How the cable cross-section calculator works
The calculator checks two limits simultaneously: first the voltage drop (for economy and yield), second the current capacity (for safety and fire protection). The recommended cross-section is the smallest from the standard series (1.5–300 mm² per IEC 60228) that meets BOTH limits. The voltage drop formula is ΔU = 2·I·L·ρ/A (DC and single-phase AC, factor 2 for forward and return) or ΔU = √3·I·L·ρ/A (three-phase AC, single length only).
The specific resistance ρ is temperature-dependent: for copper 0.0178 Ω·mm²/m at 20 °C, rising about 0.4 %/K. The calculator corrects this automatically. Current capacity tables come from DIN VDE 0298-4 (Germany), analogous to OVE E 8101 (Austria) and SIA 4002 (Switzerland). Installation-type corrections account for: free air (best cooling), wall channel (medium), conduit (restricted), buried cable (good cooling) and riser (heat buildup).
Each mode has different standard voltage-drop limits: 1 % for PV strings (industry standard), 2 % for MPPT-battery lines (relaxed due to shorter length), 0.5 % for battery-inverter connections (very strict due to DC ripple), 3 % for AC distribution (standard per VDE 0100-520). For Victron battery-inverter connections, the manual table is the primary source — see next section.
The four modes in detail
Select the appropriate mode via the tabs above. Each mode has sensible defaults for temperature, voltage-drop limit and installation type — which you can adjust anytime.
Mode 1: PV string to MPPT/inverter
From the module string on the roof to the solar charge controller or string inverter. Current = module Isc × 1.25 (sizing factor per DIN VDE 0100-712). Standard voltage drop ≤ 1 %. Cable: solar cable H1Z2Z2-K per EN 50618, UV-resistant, 1500 V DC. Typical cross-sections: 4–10 mm². Recommended fuse: gPV solar fuse with 1.5 × Isc.
Mode 2: MPPT charge controller to battery
From the MPPT output to the battery bank. Current = max. output current of the MPPT (e.g. 100 A for SmartSolar 250/100). Voltage = 12, 24 or 48 V (system nominal voltage). Standard voltage drop ≤ 2 %. Cable: H07RN-F or H07V-K fine-stranded. Typical cross-sections: 6–25 mm². Fuse: gG at the MPPT output with 1.25 × MPPT current.
Mode 3: Battery to inverter (Victron)
From the battery bank to the inverter. Here the calculator covers two product lines: on one hand inverter-chargers (MultiPlus, MultiPlus II, Quattro, Quattro II, EasySolar II) with their DC-ripple-specific manual table, on the other hand pure inverters (Phoenix Smart, Phoenix VE.Direct) with their own manual table. Switch between the two lines via the toggle "MultiPlus / Quattro" or "Phoenix" above the model selector. If the model is in the respective table, exact values come from the official manual; otherwise fallback calculation with peak factor and 0.5 % voltage drop. Typical cross-sections: 6–120 mm² (depending on inverter power and line). Fuse: NH melt fuse matching the model.
Mode 4: Inverter to AC distribution
From the inverter AC-out to the distribution in the fuse box. Single-phase (230 V) or three-phase (400 V). Standard voltage drop ≤ 3 %. Cable: NYM-J or H07V-K per OVE E 8101 / DIN VDE 0276. Typical cross-sections: 2.5–16 mm². Fuse: MCB characteristic C or K, matching the installation type and load.
Why Victron inverters need their own table
Inverters like the MultiPlus or Quattro do not draw constant DC current from the battery. The current follows the AC sine wave and oscillates at twice the grid frequency (100 Hz on a 50 Hz grid). This pulsation is called DC ripple. If the DC line has too high internal resistance, a voltage fluctuation drops with each current pulse — the inverter sees this as an "unstable battery" and in the worst case triggers a low-battery error even with a full battery.
Victron publishes an official cable recommendation table in each inverter manual. These values are recognizably more conservative than a pure voltage-drop calculation — for a MultiPlus II 48/5000 Victron recommends 70 mm² for 0–5 m DC line, while the pure ΔU formula gets by with 50 mm². The difference protects against DC ripple problems. Our calculator uses the Victron table as the primary source and falls back to pure calculation only when the model is not in the table.
