A tenant asks for a new 40 kW electrical connection. The contractor says there is a spare breaker in the nearest SMDB and wants approval to proceed. On site, the breaker is physically available — but that alone tells you almost nothing.
Where is that SMDB fed from? What size is its incomer? What is the upstream breaker rating? How many other DBs are connected? Is the supply normal power or emergency power? What is the transformer capacity? Is there enough spare load upstream? Before approving anything, an experienced electrical or FM engineer normally asks for one document:
“Show me the latest single line diagram.”
A Single Line Diagram, commonly called an SLD, gives you a simplified picture of how electrical power travels through the building — from the utility or transformer down through LV panels, MDBs, SMDBs, DBs and finally major loads.
If you work in MEP or Facilities Management, you do not need to be an electrical designer to understand an SLD. But you should be able to open one, identify the supply path, understand breaker and cable information, and recognise when something does not match the actual installation. This single line diagram explained guide shows you how to do exactly that.
What Is a Single Line Diagram?
A Single Line Diagram is a simplified drawing showing the electrical power distribution system using a single line and standard electrical symbols.
A three-phase electrical circuit physically contains several conductors, but drawing every phase conductor separately would make a large building drawing extremely complicated.
The SLD therefore represents the power circuit in a simplified form.
For example, instead of separately drawing:
- Phase L1
- Phase L2
- Phase L3
- Neutral
- Protective earth
the main three-phase power path may be represented by one line with the relevant cable information written next to it.
An SLD can show:
- Utility incoming supply
- Transformers
- Generators
- Automatic Transfer Switches
- LV switchboards
- Bus couplers
- MDBs
- SMDBs
- DBs
- Circuit breakers
- Busbars
- Cables
- Capacitor banks
- UPS systems
- Major motors
- HVAC equipment
- Earthing arrangements
- Metering
- Protection information
IEC 60617 provides the international graphical-symbol framework used for electrotechnical diagrams and currently contains more than 1,500 symbols covering conductors, switchgear, protection, measuring instruments, electrical machines and other equipment.
An SLD is not just a construction drawing. It remains one of the most useful documents for operating, maintaining and modifying a building’s electrical system. Schneider Electric similarly identifies the SLD as a foundational document for understanding electrical interconnections, maintenance, load-flow studies and equipment ratings.
Why an SLD Matters to an FM or MEP Engineer
Imagine an MDB trips and six tenants lose power.
Without an updated SLD, your team may know which MDB has tripped but not immediately understand:
- Which transformer feeds it
- What other loads share the same upstream breaker
- Whether another supply is available
- Where downstream isolation points are located
- Whether the fault can affect other tenants
- What cable and breaker sizes are installed
The SLD gives you the system relationship.
That is why it becomes important during:
- Fault investigation
- Shutdown planning
- Tenant load additions
- Electrical modification
- Breaker replacement
- Cable replacement
- Protection coordination studies
- Generator load assessment
- Emergency restoration
- Building handover
- Electrical audits
- Planned maintenance
The drawing will not diagnose the fault for you. It tells you where the equipment sits in the electrical hierarchy and how each part is connected.
For a detailed explanation of the panel hierarchy itself, refer to the existing MEP Master Guide article:LV Panel, MDB, SMDB and DB Explained
Start Reading an SLD From the Power Source
When somebody gives you a complicated SLD, do not start in the middle.
Find the source. The normal power path in a commercial building may look conceptually like this:
Utility → Transformer → LV Main Switchboard → MDB → SMDB → DB → Load
A large building may also have:
Generator → Emergency Switchboard → Emergency MDB → Essential Loads
And critical facilities may add:
Normal Supply → UPS → Critical DB → IT / Security / Control Loads
Your first question should always be:
Where is the electricity coming from?
Once you identify that, follow the line downstream one section at a time.
Step 1: Identify the Incoming Electrical Supply
At the top or left side of the SLD, you will normally find the main incoming supply.
