase Service" Means 100A Per Phase?
Short Answer:
When a service is described as “100A 3-phase”, it means 100A per line conductor (per phase) — not shared across the 3 phases.
Why?
- A 3-phase 100A service means:
- Each ungrounded conductor (L1, L2, L3) is rated to carry 100 amps.
- The disconnect, panel, and feeders must be able to carry 100A per phase.
- It's not 100A total split 3 ways — it’s 100A for each leg.
Think of It Like This:
Imagine you're wiring a 3-phase panel labeled "100A":
- You install:
- A 3-pole 100A breaker
- 3 conductors (L1, L2, L3)
- Each of those conductors must carry up to 100A.
So you must size each ungrounded conductor to handle 100A, whether it's one cable per phase, or multiple in parallel.
Example: 1/10 Difficulty
What is the minimum size RWU 90 conductor that is required for a 200A 347V/600V three-phase service, using the IEEE installation configuration detail 2 of diagram D10? Termination Temperature is 90C.
- It shows 2 cables per phase
- 200A * 2= 400A
- Going to the Table: Appendix D
- Detail 2 → 350 kcmil
Example: 4/10 Difficulty
What is the minimum size RWU 75 conductor that is required for a 200A 347V/600V three-phase service, using the IEEE installation configuration detail 2 of diagram D10?
- It shows 2 cables per phase
- 200A * 2= 400A
- Note 2 says we have to “de-rate” because the table is based on 90C.
- 400A * 0.886= 354.4A
- Going to Table: Appendix D
- Detail 2 → 300 kcmil
Example: 10/10 Difficulty
What is the minimum size RWU 75 conductor that is required for a 200A 347V/600V three-phase service with an 80% Power Factor with a length of 150m, using the IEEE installation configuration detail 2 of diagram D10 that stays within 3% voltage drop?
- It shows 2 cables per phase
- 200A * 2 = 400A
- Note 2 says we have to “de-rate” because the table is based on 90C.
- 400A * 0.886= 354.4A
- Going to Table: Detail 2: 300 kcmil
Numbers
- V = 600V
- VD = ??
- K = 0.167
- f = 1.73
- I = 177.2A
- L = 150m
VD = K*f*I*L / 1000
Use CEC Table D3 for K
- 300 kcmil copper, 75°C, Cable, 80% PF
- K = 0.167 ohms/km
What are my Amps?
- Adjusted Total Load (after de-rating) : 354.4A
- There are 2 cables per phase, and we need to size for each individual conductor:
- 354.4 A / 2 = 177.2 A
- VD = 0.167 x 1.73 x 177.2 x 150 / 1000
- VD= 7.67V
Calculate Percent Drop
- VD% = 7.67 * 100 / 600
- VD% = 1.27%
It stays.