Cable selection algorithm · PV strings, sub-arrays, array mains, battery and DC-bus circuits

DC cable selection algorithm

Sizing a d.c. cable in a PV or battery system, where neither the voltage nor the current comes from a load: the array voltage corrected to the lowest expected temperature, the minimum current rating of Table 5, the module temperature rise of 40 K, and the fault current a battery can deliver.

12 steps · clause and formula numbers cited at every node · no standard text reproduced

A d.c. cable in a PV or battery system is sized on four criteria, and the largest size any of them demands is the one applied (IEC 62548, 7.3.7.1.1): the overcurrent protection rating where one is fitted, the minimum current rating of Table 5, the voltage drop, and the prospective fault current.

Two inputs are specific to d.c. and are where most designs go wrong. The voltage is U_OC of the array corrected to the lowest expected operating temperature — up to 1,25 × at −40 °C (Table 4) — while the current is not simply 1,25 × I_SC: for anything other than a single-string array it is I_n of the nearest downstream device plus 1,25 × I_SC × (N_PO − 1). And the cable temperature for module-adjacent runs is the maximum ambient plus 40 K, so a 52 °C site means rating the cable near 92 °C.

The algorithm

Flowchart of the DC cable selection algorithm for PV and battery circuits to IEC 62548, IEC 60364-7-712, IEC 60287 and IEC 60364-5-52 DC CIRCUIT — PV OR BATTERYIdentify the circuit and collect the dataIEC 62548, 5.1.3 and Clause 7; IEC 60364-7-712, 712.3- which circuit: PV string, PV sub-array, PV array main, battery, or a DCbus between converters- module data at STC: U_OC, I_SC, the maximum series fuse rating, and thetemperature coefficients- lowest expected operating temperature, and the highest expected ambient- string, sub-array and array counts, and where overcurrent protection isprovided- for battery circuits: the maximum charge voltage and the prospective faultcurrent the battery can deliver- backfeed current the converter can push into the array under fault (ratedper IEC 62109-1)1Establish the maximum circuit voltageIEC 62548, 7.2 and Table 4; IEC 60364-7-712, 712.411.1- PV array maximum voltage is U_OC of the array corrected for the lowestexpected operating temperature- use the module maker's coefficients; failing that, Table 4 gives 1,02 at20 °C rising to 1,25 at −40 °C for crystalline silicon- below −40 °C, or for non-crystalline technologies, only the maker'sinstructions apply- for SELV or PELV, U_OC at STC replaces U_n and must not exceed 120 V d.c.2Establish the minimum current rating of the circuitIEC 62548, Table 5 and 6.5.5; IEC 60364-7-712, 712.433- single-string array with no string protection: 1,25 × I_SC of the module- otherwise: I_n of the nearest downstream device + 1,25 × I_SC × (N_PO −1); with no protection anywhere, I_n becomes zero and N_PO is every parallelstring- where string protection is provided, the criterion is the rated currentI_n of that device- sub-array with no sub-array protection: the larger of (I_n of the arraydevice + 1,25 × the short-circuit current of all other sub-arrays) and 1,25× I_SC of this sub-array- add the converter backfeed current where it can flow into the array- omitting overload protection is only allowed while the capacity stays ator above 1,25 × I_SC STC (712.433.1 and 712.433.2)3Choose the cable typeIEC 62548, 7.3.7.2 and 7.3.1; IEC 60364-7-712, 712.522.8.1; IEC 60228- rated for d.c., with a voltage rating at or above the maximum circuitvoltage of step 2- UV resistant, or protected from UV, and water resistant- above