How far should a UPS be from the battery cabinet?

There is no universal 5 m rule for the distance between a UPS and its external battery cabinet. Five installation manuals say something more useful: they do not agree on the distance, but the four covering large systems agree on the number behind it.

A 5 m limit does appear in some small DC UPS manuals, but it should not be carried over directly to a 100 kVA three-phase UPS. What governs a large battery bank is the current it delivers and how much of its voltage is lost on the way to the UPS.

Everything else follows from those two.

1. Why the battery loop is twice the distance you think

A battery circuit has two conductors. Current leaves the battery on the positive cable and returns on the negative one, so a cabinet sitting 5 m from the UPS puts 10 m of conductor into the voltage-drop calculation, not 5 m.

This is the most common error in battery cable sizing, and it is far cheaper to fix on a drawing than in copper.

The drop itself is simple:

Voltage drop = current * cable resistance

Cable resistance rises with length and falls as cross-section grows. That gives two levers with the same effect: halve the run, or double the copper, and the drop halves either way.

Two cabinets 5 m apart put 10 m of conductor between the battery and the DC bus. Quote the loop length, not the gap.

2. What a 100 kVA UPS draws from its battery bank

Battery current is the input to every calculation that follows, and it is larger than most people expect.

For illustration, take a 100 kVA UPS rated at 100 kW on a 384 V nominal bank. Ignoring conversion losses for the first pass:

100,000 W ÷ 384 V ≈ 260 A

Units with a lower output power factor land on a lower DC power, so this is an example condition, not a default.

Two things push the real figure above that. Battery voltage falls through the discharge, so the same power demands more current at the end than at the start. The inverter also takes its own losses before any of it reaches the load.

The current that matters for cable selection is therefore the rated maximum battery discharge current in the UPS manual, worked out from the rated output, the efficiency, the nominal battery voltage and the minimum battery voltage at end of discharge. A kVA rating and a division are not enough.

Start from the discharge current, and start from the rated kW behind it rather than the kVA on the front panel.

3. Voltage drop is the number that governs, not metres

Here is what the published manuals actually say about maximum DC cable length:

The limits run from 5 m to 50 m — a factor of ten. In the four large UPS examples, the published voltage-drop target is under 1%. The small DC UPS manual states a length without publishing a drop target alongside it.

That is the whole point. Distance is the result of a drop calculation, not the rule that replaces it. The published cable length is therefore tied to the manufacturer's cable sizing assumptions and voltage-drop limit, rather than being a universal distance rule.

Schneider's manual also shows where the cable sizes themselves come from: tables B.52.3 and B.52.5 of IEC 60364-5-52, assuming 90 °C conductors, 30 °C ambient, copper and installation method C. Change the ambient temperature or the installation method and the correction factors move the answer.

The spread is ten to one. The drop target behind the four large systems is not.

For a 260 A bank at 384 V, that 1% target translates directly into reach per cable size:

Those bars show voltage drop only. Final cable sizing must also satisfy ampacity, grouping, ambient temperature, installation method and protection requirements.

4. What a long DC run costs you

Voltage drop eats runtime at the wrong moment

At end of discharge the battery voltage is already falling. Cable drop on top of that means the UPS sees less than the battery terminals measure, and it reaches its low-battery shutdown threshold earlier than the battery is actually finished.

The bank was sized for a runtime. The cable quietly takes part of it back.

Copper, terminations and tray space

A longer run needs a larger cross-section, and the consequences are not only the cable price:

• larger lugs and a larger bending radius
• more space taken in the tray, which competes with everything else in it
• heavier, harder termination
• more mechanical load on the battery and UPS terminals
• more joints if the run cannot be pulled in one length

On a 120 kVA UPS with a long battery run, the additional copper, terminations and installation work can quickly outweigh the cost of improving the equipment layout during design.

Fault current and unprotected cable

A battery bank is a source. A short on the DC cable draws whatever the bank can deliver, and a long cable is also a long fault path with impedance that may keep a protective device from clearing quickly.

This is the third reason the distance question is really a layout question.

5. Why the battery cabinet often has to sit further away

If short runs were only a matter of choice, everyone would build them. They are not.

