Sizing Main Cables and Breakers by Installation Method
The same conductor size can have nearly twice the current rating simply because the installation method changes. This is a decision to settle before ordering materials, not something to sort out on site.
This job follows on from the previous article, where I investigated the main cable at a service building and found that the laundry load shared a conductor with five other zones, to the point that the insulation melted (read the diagnostic article). Once I concluded that it needed a separate feeder, the next questions were what cable size to use and what amperage breaker to choose. I had to sit down with the manufacturer's current-rating tables alongside the installation standard, and came across what I think people working on site overlook most often: the exact same cable size can carry nearly twice the current simply because it is installed differently. In this article, I have included the full table and my selection logic.
The current ratings, cable sizes, and breaker sizes in this article come from the conditions at a single building, including ambient temperature, cable length, conductor count, and installation method. They can serve as a guide to the reasoning, but an actual installation requires an electrical engineer to calculate and sign off on the design, using the manufacturer's tables for the specific cable purchased and the current edition of the Electrical Installation Standard for Thailand.
The starting point: a new 32-meter feeder across an open area
The new feeder runs from the existing main panel across a road on an existing steel truss, a distance of about 32 meters, to a new MDB under the eaves of the laundry building. It directly supplies seven washer and dryer circuits, with branch feeders to subpanels in two other zones and three spare ways.
The key point is that this 32-meter stretch is outdoors on a truss, not inside a building. That gives me several installation methods to choose from, and that choice determines what cable size I need to buy.
CV at 90 degrees versus THW at 70 degrees
Before looking at the table, I need to distinguish between the insulation types. The two cable types commonly used for mains here are CV and THW, which differ in their insulation and maximum conductor operating temperature.
Cross-linked insulation, rated for 90 degrees
The conductor can run at up to 90 degrees without degrading the insulation, so its current rating is higher at the same size, and it withstands outdoor conditions better. At medium to large sizes, the price per meter is not much different from THW, so I choose CV almost every time for sizes of 35 mm² and above.
PVC insulation, rated for 70 degrees
Slightly cheaper at small sizes and easier to find, but its current rating is noticeably lower. THW 95 mm² in conduit embedded in a wall carries only 143 A, while CV of the same size carries 197 A, a difference of 38% with exactly the same amount of copper.
The point to watch is that CV's higher current rating is available only if the equipment at the ends can also handle that terminal temperature. The breaker terminals and cable lugs must be rated for that temperature; buying 90-degree cable and bolting it to terminals designed for 70 degrees is not enough.
Current-rating table: the same cable, but an 82% difference
This is the table I used to make the decision. All values are for copper conductors at an ambient temperature of 40 degrees Celsius, with 3 current-carrying single-core conductors.
| Installation method | CV 1x95 | CV 1x120 | THW 1x95 | |---|---|---|---|---| | In conduit embedded in a wall | 197 | 227 | 143 | | In surface-mounted conduit in air | 245 | 284 | 180 | | Cable tray with a cover | 245 | 284 | — | | Uncovered cable ladder, spaced apart | 311 | 364 | 239 | | Suspended on insulators in air | 354 | 413 | 202 |
This table shows two important things. First, the exact same CV 1x120 cable can carry anywhere from 227 A to 413 A, depending on how it is installed. Second, CV 1x95 installed in the open on a cable ladder (311 A) can carry considerably more than 120 cable in conduit embedded in a wall (227 A). This means that the installation method can buy ampacity more cheaply than increasing the cable size in many cases.
The physics is straightforward: a conductor that sheds heat better can carry more current. Conduit embedded in a wall is a second layer of thermal insulation. Three cables bundled tightly in trefoil heat one another, while cables laid out with spacing on an open ladder allow air to flow around each cable and carry the heat away fully.
Calculate the feeder size first, then find the cable
The correct order is to calculate the feeder current first, use it to select the breaker, and then choose a cable rated above the breaker, rather than choosing the cable first and forcing a breaker to fit. The feeder formula I use adds all the loads, then adds a 25% allowance for the largest load only.
