DC vs AC Cable Sizing for Solar: Why Gauge and Length Both Matter
Cable gauge is not a number an installer picks once and reuses for every job. DC and AC runs are sized by genuinely different logic, and Singapore's SS 638 adds its own specific requirements on top.
Quick answer
Cable sizing for solar is a calculation, not a fixed gauge reused for every job, balancing current, run length and an acceptable voltage drop, typically around 2% DC and 1% AC. DC runs use the round-trip distance current actually travels, roughly double the straight-line cable length, which AC calculations do not always require. SS 638 adds Singapore-specific rules: cable exposed to sun must be UV-resistant, and DC cable must run in metal trunking or conduit.
2% + 1%
Common industry target for maximum voltage drop: up to 2% on the DC side, 1% on the AC side, for roughly 3% total across the system
Cable gauge in a solar installation is not one fixed number an installer reuses for every job. It is a calculation, balancing the current the cable carries, how far it has to run, and how much voltage drop, the gradual loss of voltage along the cable's length, is acceptable before it starts costing real generation or creating a safety concern.
What is voltage drop, and why does it matter?
Every metre of cable has some electrical resistance. Current flowing through that resistance loses a small amount of voltage along the way, converting the difference into heat rather than usable electricity.
A small amount of this is unavoidable and normal. Too much means real generation lost as waste heat, and in a persistent worst case, cable running hotter than it safely should under sustained load.
Industry practice commonly targets keeping voltage drop to around 2% on the DC side and 1% on the AC side, roughly 3% total across the full system. That target balances real efficiency against the cost of using thicker, more expensive cable everywhere by default.
Why aren't DC and AC runs sized by the same logic?
DC voltage drop calculations use the round-trip distance current actually travels, out along one conductor and back along the other, which works out to roughly double the straight-line cable length between two points. Some AC calculations don't require this same doubling, depending on the specific circuit configuration. So a DC and an AC run of the same physical length on a roof aren't automatically sized the same way.
This distinction matters most on a larger roof with panels spread across multiple orientations, where DC run lengths from the farthest string back to the inverter can be considerably longer than the AC run from the inverter to the distribution board. Each needs its own calculation, rather than one gauge assumed to cover both.
What does SS 638 specifically require for solar cabling in Singapore?
Beyond the general voltage-drop calculation, Singapore's SS 638 sets its own explicit requirements for solar-specific cabling. Any cable exposed to direct sun has to be UV-resistant, since standard cable insulation degrades under sustained UV exposure in a way that isn't obvious until it has already started failing. DC cable specifically has to run inside metal trunking or conduit for mechanical protection, a requirement that goes beyond what a general household wiring circuit needs.
It's worth confirming an installer's wiring plan meets these specific SS 638 requirements, not just that cable is present and connected. That's a concrete, checkable question, not a vague assurance to take on trust.
What does a marginal voltage drop actually cost over a system's life?
A 3% total voltage drop sounds small enough to dismiss. Applied against a system generating for 25 years, though, that percentage represents real, permanently lost generation every single day the sun is up, not a one-time or occasional loss.
Unlike a fault that shows up as an obvious trip or error, undersized cable degrading output through voltage drop produces no alarm and no error message. It's just a monitoring app quietly reporting numbers that never quite match what the system's specifications suggested.
This is exactly why proper sizing at installation matters more than it might seem from the size of the percentage alone: calculated for the actual run lengths on a specific roof, not assumed from a generic rule of thumb. A homeowner comparing two quotes with identical panels and inverters has no way to see this difference without asking directly what cable gauge and expected voltage drop each installer actually calculated.
Why isn't simply oversizing every run free?
Using a thicker gauge everywhere by default would solve voltage drop with margin to spare. But thicker cable costs more, is physically harder to route through tight spaces, and isn't automatically stocked in every gauge an installer might want on a given day. Proper sizing calculates what a specific run actually needs, rather than either underspecifying to save cost or overspecifying past the point of genuine benefit.
Cable sizing is a calculation done for each installation's actual run lengths and loads, not a single default an installer applies without checking. It's the same principle that governs ballast weight calculations on the structural side of an installation.
What should you check in a quote?
Ask for the specific cable gauge proposed for both the DC and AC runs, and whether the expected voltage drop for your actual roof layout and run lengths was calculated, rather than assumed from a generic figure applied regardless of the property. Confirm the cable meets SS 638's UV-resistance and DC mechanical-protection requirements explicitly, the same standard Balance of System quality depends on more broadly.
Ask to see the figures in writing, rather than accepting a verbal assurance that everything has been properly sized. Run the Sunnify solar estimate to start planning a properly specified system.
FAQ
Frequently asked questions
Resistance increases with length, and that added resistance is what turns into voltage drop and wasted energy as heat along the run. A thicker conductor carries the same current with less resistance per metre, which is the direct trade-off against a longer physical run rather than a separate, unrelated consideration.
Both, and the safety side matters more. Undersized cable running hotter than it should under sustained load is a genuine fire risk over years of operation, not just a source of quietly reduced generation numbers on a monitoring app. Efficiency loss is the more visible symptom; heat is the underlying hazard it comes bundled with.
Mechanical protection, since DC cable exposed to physical damage, from foot traffic, tools, or general wear during future roof work, carries a different risk profile than AC cable already protected inside a building's walls. SS 638 addresses this directly for solar-specific DC runs rather than leaving it to general practice.
Not meaningfully by inspection, since gauge is not something visible or verifiable from outside the finished installation. Asking the installer to state the specific gauge and expected voltage drop for your system's actual run lengths, in writing, is the practical way to hold a quote accountable rather than trying to inspect cable after the fact.
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