The one-line diagram drives some of the largest numbers in an electrical bid: service, switchgear, panels, transformers and feeders. It is also where a missed note or a stale gear quote costs the most. Here is how to read it for estimating, what to check against your vendor quotes, and the items that most often get missed.
The one-line is the backbone of the distribution price: gear, feeders and transformers are some of the largest lines in an electrical estimate. Reading it in order, from the utility to the last panel, keeps you from skipping a branch.
Find the service point first: utility transformer, metering, service entrance equipment. Note what the drawing says is by the utility and what is by the contractor.
Trace every path from the main switchboard down. Mark each piece of equipment off on the one-line as you add it to your takeoff, so nothing is left unmarked at the end.
Read the general notes and the legend on the one-line sheet. Notes often add requirements, such as surge protection, metering or spare breakers, that aren't drawn.
Check the one-line against the riser diagram and the floor plans. Equipment shown on a plan but missing from the one-line, or the reverse, is an RFI.
2 · Gear: what to check against the quote
Switchgear and panelboard quotes are priced from the one-line and the panel schedules. If the vendor read a different revision, or left out items in the notes, the gap is yours.
Ratings: compare ampacity, voltage, short-circuit rating and main device type on the one-line with the gear quote, line by line.
Breaker counts and spaces: check feeder breakers, spares and spaces against the drawing. Spares and 'provisions for future' are easy to drop.
Accessories: metering, surge protection, ground fault protection, key interlocks, shunt trips and special enclosures are often in notes or specs, not on the symbol.
Ask the vendor which drawing revision and which spec sections their quote is based on, and get it in writing. Then check it again after every addendum.
Note who carries studies and labelling. Short-circuit, coordination and arc-flash work is sometimes in the gear quote and sometimes not.
3 · Feeders: sizes on the one-line, lengths on the plans
The one-line usually tells you feeder sizes but not lengths. Lengths come from the plans, and the difference between a straight-line estimate and the real route is money.
Use the feeder schedule if there is one, and check that its tags match the one-line. Mismatched tags between the two are common after revisions.
Measure feeder lengths along the likely route, including vertical rises, offsets and the run into the gear. Add an allowance for terminations and slack.
Watch for parallel sets, separate neutrals and full-size grounds. Each set multiplies wire, conduit and termination labor.
Check whether voltage drop is likely to push a long feeder to a larger size. If the drawings are silent on a very long run, ask.
Note the conduit type each feeder needs where it runs: underground, in slab, exposed or in hazardous areas can each change material and labor.
4 · Transformers, generators and what hangs off them
Each transformer or generator adds equipment, pads, grounding and connections that a quick read of the one-line can skip.
Dry-type transformers: count them, check kVA and voltage against the quote, and add primary and secondary feeders, grounding and mounting or pads.
Generator and transfer switches: check who furnishes the generator, fuel and exhaust, and who carries the transfer switches, controls wiring, load bank testing and startup.
Emergency and standby distribution: separate panels, separate feeders and separate raceway requirements in the specs all add cost. Count them as their own system.
Equipment shown 'by others' on the one-line still needs feeders, terminations and a connection. Check whether those are yours.
5 · The misses that show up after award
These are the items estimators most often find on the one-line only after the job is won.
Future or spare breakers and spaces the drawings require, missing from the gear quote.
A feeder shown on the one-line but not on any plan, so it never got a length.
A late addendum that reissued the one-line, and a gear quote still based on the original.
Notes requiring metering, monitoring or surge protection at multiple points, priced only at the main.
Utility scope assumed to be the utility's, when the drawings or specs put it on the contractor.
Make the one-line your checklist
Print the one-line, or keep a marked-up copy, and highlight every piece of equipment and every feeder as it goes into your takeoff. When you are done, anything not highlighted is either a miss or a question. Do the same with the gear quote: highlight each item the vendor carried, and the unhighlighted items are what you still need to price, RFI or clarify. When an addendum reissues the one-line, start a fresh copy and re-check the quote using the addendum review checklist.
Questions estimators ask
What is a one-line diagram used for in electrical estimating?
A one-line (single-line) diagram shows the power distribution system from the service to each panel: equipment, ratings, feeders and transformers. Estimators use it to build the distribution portion of the bid and to check the switchgear and panelboard quotes.
Does the one-line show feeder lengths?
Usually not. It shows sizes and connections. Lengths come from routing the feeders on the floor plans and site plans, including vertical rises and the run into each piece of equipment.
What should I do if the one-line and the panel schedules disagree?
Note the conflict, price the more conservative option or send an RFI before bid day, and make sure your gear vendor knows which version they are quoting. If there is no answer by bid close, clarify your assumption in writing.
How can I check the one-line against the rest of the package faster?
Work from the service downstream and mark off every item as it goes into your takeoff. Bidprobe reads the whole bid package and flags scope gaps, spec conflicts, missed addendum items and exclusions; most reviews come back in 5 to 15 minutes.
Check the whole package, not just the one-line.
Bidprobe reads the whole bid package and flags scope gaps, spec conflicts, missed addendum items and exclusions. Most reviews come back in 5 to 15 minutes, and your first bid check is free with no card.