For current calculation the calculator uses peak factors: ×1.3 for lithium batteries, ×1.2 for lead batteries, ×1.5 conservative. These factors represent the AC sine effect in the DC current. A MultiPlus II 48/5000 in 5000-VA full-load operation draws an average current of about 110 A — but peaks up to 165 A with lithium. The cable must carry this peak without significant voltage drop.
Phoenix vs. MultiPlus/Quattro — which line do I need?
Victron offers two fundamentally different inverter families: on one hand inverter-chargers like MultiPlus, MultiPlus II, Quattro and Quattro II, on the other hand pure inverters of the Phoenix series. Both convert battery DC to 230 V AC, but the application areas and thus the cable requirements differ significantly.
MultiPlus and Quattro additionally contain a charger section and can therefore also feed power from the grid or a generator back into the battery. They are ESS-capable (Energy Storage System), i.e. they can be integrated into a PV self-consumption installation with grid feed-in. The Phoenix series cannot do this — it is a pure DC-to-AC converter without charger function. In return, the Phoenix series is often more compact, lighter and cheaper.
For the cable cross-section calculator this means: both lines have their own official manual tables. The MultiPlus/Quattro manuals are conservatively sized due to DC ripple (continuous AC sine characteristic in the DC input, see section above). The Phoenix manuals specify values for different length classes (up to 6 m for the 1600-2000 VA models, staggered 0-5 m / 5-10 m for the 3000-5000 VA models).
When is Phoenix the right choice?
Phoenix inverters are ideal for applications without ESS connection: RV, boat, mountain cabin, emergency power backup, simple off-grid cabin. If you have a 12 V on-board battery and want to operate a single 230 V appliance (kettle, laptop charger, small tools), you go for the Phoenix VE.Direct series (250-1200 VA). For medium-sized off-grid systems with battery bank without grid connection, the Phoenix Smart series (1600-5000 VA) comes into question. However, as soon as grid feed-in, self-consumption optimization or a charging concept is required, MultiPlus or Quattro is the right choice.
Phoenix VE.Direct — factory cable and external fuse
An important feature of the smaller Phoenix VE.Direct models (250-1200 VA): they come from the factory with a finished cable set of 1.5 m length and have an internal DC fuse mechanism. As long as you use the supplied cable and the battery is less than 1.5 m away from the inverter, you do not need an external fuse. If the cable is extended or replaced by your own, an external DC fuse near the battery must be installed mandatorily — with the Phoenix Smart models 1600-5000 VA anyway (no internal fuse). The exact fuse values (e.g. 200 A for Phoenix Smart 12/1600, 100 A for 48/3000) are provided by our calculator directly from the manual.
Common errors in cable sizing
- Forward and return path not considered. For DC and single-phase AC, current flows forward AND back through two separate lines. Voltage drops on both paths — so use double the length in the formula. Our calculator does this automatically, but those working with online calculators that take only the single length end up with too small cross-sections.
- Current capacity forgotten. Voltage drop is not the only limit — the cable must also carry the current without overheating. For short lines, current capacity is often the harder limit. Example: 4 mm² for 200 A would still be OK for voltage drop at 0.5 m, but the cable would burn immediately (capacity ~42 A).
- Temperature correction ignored. Solar cables on the roof get 60–80 °C hot in summer. At these temperatures the current capacity drops to 60–70 % of the datasheet value. The specific resistance of copper also rises by 15–20 % — the voltage drop is therefore higher than calculated at 20 °C. Always calculate with real operating temperature.
- Trying to undercut Victron recommendations. For inverter cables, many DIYers try to undercut the "comically large" Victron recommendations with their own voltage-drop calculation. This often works technically — until the inverter throws low-battery errors at full load. Victron is intentionally conservative due to DC ripple. In warranty cases, Victron refers to the manual values.
- Choosing fuse too large. A fuse must be smaller than the current capacity of the cable, otherwise it does not protect. A 6 mm² cable (54 A capacity) with a 63 A fuse would burn in a fault before the fuse trips. Rule of thumb: fuse ≥ operating current and fuse ≤ cable capacity. For PV strings also use gPV type instead of gG.
Frequently asked questions
Why double the cable length for DC?
For direct current (DC) and single-phase AC (AC1), the current flows forward in one line and back in a second. The voltage drop occurs on both paths. If the single distance from MPPT to battery is 3 m, the total conductor length in the circuit is 6 m. Our calculator doubles this automatically — you enter only the single length.
Which installation type should I choose?