Depending on the project, this could be:
- Utility LV supply
- 11 kV utility supply
- 22 kV supply
- 33 kV supply
- Transformer secondary
- Generator
- Solar PV connection
- Another building’s main switchboard
For a large commercial facility, a common arrangement may be:
11 kV Utility → 11/0.4 kV Transformer → LV Main Switchboard
The transformer label may look something like:
TR-01
1500 kVA
11/0.415 kV
Dyn11
You do not need to understand every transformer-design detail just to read the SLD.
For normal FM review, first understand:
- Transformer identification
- Transformer capacity
- Primary voltage
- Secondary voltage
- Which LV board it supplies
If you have two transformers, confirm whether each supplies a separate bus section or whether they can operate through a bus coupler.
Step 2: Identify the Main LV Switchboard
After the transformer, you will normally reach the LV Main Switchboard or LVMSB.
This is the main low-voltage distribution point.
A typical drawing might show:
Transformer TR-01 → ACB → LVMSB-01
The incoming ACB could be labelled:
2500 A, 4P, 65 kA
This tells you several important things.
2500 A
The circuit breaker rated current is 2500 amperes.
That does not mean the board is actually consuming 2500 A.
It means the breaker has been selected/rated around that current level subject to its specific configuration and settings.
4P
Four-pole.
In a three-phase, four-wire system this generally means the breaker switches three phases plus neutral.
65 kA
This normally relates to the breaker’s short-circuit breaking capability.
Do not confuse:
Current rating = A
with:
Short-circuit breaking capacity = kA
A breaker could therefore be rated:
1600 A, 4P, 50 kA
The two values serve completely different purposes.
Step 3: Understand the Busbar
Inside the switchboard you may see a thick horizontal line.
This usually represents the busbar.
The busbar receives the incoming supply and distributes it to outgoing feeders.
You may see a notation such as:
2500 A Cu Busbar
This indicates a copper busbar system with a stated rating of 2500 A.
Large switchboards sometimes have two bus sections:
Bus Section A | Bus Coupler | Bus Section B
Each side may normally be supplied from a separate transformer.
The bus coupler allows the operating arrangement to be changed under the approved system design and switching procedure.
For an FM engineer, this matters greatly during shutdown planning.
If Transformer 1 or one LV section is being isolated, you need to understand whether the other transformer and bus section are designed to support any transferred load.
Never assume that because a bus coupler exists, you can simply close it.
The approved switching procedure, interlocks, transformer capacity, protection scheme and operational limitations must all be considered by competent electrical personnel.
Step 4: Follow the Outgoing Feeders
From the main busbar you will see several outgoing lines.
Each may feed:
- MDB
- Chiller
- Fire pump panel
- Lift panel
- Capacitor bank
- Generator synchronising panel
- UPS
- Mechanical plant
- Tenant electrical system
For example:
LVMSB → 630 A MCCB → MDB-HVAC-01
Follow that feeder.
You may then see:
MDB-HVAC-01 → 250 A MCCB → SMDB-AHU
Then:
SMDB-AHU → 63 A MCB → AHU-01
The SLD has now shown you the complete electrical relationship.
If AHU-01 loses power, you know which protective devices exist upstream.
That information is extremely useful before technicians start opening panels randomly looking for the supply.
Step 5: Read Circuit Breaker Information Correctly
Common protective devices shown on SLDs include:
- ACB — Air Circuit Breaker
- MCCB — Moulded Case Circuit Breaker
- MCB — Miniature Circuit Breaker
- RCBO — Residual Current Breaker with Overcurrent protection
- RCD/RCCB — Residual Current Device
A simplified reference is:
| Device | Typical Application | What to Read |
| ACB | Main LV incomers / large feeders | Current, poles, fault rating, protection |
| MCCB | MDB/SMDB and major loads | Current, poles, fault rating, trip unit |
| MCB | Final circuits / small loads | Current, poles, trip curve |
The drawing may show:
MCCB 250 A, 4P, 36 kA
Before approving any modification, do not look only at the 250 A rating.
You must also consider:
- Actual operating load
- Cable capacity
- Upstream protection
- Downstream protection
- Short-circuit level
- Discrimination/selectivity
- Breaker settings where adjustable
This is especially important when additional tenant loads are proposed.
Step 6: Read Cable Information
A cable description may appear as:
4C × 120 mm² Cu XLPE/SWA/PVC
Breaking this down:
4C
Four-core cable.