DVC-A, selected to minimise the risk of earth faults andshort-circuits — normally double-insulated or reinforced single-coresheathed cable- class 5 flexible conductor where movement is expected (string cables,trackers, plug connections); class 2 or class 5 where it is not- flexible types to EN 50618 or UL 4703; IEC 62930 was under developmentwhen IEC 62548 was published4Set the design operating temperatureIEC 62548, Table 5 footnote a, 7.3.7.2 and 5.1.9- for cables in contact with or near modules, take an operating temperatureof at least the maximum ambient plus 40 K- the insulation temperature rating has to match that, not the ambient- modules facing open sky can also sit up to 5 °C below ambient — which iswhy the voltage check uses a different temperature from the current check5Take a trial cross-sectionIEC 62548, 7.3.7.1.1; IEC 60228- one trial size per circuit, from the standard sizes- the standard's rule: the largest size that any of the criteria belowdemands is the one to apply6Calculate the current-carrying capacityIEC 62548, 7.3.7.1.2; IEC 60287 (all parts); IEC 60364-5-52 for the derating- capacity from IEC 60287, at the design temperature of step 4- derate for the installation method and location per IEC 60364-5-52 —trays, conduit, buried duct, bunched under modules- the cable maker's own rating data for that installation method has to betaken into account7Capacity at least the step 3 current?IEC 62548, 7.3.7.1.1 b)8yesVoltage drop within the project limit?IEC 62548, 7.3.7.1.1 c) requires it to be considered but sets no figure9yesWithstands the prospective fault current?IEC 62548, 7.3.7.1.1 c) and 6.5.6; IEC 60364-4-43, 434.5.2 and Table 43A10yesSize and place the overcurrent protectionIEC 62548, 6.5.3 to 6.5.7- string device: I_n above 1,5 × I_SC of the module, below 2,4 × I_SC, andnot above the module's maximum series fuse rating- sub-array device: above 1,25 and not above 2,4 × the sub-arrayshort-circuit current; array device the same against the array current- battery-connected systems always need overcurrent protection, rated tobreak the battery fault current- devices go at the end of the cable furthest from the source they protectit against (6.5.7)11Check the erection requirementsIEC 62548, 7.3.7.3, 7.3.8, 7.4.3; IEC 60364-7-712, 712.444.4.4- general requirements of IEC 60364-5-52 apply to the erection- support against wind and snow fatigue, protection from sharp edges, bendradius per the maker; cable ties are not a primary support unless rated forthe plant life- wiring loops kept to the minimum area, to limit the voltage induced bylightning- segregation between d.c. and a.c. circuits to the same requirement asbetween different voltage levels12LARGEST SIZE FROM ALL CRITERIA GOVERNSIncrease the cross-section, or improve theroutebunched string cables under a module table are agrouping case like any other; separating them is oftencheaper than a size upnoIncrease the cross-section, or re-route toshorten the runno IEC limit exists for a PV d.c. circuit; the limitbelongs to the project's loss budget. Compute it at theconductor's operating temperature, not at 20 °C — thatis what turns a 'compliant' 1 % into 1,3 %noIncrease the cross-section, or addprotection nearer the sourcea PV array is current-limited, but a battery is not: thebattery fault current governs the array main cable andevery device has to be able to break it (6.5.6)no
Spine down the left is the path a compliant design takes. Every branch to the right is a failed check and the change it forces, with the dashed arrow showing where the algorithm restarts. The table below carries the same content in text.