For PV strings mostly "free air" (roof with good ventilation). For MPPT-battery and inverter connections often "wall channel" or "conduit". For AC distribution in the house typically "wall channel" or "conduit". For garage/outdoor installation "buried cable". For vertical risers choose "riser" — due to heat buildup there is a 25 % derating.
What is the difference between voltage drop and current capacity?
Voltage drop is an economic limit: too little arriving voltage means yield loss. Current capacity is a safety limit: too high current overheats the cable and can cause fire. Our calculator checks both simultaneously. The recommended cross-section is the smallest that meets both limits.
Why are Victron recommendations so large?
Victron calculates not only the average current but also the DC ripple: in an inverter, the DC current follows the AC sine wave at twice the grid frequency. The cable must transmit these pulses without voltage fluctuation, otherwise there are low-battery errors. The larger cross-sections protect against this problem. It is not over-caution but technically justified.
Which fuse fits which cross-section?
Standard values (installation type "free air"): 1.5 mm² → 16 A, 2.5 mm² → 25 A, 4 mm² → 32 A, 6 mm² → 40 A, 10 mm² → 63 A, 16 mm² → 80 A. With other installation types the permissible fuse is reduced according to capacity. PV strings: gPV solar fuse instead of gG. Inverter main cable: NH melt fuse in the size from the Victron manual.
Can I use multiple thin cables in parallel instead of one thick one?
Yes, often practical for large cross-sections. Victron recommends e.g. for MultiPlus II 48/8000 either 1×95 mm² or 2×50 mm². Important: both parallel conductors must be the same length (otherwise uneven current distribution), same cross-section, same material. Connection to the same terminal (or via separate lugs on the same bolt).
How accurate are the capacity values?
The values come from DIN VDE 0298-4 and are industry standard. They apply for 30 °C ambient, 2 loaded conductors, standard-compliant installation. For deviations (higher temperature, multiple cables together, different installation) there are correction factors — our calculator considers installation type and temperature automatically. For very specific conditions (e.g. accumulation of 6+ cables) a professional may find additional factors necessary.
Do I need a separate fuse for each string?
For a single string NO (there is no source that could supply return current). For three or more parallel strings YES — with three strings, two could feed back into a shorted third and overload it. Rule of thumb: from 3 parallel strings, one gPV fuse per string with 1.5 × Isc. For two strings it is a design decision — many professionals still do it.
What is the difference between Phoenix and MultiPlus in cabling?
Both have official manual tables for the DC cables between battery and inverter, but the values differ. MultiPlus/Quattro are slightly more conservatively sized due to DC ripple — the same power often requires a larger cross-section. For Phoenix Smart 48/3000, 35 mm² is sufficient for 0-5 m; for MultiPlus II 48/3000 it would also be 35 mm² but with different fuse values (125 A instead of 100 A). Always use the manual for your specific line — the calculator does this automatically via the line toggle.
Do I need an external fuse with Phoenix VE.Direct?
The Phoenix VE.Direct models (250-1200 VA) have an internal DC fuse and come with a 1.5 m factory cable set. If you use exactly this cable and the battery is near the inverter, you do not need an additional external fuse. However, as soon as you replace the cable, extend it or do not use the factory set, an external DC fuse near the battery is mandatory — the value comes from the manual (e.g. 30 A for Phoenix 12/250, 50 A for Phoenix 12/500). For the larger Phoenix Smart models 1600-5000 VA, an external NH fuse is always mandatory because there is no internal fuse installed there.
Related tools
The cable cross-section calculator is part of our tool suite for photovoltaic planning:
MPPT Calculator for Victron
Verification of string configuration against all Victron MPPT charge controllers. Voc/Vmpp/Isc match per ÖNORM.
PV wizard for buildings
Complete system planning in 7 steps — roof measurement, statics, bill of materials.
Tool overview
All available configurators and tools at a glance.
Sources and calculation basis
The calculation algorithms are based on the following sources and standards:
- DIN VDE 0298-4:2013 Current capacity of cables and lines
- OVE E 8101:2019 Installation of electrical systems up to 1000 V
- DIN VDE 0100-712:2024 Solar photovoltaic power supply systems
- Victron Energy MultiPlus II / Quattro manuals 2024 ("Cable connection" sections)
Important note: This calculation serves as initial orientation. For binding sizing and construction of a photovoltaic or storage system, assessment and execution by a licensed electrician is required. For Victron inverter systems, also note the original manuals — values may differ for newer models.