120 mm²
Cross-sectional area of each conductor.
Cu
Copper conductor.
XLPE
Cross-linked polyethylene insulation.
SWA
Steel wire armour.
PVC
Outer sheath.
Another drawing may show parallel cables such as:
2 × 4C × 240 mm²
This normally means two parallel sets of four-core 240 mm² cables.
That is very different from one 240 mm² cable.
When checking an SLD against the actual installation, verify the quantity as well as the cable size.
Cable selection is not determined by load current alone. Installation method, ambient temperature, grouping, voltage drop and fault withstand can all affect the required size.
Your existing cable-sizing guide covers that calculation in more detail:How to Size Electrical Cables for Buildings
Step 7: Understand MDB, SMDB and DB Hierarchy
A common mistake for engineers from non-electrical backgrounds is assuming every panel is basically the same.
The SLD shows the hierarchy.
For example:
LVMSB
↓
MDB-F1
↓
SMDB-F1-A
↓
DB-Lighting-01
↓
Lighting circuits
If DB-Lighting-01 trips at its incomer, only that DB may be affected.
If MDB-F1 trips, several SMDBs and DBs could lose power simultaneously.
That is why the location of the protective device matters.
A 100 A breaker at DB level and a 1000 A breaker at MDB level have completely different consequences when they trip.
Understanding this hierarchy is particularly important when investigating repeated power interruptions.
Step 8: Identify Normal and Emergency Power
Buildings often have more than one electrical source.
The SLD may distinguish between:
- Normal utility power
- Generator emergency power
- UPS-backed power
- Life-safety power
Look for equipment such as:
ATS — Automatic Transfer Switch
An ATS transfers the connected load between the normal and standby supplies according to the system design and control logic.
Typical essential loads may include:
- Fire alarm
- Emergency lighting
- Fire pumps or related controls
- Smoke-management systems
- Security
- Selected lifts
- Data systems
- Critical medical loads
- BMS controls
Do not assume every load connected to an emergency board will remain continuously energised during the changeover.
Generator starting and transfer arrangements can create a short interruption unless the load is supported by a UPS or another no-break system.
Step 9: Recognise UPS Systems on the SLD
A UPS may be shown between the incoming electrical supply and critical distribution board.
For example:
Normal MDB → UPS-01 → Critical DB → Server Racks
The UPS may also have:
- Input breaker
- Output breaker
- Static bypass
- Maintenance bypass
- Battery bank
From an FM perspective, understanding the bypass arrangement is important.
During UPS maintenance, the load may be transferred to bypass so that equipment can remain powered.
But UPS isolation and bypass switching must follow the manufacturer-approved procedure and be handled by competent personnel because an incorrect sequence can interrupt critical loads.
Step 10: Identify Generator Connections
A generator SLD section may show:
Generator → Generator Breaker → ATS / Synchronising Panel → Emergency MDB
The drawing can help you understand:
- Generator rating
- Number of generators
- Which board they supply
- Whether generators operate independently or in parallel
- Which loads are emergency loads
- Whether load shedding exists
- Where ATS equipment is installed
This becomes especially useful during generator testing.
If a generator test requires an actual building transfer, the SLD tells you which downstream systems may be affected.
You already have a detailed generator PPM guide on MEP Master Guide: Generator Preventive Maintenance Checklist
Step 11: Look for Metering and CT Information
You may see:
- Ammeter
- Voltmeter
- Energy meter
- Multifunction meter
- CT — Current Transformer
A CT ratio might be written:
800/5 A
This means the CT converts the high primary current to a smaller secondary current suitable for metering or protection.
If an energy meter displays an obviously incorrect value after modification work, checking the CT ratio programmed into the meter is one of the items worth investigating.
Metering information is particularly important in:
- Tenant billing
- Energy management
- Building load analysis
- Transformer loading assessment
Step 12: Understand Connected Load and Design Load
An SLD may contain load information beside each feeder.
For example:
Connected Load: 120 kW
Maximum Demand: 82 kW
These are not the same.
Connected load represents the combined installed load.
Maximum or design demand accounts for how the installation is expected to operate.