I feeder = 1.25 x I (โหลดใหญ่สุด) + ผลรวม I (โหลดที่เหลือ)
โหลดใหญ่สุด DE-100 = 119.0 A
โหลดที่เหลือ IM50 45.6 + SPE-50 36.5
+ ห้องยา 9.0 + สำนักงาน 13.5 = 104.6 A
I feeder = 1.25 x 119.0 + 104.6 = 253.4 A
# เช็คลำดับ In <= Iz ให้ครบทุกครั้งก่อนสั่งของ
i_calc = 253.4 # กระแสที่คำนวณได้
breaker = 350 # พิกัดเบรกเกอร์ที่จะสั่ง
cable_iz = 364 # พิกัดสายตามวิธีติดตั้งที่จะเดินจริง
assert i_calc <= breaker, 'เบรกเกอร์เล็กกว่าโหลด จะทริปเวลาทำงานปกติ'
assert breaker <= cable_iz, 'เบรกเกอร์ใหญ่กว่าสาย เครื่องป้องกันคุ้มครองสายไม่ได้'
print('ผ่าน:', i_calc, '<=', breaker, '<=', cable_iz)
The condition In less than or equal to Iz is at the heart of safety here. If the breaker rating exceeds the cable rating, the cable can overheat and fail without the breaker even reaching its trip threshold, which is the same condition that already melted the original cable's insulation.
Choosing a 350 A main breaker and CV 1x120 cable
The calculation came to 253.4 A, so I chose the next standard main breaker size, 3P 350 A, and required the cable to carry more than that. The option that fit was CV 1x120 laid out with spacing on an open cable ladder, rated at 364 A, just above the breaker.
| Item | Selected value | Reason |
|---|---|---|
| Calculated current | 253.4 A | Feeder formula, with a 25% allowance for the largest load |
| Main breaker | 3P 350 A | Next standard rating above the calculated value |
| Conductors | CV 1x120 x4 + G 1x35 | Rated for 364 A, above the breaker |
| Actual operating peak | 224 A | 64% of the breaker rating, leaving headroom |
| If changed to a conveyor type | 296 A | Still below 350 A, so it can be supported |
| Voltage drop over 32 meters | ~0.7% | Well below the limit |
The 364 A rating applies only under the condition "open cable ladder, no cover, cables laid out with spacing of at least one cable diameter." If the installers bundle the cables neatly together on installation day, or the owner later orders a tray cover to keep dust out, the cable rating drops to 284 A, immediately below the 350 A breaker rating. The system then fails the requirement for the protective device to protect the cable. This condition must be written into the drawings and reinforced with the site crew every time.
The cheaper option: downsize to 95 and compensate with operating rules
The owner asked whether the cable could be downsized to CV 1x95. The answer is yes, but there are conditions to accept in exchange. Size 95 cable on an open cable ladder carries 311 A, so the main breaker must be reduced to 250 A to stay below the cable rating. Once the main is reduced to 250 A, all the machines can no longer run at the same time, and operating rules must be introduced.
| Operating mode | Machines allowed to run | Total current | Result with a 250 A main |
|---|---|---|---|
| Washing mode | SP120 + SP125 + HE-60 + pharmacy room | ~179 A | Pass |
| Drying mode (after washing has stopped) | DE-100 + IM50 + SPE-50 + pharmacy room | ~210 A | Pass |
| Overlap period | DE-100 drying plus only 1 washer | ~200 A | Pass |
| All washers + all dryers at once | All machines | ~381 A | Main trips, entire room loses power |
เช็คสองแบบด้วยเงื่อนไขเดียวกัน I calc <= In (เบรกเกอร์) <= Iz (สาย)
แบบ CV 1x120 253.4 A <= 350 A <= 364 A ผ่าน ไม่ต้องมีกติกา
แบบ CV 1x95 210.0 A <= 250 A <= 311 A ผ่าน เฉพาะเมื่อทำตามกติกา
หมายเหตุ: แบบ 95 ใช้ค่าโหลด 210 A ของโหมดอบเป็นตัวตั้ง ไม่ใช่ 253.4 A
เพราะออกแบบบนสมมติฐานว่าซักกับอบไม่ทับกัน สมมติฐานพังเมื่อไร ตัวเลขพังตาม
No operating rules needed
All machines can run at once without tripping, with about 90 A of headroom for future additions. The cable costs more, but it is a one-time expense that settles the issue without relying on the discipline of site staff.
