How to document an existing electrical facility that has no drawings

A practical walk-through workflow for as-builts when no one-line exists. Written by a licensed PE.

Every electrical engineer who does existing-facility work has had this conversation. You show up for an arc flash study, an upgrade scope, or a safety audit. You ask the facility manager for the one-line. They look at you, apologetic, and say one of three things: we lost it, we never had it, or the last one was from 1987 and none of it is current.

In my experience doing arc flash and existing-conditions work across a mix of commercial, industrial, and mixed-use facilities, somewhere north of 80 percent of buildings more than 15 years old do not have drawings that accurately reflect their current electrical system. If they ever did, the drawings are missing, they are three renovations out of date, or they are rolled up in a tube somewhere no one has seen since the GC finished punch list.

So documenting the existing system falls to you, in the field, on one site visit, with a phone and a notebook. This post walks through the workflow I use to come back from a single facility walk with enough data to build a complete one-line and an equipment list that is ready for an arc flash study, a panel schedule, an upgrade design, or anything else the client asks for next.

Why getting this right on one visit matters

A second site visit for missed data is the most expensive hour in this business. You pay for it twice: once in the hourly fee you probably cannot bill, and once in the two to three weeks of delay while you wait for client schedules, plant shutdowns, or an escort to get you back in the room. Everything downstream of a missed field measurement sits idle. Arc flash calculations cannot run without the utility available fault current. One-lines cannot close without the feeder size between MSB and Panel LP-3. Equipment reports cannot deliver without an interrupting rating on every breaker.

The goal of the walk is not to fill in every optional field. The goal is to never have to come back.

Pre-visit prep (15 minutes, at your desk)

Before you drive anywhere, do these five things. They cost nothing and collectively save hours of on-site confusion.

• Pull the utility source data. Call or email the utility and request the available fault current and X/R ratio at the service point. Utilities have standard forms for this. Start the request early because some regional utilities take 5-10 business days to respond.
• Get the site address, gate codes, and contact. Many industrial facilities have multiple gates, electrical rooms on the far side of the complex, and no cell service inside the MEP closets. Know who to call before you are standing outside at 7 AM.
• Find the latest panel schedules the client has, even if they are wrong. An outdated schedule is still a starting skeleton. Take a photo of each one.
• Bring the right PPE. For arc flash work, dress for the worst rating you might encounter. If you do not know, dress for Category 2 and carry Category 4 in the truck.
• Charge your phone and a backup battery. Obvious. People forget. Nameplate photos are the backbone of the deliverable.

The walk order: top down, always

The single most useful habit I have developed is starting at the utility service and walking downstream. It matches how the one-line reads and it keeps your brain aligned with the physical topology while you are moving through the building.

1. Utility service (pad transformer or service head). Capture the utility nameplate, primary and secondary voltages, kVA, and percent impedance if available.
2. Main switchgear or main distribution panel. Main breaker or main lug, bus rating, voltage, incoming feeder size and length, and every branch breaker with its trip rating, frame size, interrupting rating, and what it feeds.
3. Sub-distribution panels and MCCs. Same capture as MSB, plus motor nameplate data for any MCC bucket.
4. Branch panels. Mains, branch breakers, connected loads, and any notable loads (HVAC, life safety, elevators) that will need to be called out in the deliverable.
5. Standalone equipment. Generators, transfer switches, UPSes, rooftop units, standalone disconnects for specific loads.

If you walk in random order or branch-out order you will miss connections between pieces of equipment, and you will catch it three days later when you are trying to build the one-line and there is a panel hanging with no source.

What to capture, by equipment type

Nameplate photos are the minimum. The field is full of practices that stop at "take a picture," but a photo alone will not let you recall, three weeks later, which disconnect fed which pump, or whether the breaker at position 14 was a 50 amp or a 150. Capture the photo plus structured data per equipment type.

Utility and transformers

• Nameplate photo
• Primary voltage, secondary voltage, kVA rating, percent impedance (if shown)
• Winding configuration (delta-wye is most common, but confirm)
• Location and accessibility for maintenance

Switchgear and main distribution

• Nameplate photo plus a clean photo of the breaker interior or panel schedule if accessible
• Bus rating, voltage, number of phases
• Main breaker or lug, trip rating, interrupting rating, and frame size
• Every branch position, what it feeds, trip rating, and interrupting rating
• Feeder conductor size and approximate length to each downstream bus

Panels and panelboards

• Nameplate photo plus clean panel-door photo with the schedule visible if present
• Bus rating, main breaker or MLO, voltage
• Every branch breaker position, trip rating, number of poles, and connected load description
• Any spare positions for future work

Motors and motor control

• Motor nameplate photo
• HP, FLA, voltage, service factor, locked rotor code
• MCC bucket or local starter, including overload setting
• Connected driven equipment (pump, fan, compressor, etc.) and HVAC or process tag if present

Generators, transfer switches, UPS

• Generator nameplate: kW, kVA, voltage, fuel type
• Transfer switch: rating, number of poles, type (contactor, breaker, closed-transition), connected emergency and normal sources
• UPS: rating, battery runtime, downstream loads
• Which loads are on emergency power versus normal power, called out explicitly

Common mistakes that force a second site visit

After years of doing this, the repeat offenders are always the same five things:

1. No interrupting rating captured on branch breakers. Trip rating is easy to remember. AIC is on the nameplate but people forget to zoom in. Without it, you cannot do any arc flash downstream of that breaker.
2. Feeder lengths not measured. A guess in the field becomes a meaningful error in the arc flash calculation. Pace it off or use a laser measure. Write it down.
3. Missing percent impedance on transformers. If it is not on the nameplate photo, go back and get it. Without %Z you cannot compute available fault current at the secondary bus.
4. Unlabeled spare breakers. If a breaker in the panel is flipped and has no load, note it as spare. Missing this creates phantom loads on the one-line later.
5. Emergency source connections not documented. Transfer switches that are fed by both normal and emergency sources need both connections mapped. Half the time people only photograph the normal side.

What a complete field package looks like

Your deliverable back to the office should include, at minimum:

• A nameplate photo for every piece of equipment, named with the equipment identifier
• A structured equipment list with every field from the type sections above
• Feeder runs between every pair of connected equipment, with conductor size and length
• A rough connectivity sketch, even hand-drawn on an iPad, that you can turn into the one-line without guessing
• Notes on anything that did not match expectations (phantom loads, breakers with no visible downstream, spare positions, nameplates that did not match the schedule)

If you have all of that, the one-line builds itself at your desk without another visit.

How I run this workflow now

I spent years running exactly the workflow above with a clipboard, a phone, and a three-column spreadsheet I maintained by hand. It worked, but the data reassembly at the desk was always the slowest part. Mapping 300 photos and a stack of penciled notes into a clean one-line and equipment list took me one to two work weeks per facility.

I built AmpSketch because I wanted the structured capture to happen on the walk itself, not as a reconstruction job afterward. You open it on your phone at the service entrance, snap the utility nameplate, the AI reads the values, you verify, and move to the next piece of equipment. The one-line builds on your screen as you walk, connected upstream to the right breaker. When you leave the site, the one-line is done. Not started. Done.

A CSV equipment list export (for hand-off to arc flash software) is currently on the roadmap, and the overall arc flash workflow is the next area of focus. For now, the field capture, one-line build, and on-site NEC flagging all work in the browser with no signup.