A DB with 120 kW of connected equipment does not necessarily consume 120 kW continuously.
Before approving a new load, check:
- Existing connected load
- Actual maximum demand
- Diversity/demand assumptions
- Breaker capacity
- Cable capacity
- Upstream capacity
- Required spare margin
For the calculation process, use:Electrical Load Calculation Explained
You can also use the site’s: Free Electrical Load Calculator
Practical Example: Reviewing a New Tenant Load
Suppose a tenant requests:
65 kW additional HVAC load
The contractor identifies a spare 125 A breaker in an SMDB and says the connection is acceptable.
Do not approve based only on the spare breaker.
Open the SLD and trace:
Transformer → LVMSB → MDB → SMDB → Proposed Breaker
Then verify:
1. SMDB incomer
If the SMDB has a 250 A incomer and is already operating near its capacity, a spare 125 A outgoing way does not mean 125 A of spare capacity exists.
2. Upstream MDB
Check what other SMDBs and major loads share the MDB.
3. Cable
Confirm the proposed feeder cable is appropriate for the design current and installation conditions.
4. Load schedule
Compare the SLD with the latest load schedule.
5. Actual demand
Where suitable, review actual measured maximum demand or trend data rather than relying only on old design figures.
This is a very common difference between physical spare space and electrical spare capacity.
A spare breaker way is not automatically spare electrical capacity.
How to Read an SLD During a Power Failure
During a power failure, avoid jumping directly to the affected DB.
First establish the affected area.
For example:
One office lost power
Likely investigation area may be a local DB or final circuit.
One floor lost power
Investigate the relevant DB, SMDB and their upstream supply.
Several tenants lost power together
Look for a common upstream panel or feeder.
Half the building lost power
Investigate main distribution section, transformer, bus section or source arrangement.
The SLD helps you identify what all affected loads have in common.
That shared upstream point often gives you the fastest direction for investigation.
Never use an SLD as permission to perform live switching or electrical work. Isolation, testing and switching must be performed by authorised competent personnel under the applicable site electrical-safety procedure.
SLD vs Load Schedule
These two documents are connected but serve different purposes.
| Document | Main Purpose | Typical Information |
| SLD | Shows distribution relationship | Sources, breakers, cables, panels |
| Load Schedule | Shows load allocation | kW, current, demand, breaker size |
When reviewing a modification, use both.
The SLD tells you where the load connects.
The load schedule tells you how much load is assigned.
If the two documents do not agree, do not simply choose the document that supports the contractor’s proposal.
Find out which reflects the actual installation.
SLD vs Panel Schedule
A panel schedule normally gives detailed information for individual circuits inside a DB or panel.
For example:
- Circuit number
- Description
- Breaker rating
- Phase
- Connected load
The SLD normally remains at a higher distribution level.
Think of it this way:
SLD = electrical road map
Panel schedule = detailed street listing inside one area
Both are needed for proper FM documentation.
Common SLD Abbreviations
| Abbreviation | Meaning | Typical Use |
| LVMSB | Low Voltage Main Switchboard | Main LV distribution |
| MDB | Main Distribution Board | Major distribution |
| SMDB | Sub-Main Distribution Board | Intermediate distribution |
| DB | Distribution Board | Final distribution |
| ACB | Air Circuit Breaker | Main/high-current protection |
| MCCB | Moulded Case Circuit Breaker | Feeder protection |
| MCB | Miniature Circuit Breaker | Final circuit protection |
| ATS | Automatic Transfer Switch | Normal/emergency transfer |
| UPS | Uninterruptible Power Supply | Critical no-break supply |
| CT | Current Transformer | Metering/protection |
Project terminology can vary, so always check the drawing legend.
Common Mistakes Engineers Make When Reading an SLD
Mistake 1: Looking only at the breaker rating
A 125 A breaker does not prove that 125 A spare capacity exists.
Check actual loading and upstream capacity.
Mistake 2: Ignoring the upstream system
Every DB ultimately depends on another board.
Always trace the feeder back toward the source.
Mistake 3: Assuming the drawing is current
Buildings change.
Tenants modify spaces.
Panels are added.
Cables are rerouted.
Breakers are replaced.