Lower cable cost in exchange for rules
The wash-before-dry sequence must be enforced throughout the installation's service life. There is no room for new machines, and whenever someone breaks the rules, the main will trip and shut down the entire room, leaving laundry stranded mid-cycle in both washers and dryers.
An often-overlooked advantage of the 95 option is that the 250 A breaker enforces the rules itself. If someone quietly switches on more than agreed, the main trips immediately instead of letting heat gradually build up in the cables, as in the old system where nothing stopped it. From a safety standpoint, 95 cable with a 250 A main is still valid as long as In remains below Iz. The rest comes down to convenience and cost, and the owner can choose.
On price, I will say broadly that CV 1x120 costs about a quarter more per meter than 1x95 at market prices. Multiply that by four cables and the distance, and it becomes a noticeable sum on the quotation. But in this case, the run is only 32 meters, so the difference is not large enough for me to recommend taking a chance on operating rules. If the run were two hundred meters, that would be a different discussion.
Installation work dictated by the numbers
Choosing an open cable ladder with spaced cables determines a whole set of installation requirements. These are the parts that often get cut during price negotiations, leaving the calculated current rating inapplicable to the actual installation.
- A 300x100 mm cable ladder along the entire route, mounted on the existing steel truss, rather than tying the cables directly to the truss.
- Steel angle brackets supporting the ladder approximately every 2 meters; this job needs 17 supports over 32 meters, all coated with rust-inhibiting paint.
- Cables laid out with spacing of at least one cable diameter. No bundling and no tray covers.
- Outdoor sections need sunshades. Direct sunlight raises the ambient temperature above the 40 degrees used for the table, invalidating the calculated rating.
- An earth conductor runs the full route, with an earth rod at the destination panel, and earth resistance measured to meet the limit before handover.
Do not mix cable sizes in the same circuit
One last issue I often encounter when fixing old installations is different cable sizes joined together in the same circuit. For example, someone runs 120 cable for thirty meters, then uses leftover 95 from stock for the final ten meters. This is strictly prohibited.
The reason is simple: a circuit has one breaker, and that breaker is set to suit the larger cable's rating. Once there is a section of smaller cable, that section is left without protection. A current that 120 cable carries comfortably can overheat 95 cable, and the breaker will not trip because the current has not reached its own rating.
Beyond the rating issue, every joint adds resistance and becomes a source of heat. The old system's main cable in this case had a joint at every pole. Over time, the joints gradually loosened, resistance increased, and they grew hotter, repeating this cycle until the terminals burned. I therefore designed the new feeder as a single continuous run from panel to panel, without even one intermediate joint.
Selection method recap
Briefly, the sequence I use on every job is to collect the actual loads from the nameplates first, calculate the feeder current using the formula that adds 25% for the largest load, select the next standard breaker rating, and then look up a cable whose rating for the actual installation method exceeds the breaker rating. Do not pick the table column with the most attractive numbers and then hope the site crew installs it accordingly.
As for whether downsizing the cable is a sensible way to save money, my answer is that it can be done if the operating rules are acceptable. But those rules must be documented and posted at the panel, not just explained verbally to the technician present during installation, because the person pressing the start button five years from now will certainly be someone else. It depends on each site's budget and convenience.
The remaining issue is the upstream problem that prevented the whole system from balancing its phases in the first place. I cover that separately in the article on a neutral shared across transformers. If I have missed anything in the explanation, or a newer table has different values, I will come back and update this later.
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