Should you size pumps and cables in a spreadsheet or in an engineering tool?

The short answer

Size in a spreadsheet when the calculation is one-off, unusual, offline or reviewed only by you. Size in an engineering tool when the calculation is recurring and someone else has to trust it. The deciding factor is not accuracy — a careful spreadsheet can be exact — but whether the method is visible, fixed and reviewable, and whether an error would announce itself or hide behind a plausible number.

Side-by-side comparison

A concrete example: the fixed friction factor

A common spreadsheet shortcut is a fixed Darcy friction factor, often f = 0.02, typed into one cell and reused for every line. The real friction factor depends on the Reynolds number and the relative roughness and comes from Colebrook-White:

1/sqrt(f) = -2·log10( (ε/D)/3.7 + 2.51/(Re·sqrt(f)) ), with head loss h_f = f·(L/D)·(v²/2g).

Inputs (same for every case): water at about 20 °C, kinematic viscosity ν = 1.004 × 10⁻⁶ m²/s; length L = 120 m; g = 9.80665 m/s²; new seamless carbon steel ε = 0.045 mm (aged steel: ε = 1.0 mm); internal diameters from ASME B36.10M Schedule 40 (2” = 52.50 mm, 4” = 102.26 mm, 8” = 202.71 mm). The Colebrook-White friction factor was computed with the same LOGOS friction module used by the calculators.

Three things stand out. First, the error changes sign: the same constant overestimates the loss in large lines and underestimates it in small ones, so it does not even fail in a consistent direction. Second, the worst case is the aged line, where the spreadsheet reports 3.42 m of friction loss and the pipe actually consumes 6.50 m — about 3.1 m of head missing from the pump specification on only 120 m of pipe. Third, nothing in the spreadsheet looks wrong: 3.42 m is a perfectly plausible number.

In LOGOS the friction factor is never a typed constant: every calculator solves Colebrook-White for the actual Reynolds number and the roughness of the selected material, and the results show the friction factor used in each segment. See the pump sizing method and the pipe roughness table.

Where the spreadsheet is the right choice

• One-off or highly specific calculations — an unusual correlation, a vendor-specific curve, a geometry no tool covers.
• Non-standard topologies — LOGOS solves defined industrial arrangements (single lines, pumps in series or parallel, multi-branch networks with one pump, closed supply/return loops), not arbitrary networks.
• Offline work — on a site without internet access, a spreadsheet works and LOGOS does not.
• No account or license allowed — some companies restrict cloud tools; a spreadsheet already on the computer has no entry barrier. (The LOGOS quick hydraulic calculations and some instrumentation tools run without an account, but saving projects and the .docx report do not.)
• Exploration and teaching — building the equations yourself is a good way to learn them.

Where LOGOS is the right choice

• Recurring sizing — pumps, gravity lines, control and relief valves, orifice plates, water hammer, cable lists, short circuit, grounding grids, panels.
• Work that someone else will review — the reviewer reads named methods and a report generated from the same server code, instead of auditing cells.
• Teams — everyone uses the same method and the same version, instead of personal copies of a spreadsheet.
• Documentation — the .docx calculation report comes out of the calculation, not out of copy and paste.

Limits of this comparison

The numbers above isolate one shortcut — a fixed friction factor — on straight pipe only; minor losses, static head and the pump curve were deliberately left out so the error can be read directly. A well-built spreadsheet that solves Colebrook-White gives the same friction factors as LOGOS. Neither a spreadsheet nor LOGOS replaces the professional responsibility of the engineer who signs the design.

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