If the SLD has not been updated, the drawing can become misleading. Maintaining accurate SLDs is specifically highlighted by electrical-system specialists because outdated drawings make safe operation, engineering studies and future modifications harder.
Mistake 4: Confusing breaker frame rating and protection setting
Adjustable MCCBs and ACBs can have settings that differ from the breaker frame rating.
Seeing “800 A” on the breaker body does not necessarily tell you its actual protection setting.
Mistake 5: Ignoring cable quantity
2 × 4C × 240 mm²
is not the same as:
1 × 4C × 240 mm²
Parallel cable arrangements matter.
Mistake 6: Treating the SLD as an isolation procedure
An SLD helps you understand the network.
It does not replace:
- Approved switching procedures
- Permit to Work
- Lockout/Tagout requirements
- Voltage verification
- Competent electrical personnel
What FM Teams Should Check During Building Handover
When taking over a new building, request the latest as-built SLD, not only the tender or IFC drawing.
Check that:
- Transformer numbers match site labels
- LVMSB labels match drawings
- MDB and SMDB names are correct
- Breaker ratings match installation
- Cable sizes match approved records
- Generator connections are included
- UPS systems are shown
- ATS connections are shown
- Capacitor banks are included
- Major mechanical equipment is identified
- Final revisions are incorporated
- Drawing revision status is clear
Then keep the controlled copy accessible to the FM team.
Keep the SLD Updated After Every Modification
This is one of the most overlooked FM responsibilities.
Imagine the original SLD shows:
MDB → 160 A MCCB → Tenant A
Two years later the tenant expands.
The contractor upgrades the breaker, changes the cable and installs another panel.
The physical installation changes — but nobody updates the SLD.
Three years later, another engineer uses the old drawing for shutdown planning.
Now the drawing is no longer documentation. It is misinformation.
Whenever an approved permanent electrical modification changes:
- Breaker
- Cable
- Panel
- Transformer
- Generator
- ATS
- UPS
- Major connected load
- Distribution route
the as-built electrical documentation should be revised through the project’s document-control process.
A good FM team treats the latest SLD as a controlled operational document.
Frequently Asked Questions
1. What is a single line diagram in electrical systems?
A single line diagram is a simplified representation of an electrical power distribution system. It shows how sources, transformers, switchboards, breakers, cables and major loads connect to one another without drawing every conductor individually. For FM and MEP engineers, it acts as the main road map of the building’s electrical network.
2. Why is it called a single line diagram when the system is three-phase?
A three-phase system physically uses multiple conductors, but showing every phase separately would make large electrical drawings unnecessarily complicated. The SLD represents the main three-phase power path using one line and adds ratings and equipment information beside it. Detailed conductor arrangements can be shown in other drawings where required.
3. What should I check first when reading an SLD?
Start with the source of supply — utility, transformer or generator — and follow the power path downstream. Identify the LV main switchboard, MDB, SMDB and final load you are interested in. Then read the breaker ratings, cable information and connected equipment along that path.
4. Can I determine spare electrical capacity from an SLD?
An SLD helps you understand the network and equipment ratings, but it is normally not enough by itself to confirm available spare capacity. You should also review the load schedule, actual maximum demand, breaker settings, cable capacities and upstream loading. A physically spare breaker does not automatically mean the electrical system has equivalent spare capacity.
5. What is the difference between an SLD and an as-built drawing?
An SLD describes the electrical distribution arrangement, while “as-built” describes the revision status of a drawing. An as-built SLD should reflect the installation as it was actually constructed and subsequently revised through controlled changes. For FM operation, the latest verified as-built version is far more useful than an old tender or design-stage SLD.
Conclusion
You do not need to design an entire electrical system to use a single line diagram effectively. Start at the source, follow the power downstream, understand the panel hierarchy, read breaker and cable information carefully, and always compare the drawing with the latest load data and actual site installation.
Once you become comfortable reading SLDs, troubleshooting, shutdown planning and reviewing contractor load requests become much easier.
If you are still building your electrical-system fundamentals, continue with the LV Panel, MDB, SMDB and DB Explained guide and use the Electrical Load Calculator when reviewing proposed building loads.
