Weekly digest #124: price changes in copper

Copper spot moved again this week, and it is hitting job costs faster than most shops can reprice bids. If you pulled THHN pricing from your supply house thirty days ago and are bidding off that sheet, you are losing margin on every foot. Here is what changed, what to do about it, and how the code still governs your conductor choices when the spreadsheet pressure starts pushing you toward substitutions.

What the market did this week

COMEX copper closed the week above the $4.60/lb range after another round of supply tightening out of Chile and a draw on LME warehouse stocks. Distributors are passing through increases on building wire within days now, not weeks. Southwire and Encore both issued letters mid-week. If your PO is older than ten days, call and verify before the truck rolls.

Aluminum building wire has not moved as sharply. The spread between copper and aluminum conductors for feeders and service entrance has widened again, which is pulling more estimators toward XHHW-2 aluminum for anything 1/0 and up. That is a legitimate substitution when the engineering supports it, but do not let the price sheet drive the conductor selection without checking the terminations.

Where the code pushes back on substitutions

Before you swap copper for aluminum on a repricing panic, confirm the terminations are rated for the conductor you want to land. NEC 110.14(A) and (B) are explicit about this, and failed terminations are still one of the top callback causes on service upgrades.

• Check the lug or breaker marking for CU, AL, or CU/AL (also marked AL9CU on newer gear).
• NEC 110.14(C) governs the temperature rating you size the conductor from, usually 75C for terminations on equipment rated 100A or less unless marked otherwise.
• For aluminum, use a listed antioxidant compound where required by the lug manufacturer's instructions, and torque to the value on the label, not from memory.
• Do not mix dissimilar conductors in the same termination unless the connector is listed for it.

If the panel is older and the breakers are CU only, the aluminum savings evaporate the minute you start changing out breakers or the whole panel. Price the real job, not the conductor-only delta.

Ampacity and the 310.16 table, again

When copper prices squeeze, the temptation is to size down to the smallest conductor the ampacity table allows. That works until you remember the adjustment and correction factors. NEC 310.15(B) and (C) are where estimators quietly lose money, because they sized off the base table without applying ambient temperature or conduit fill corrections.

Rooftop runs in conduit are the classic trap. NEC 310.15(B)(1) and Table 310.15(B)(2)(a) give you the adder for solar heating above rooftops, which can push an effective ambient well past 40C in summer. If you sized #10 THHN copper tight to a 30A circuit without that adder, you are installing an undersized conductor the minute the sun hits the EMT.

Field tip: when in doubt on a rooftop run, size up one AWG and skip the argument with the inspector. The copper premium on 50 feet of #8 versus #10 is a fraction of a callback.

Service and feeder resizing on change orders

If a GC is asking you to re-bid a service or feeder because the copper number blew up, walk through the load calculation before you just substitute materials. NEC 220 governs the calculated load, and there is often room to tighten the calculation before you touch the conductor.

1. Re-run the dwelling or commercial load calc with the actual connected loads, not the rule-of-thumb number from the bid sheet.
2. Apply demand factors per NEC 220.42, 220.52, 220.54, and 220.55 where they legitimately apply.
3. Check whether the existing service can be re-used if you are on a renovation. NEC 230.79 still sets the minimum rating, but you may not need the size the architect specified.
4. Verify neutral sizing per NEC 220.61. The neutral can often be reduced on a three-phase feeder serving mostly linear loads, which is real copper savings.

For parallel feeders, NEC 310.10(G) requires conductors in each phase to be the same length, material, size, insulation type, and terminated the same way. Do not try to save copper by running one parallel set in aluminum and one in copper. It is not code legal and it will not share current correctly.

Grounding and bonding do not get cheaper

The equipment grounding conductor and the grounding electrode conductor are sized off NEC 250.122 and 250.66 respectively, and they do not scale down just because the ungrounded conductors got expensive. If you upsize the phase conductors for voltage drop, NEC 250.122(B) requires the EGC to be proportionally upsized as well. Missing that is a common plan-review comeback.

Field tip: on long runs where you are already upsizing for voltage drop, calculate the EGC increase up front and add it to the take-off. It is a line item most estimators forget until the material list comes back short.

For the GEC on a service, NEC 250.66 caps the required size based on the ungrounded service conductor, with specific limits for connections to rod, pipe, plate, concrete-encased, and ground ring electrodes. Read the section, not the summary. The caps save real money on larger services.

What to do Monday morning

Pull every open bid older than two weeks and flag the ones heavy on copper feeders, service entrance, or long branch circuit runs. Call your primary supplier and get written confirmation on pricing for anything you have not released. If a job is going sideways on material cost, get the change order in writing before you pull wire, not after.

• Lock pricing in writing for any job releasing in the next 30 days.
• Re-run load calcs on service upgrades before substituting conductor material.
• Verify terminations match the conductor you are actually installing, per NEC 110.14.
• Do not shortcut grounding and bonding sizing to chase copper savings.

Copper will keep moving. The code will not, at least not until the next cycle adoption in your jurisdiction. Build your bids around what the NEC requires and let the material pricing be the variable, not the conductor selection.