The steps in words

The same algorithm as a table: what is checked at each step, the criterion, and the clause it comes from.
#Check or actionCriterionReference
1Identify the circuit and collect the dataA DC circuit cannot be sized until it is classified — the current criterion is different for each type.IEC 62548, 5.1.3 and Clause 7; IEC 60364-7-712, 712.3
2Establish the maximum circuit voltageMaximum voltage = U_OC ARRAY × the correction for the lowest expected operating temperature.IEC 62548, 7.2 and Table 4; IEC 60364-7-712, 712.411.1
3Establish the minimum current rating of the circuitThe Table 5 current for that circuit type and that protection arrangement — not simply 1,25 × I_SC.IEC 62548, Table 5 and 6.5.5; IEC 60364-7-712, 712.433
4Choose the cable typeDC-rated, UV and water resistant, reinforced or double-insulated above DVC-A, class 5 wherever the cable moves.IEC 62548, 7.3.7.2 and 7.3.1; IEC 60364-7-712, 712.522.8.1; IEC 60228
5Set the design operating temperatureCable design temperature ≥ maximum ambient + 40 K for module-adjacent runs.IEC 62548, Table 5 footnote a, 7.3.7.2 and 5.1.9
6Take a trial cross-sectionA standard size, to be raised by whichever check governs.IEC 62548, 7.3.7.1.1; IEC 60228
7Calculate the current-carrying capacityCapacity at the real temperature and the real installation method, not the flat catalogue figure.IEC 62548, 7.3.7.1.2; IEC 60287 (all parts); IEC 60364-5-52 for the derating
8Capacity at least the step 3 currentbunched string cables under a module table are a grouping case like any other; separating them is often cheaper than a size upIEC 62548, 7.3.7.1.1 b)
9Voltage drop within the project limitno IEC limit exists for a PV d.c. circuit; the limit belongs to the project's loss budget. Compute it at the conductor's operating temperature, not at 20 °C — that is what turns a 'compliant' 1 % into 1,3 %IEC 62548, 7.3.7.1.1 c) requires it to be considered but sets no figure
10Withstands the prospective fault currenta PV array is current-limited, but a battery is not: the battery fault current governs the array main cable and every device has to be able to break it (6.5.6)IEC 62548, 7.3.7.1.1 c) and 6.5.6; IEC 60364-4-43, 434.5.2 and Table 43A
11Size and place the overcurrent protectionDevice ratings inside the 1,5 to 2,4 × I_SC window for strings, 1,25 to 2,4 × I_SC for sub-arrays and arrays.IEC 62548, 6.5.3 to 6.5.7
12Check the erection requirementsSupport, bend radius, minimum loop area and a.c./d.c. segregation all as installed.IEC 62548, 7.3.7.3, 7.3.8, 7.4.3; IEC 60364-7-712, 712.444.4.4

What actually governs the size

Running the algorithm is mechanical. Knowing which check will bind before you start is what makes it quick — and what tells you whether a schedule someone else produced was ever checked at all.

CaseWhat binds
String cablesTemperature, not current. A 4 mm² or 6 mm² string cable rarely fails on ampacity, but rating it at ambient rather than ambient + 40 K overstates its capacity by a wide margin, and bunching a harness under the table adds a grouping factor on top.
Array main cables and long DC runsVoltage drop. IEC 62548 requires it to be considered but sets no figure, so the limit is the project's loss budget — and it must be computed at the conductor's operating temperature, which is what turns a nominal 1 % into 1,3 %.
Anything with a batteryThe fault current. A PV array is current-limited; a battery is not. IEC 62548, 6.5.6 requires overcurrent protection in every battery-connected system, able to break the full prospective battery fault current, and the array main cable has to withstand it.
Strings grouped under one deviceThe protection window. I_n must exceed 1,5 × I_SC, stay below 2,4 × I_SC, and not exceed the module's maximum series fuse rating — which in practice means strings can only be grouped when that rating is above about 4 × I_SC.
Everything exposedThe type. DC-rated, UV resistant, water resistant, class 5 flexible where it moves, and reinforced or double-insulated above DVC-A — a cable that passes every number and fails on UV is still the wrong cable.

Standards this algorithm is built from

StandardWhat it supplies here
IEC 62548:2016Photovoltaic (PV) arrays — design requirements; Table 4 voltage correction, Table 5 minimum currents, 7.3.7 cables
IEC 60364-7-712:2002Solar photovoltaic power supply systems — 712.433 overload, 712.522.8.1 wiring
IEC 60364-5-52:2009Wiring systems — installation methods and the derating factors IEC 62548 refers to
IEC 60287 seriesCurrent-carrying capacity, which IEC 62548, 7.3.7.1.2 requires for the CCC
IEC 60364-4-43:2008Short-circuit withstand k²S² and the k values of Table 43A
IEC 60228:2004Conductor classes — class 2 stranded, class 5 flexible
IEC 62109-1Converter safety — the backfeed current rating this algorithm needs

The clause and formula numbers above were read from the standards themselves. What is not reproduced anywhere on this site is the text of a standard: if you calculate for a living, buy the document from IEC. How each engine here is checked against the standards' own published values is on the validation page.

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