In brief
- A low cable bid usually means the sizing calculation, the derating factors or the testing were left out, not that the contractor found cheaper cable.
- Ask for the cable schedule, the current calculation and the proposed installation method before comparing totals.
- Undersized or poorly installed conductors run hot, and the heat shows first at terminations and in the cable trays.
Contents
A low cable sizing bid in Garowe usually means the estimate left something out, not that the contractor found cheaper cable. Copper and insulation cost what they cost. When a total comes in well below the others, the gap normally sits in the current calculation, the derating factors, the earth and bonding, or the testing and records. A cable sizing underbidding risk is a risk to the installation, not a saving.
What does a low cable sizing bid in Garowe usually leave out?
The omissions are predictable. Ask for the cable schedule and the current calculation before you compare prices, and you will usually find one or more of these missing.
- Conductor size based on design current, protective device rating and voltage drop over the actual route length.
- Derating for grouping, for cables in conduit or trunking, and for the ambient temperature of the space the cable runs through.
- Cable tray, conduit, glands, cleats and the labour to pull and terminate.
- Earthing conductor, main bonding and the continuity testing that proves it.
- Insulation resistance and continuity test records handed over at completion.
- Allowance for cable routing around structure, and for the extra length that comes with it.
Any one of these can move a total. Ask the estimator to show the line. If the line is not there, the price is not there. Confirm the ambient design temperature and the cable installation method against the project specification or the design engineer, because those two inputs drive the derating and the final size.
Why do undersized conductors run hot, and where does it show?
A cable carrying more current than its installation allows will run hotter than its insulation is rated for. Where cables may run through roof spaces and along external walls, the ambient heat adds to the conductor heat, so the margin matters more.
The damage shows at the weakest points first. Look at terminations, gland plates, joints and where cables bunch in a tray or conduit. Heat there dries and cracks insulation, loosens screwed connections and raises resistance, which raises heat again. On a new build the early signs are discoloured insulation, a warm distribution board after a few hours of load, and protective devices that trip without an obvious fault. Ask for the test records: insulation resistance, earth continuity and, where the specification calls for it, a thermographic check of the main board under load. Confirm which tests the specification requires and who signs them off.
How do you check the sizing and the omissions before award?
Check three things in writing, not at the meeting. First, the cable schedule: every circuit with its design current, protective device, cable type, size, route length and voltage drop. Second, the method statement: how the cable is fixed, how it is derated, and what the installation temperature is assumed to be. Third, the test and handover list, naming the tests and the records the client receives.
Then walk a recent site the bidder completed. Ask to see a distribution board, a tray run and the test certificates for that job. If the bidder cannot produce records for past work, the same gap will appear on yours.
The contract should say what happens when the schedule and the site do not match. Check what your contract allows before you sign, and keep the sizing calculation as a reviewed document rather than a note in a file. If the totals differ by a wide margin, ask each bidder to price the same schedule and the same test list, so the comparison is like for like. For local requirements on inspection and sign-off, confirm what the relevant local authority expects to see on the electrical installation.
What to do next
Write a one-page cable schedule with circuit, load, device rating, cable size, route length, installation method and the tests required. Send it to every bidder with the instruction to price that schedule and list any exclusion in writing. Ask each bidder for test records from one completed job of similar size, then check one board and one tray run on site.
Frequently asked questions
Can I compare cable bids on the total price alone?
No. Ask each bidder to price the same cable schedule, method statement and test list, then compare the lines. A total without those documents cannot be compared with another total.
What records should the client receive for a cable installation?
Ask the design engineer and the relevant local authority which tests the project requires. A common set covers insulation resistance, earth continuity and the as-built cable schedule, signed by the person who carried out the test.
Local reporting: this article is written for building work in Garowe and Puntland. Ground conditions, prices and rules differ between places; confirm the details for your own site.