IEC 62485-2, which covers safety requirements for stationary secondary battery installations up to 1,500 V DC, addresses ventilation, electrical hazards, short-circuit protection and installation clearances. In practice, floor loading, room layout and battery temperature requirements also affect where the battery cabinet can sit.

A battery cabinet is heavy enough that floor structure becomes a siting constraint before cable length does, and a room that has to be ventilated is not always the room next to the UPS. Those requirements pull the battery away from the UPS. The drop target pulls it back. The layout is the negotiation between them, and the cable size is what you pay for the outcome.

6. Four checks before you accept a longer run

1. Confirm the maximum discharge current. Take it from the UPS manual, at minimum battery voltage, not from the kVA rating.
2. Check ampacity and voltage drop separately. A cable has to carry the current without overheating and keep the drop inside the target. Grouping, ambient temperature, tray or conduit, insulation rating and terminal size all move the ampacity answer. There is no rule that says a given kVA always takes a given cable size — the run length and the DC bus design carry as much weight as the rating.
3. Keep parallel cables identical. Where a polarity needs two or more cables in parallel, they have to share equal length, cross-section and route, plus the same conductor material and type. A shorter or differently routed cable has lower resistance, carries more than its share, and runs hotter. Light-load testing will not show it.
4. Read the installation manual. Battery voltages, discharge-current limits, terminal sizes and protection arrangements differ between makers, and the manual overrides any site rule about metres.

7. Route the positive and negative together

Positive and negative should follow the same route and stay close to each other. Running one down each side of a room because it is convenient enlarges the loop area, adds inductance, and makes the installation harder to inspect and to fault-find later.

On high-current DC, routing is part of the electrical design.

8. Put the protection next to the battery

The battery breaker, DC fuse or isolation switch belongs close to the battery terminals. Its job is to shorten the length of cable sitting between an energy source and the device that can disconnect it. A protection device at the UPS end leaves the longest possible section of the run unprotected.

Which device, and whether it sits inside the battery cabinet or in a separate DC enclosure, is set by the UPS and battery system design. The test is simple: no long stretch of cable between the battery and something that will open.

Start with the layout, not the cable

Where the room allows it, place the UPS and the battery cabinet close together while keeping the service clearances the equipment needs. Short DC runs give lower drop, smaller cable, lower cost, fewer joints and easier inspection — which is why experienced engineers push for it at the drawing stage rather than after delivery.

The five-metre figure is a useful prompt. It is not the specification.

When you review a layout, ask four things instead of measuring the gap: what is the maximum DC current, what cross-section carries it, what is the calculated drop over the loop length, and where does the protection sit. Get those four right and a longer run is fine. Get them wrong and a short one still causes trouble.

Send a supplier the load list, the runtime and the room you have — the battery voltage, the available cable route, the floor the cabinet has to stand on and the clearance it needs — and the cable size comes back as a calculated result. TAFENG engineers size the DC cable from the UPS battery voltage, maximum discharge current, cable length and site installation conditions rather than applying one fixed distance rule.

FAQ

How far can a UPS be from the battery cabinet? Published manufacturer limits range from 5 m on some small DC UPS systems to 50 m on large UPS installations. For several large UPS ranges, the stated design target is a DC voltage drop below 1%. Always check the installation manual for the specific model.

Why is the battery cable loop twice the distance? Current travels to the UPS on the positive conductor and returns on the negative one. A cabinet 5 m away therefore puts 10 m of conductor in the voltage-drop calculation, and resistance — not the gap — is what produces the drop.

What voltage drop is acceptable on a UPS battery cable? Under 1% of nominal DC voltage is the convention across the large-UPS manuals checked here, usually stated at rated battery current. That figure is a maker's design target, so the manual for your unit takes priority.

How much current does a 100 kVA UPS draw from its battery? About 260 A for a 100 kVA unit rated at 100 kW on a 384 V nominal bank, before conversion losses. The real figure is higher, because battery voltage falls through the discharge and the inverter takes its own losses, so size from the rated maximum discharge current in the manual.

Where should the battery breaker be installed? Close to the battery. Protection sitting at the UPS end leaves the full cable run between the battery terminals and the device that can interrupt a fault.

Do parallel battery cables have to be the same length? Yes — same length, cross-section, route, material and cable type. A shorter or differently routed conductor has lower resistance and carries more than its share of the current.