原有電錶容量不足，即使加裝 7kW 亦會跳掣。我們代辦三相電升級申請，獲批後安裝 22kW，充電時間大幅縮短。The old meter couldn't even take 7kW without tripping. We filed the three-phase upgrade; once approved, 22kW cut the charge time sharply.
A detached house in Yuen Long. A couple with one EV each, sharing a single bay (parked nose to tail). The existing supply was single-phase 60A, serving two air conditioners, an electric water heater, an oven, and a washer-dryer.
The owner originally just wanted one 7kW charger. After the load assessment, we suggested pausing.
The hard part
The assessment said even 7kW wouldn't fit.
The maths:
Meter capacity: 60A
Measured household peak demand: about 38A (summer evening — two aircons, water heater, cooking)
Headroom: about 22A
A 7kW charger needs: 32A
So charging on a summer evening with the aircon on would trip, guaranteed. And with two cars, real demand was higher still.
Installing anyway would have meant the customer tripping breakers all through their first summer and then blaming the installer. We'd rather say it up front.
Three options, laid side by side
Option A: fit 7kW but limit it to 16A (3.5kW). Cheapest, same-day. But with the cars taking turns, each night adds only about 35 kWh and they'd still have to watch their usage in summer. The owner felt it was too much of a compromise.
Option B: enlarge the single-phase meter to 100A. Mid-price. Supports one 7kW charger, but still can't run both cars together, and leaves no headroom to upgrade later.
Option C: go three-phase. Most expensive, longest lead time. But it unlocks 22kW, charges both cars quickly in turn, and solves the whole house's capacity in one move.
The owner chose C, having worked out that two cars over the long run justified the difference.
What we did
Filed the three-phase application. Load calculations, single-line diagram and application documents to the utility. This is the slowest part of the whole job — it waits on the utility's scheduling and approval.
Replaced the meter box and main supply. Three-phase needs a new supply cable, meter box and main switch. Coordinated the outage window with the utility and completed it in a day.
Reorganised the distribution board. Added a dedicated three-phase circuit for the charger with a Type A 30mA RCD, and redistributed the existing circuits so the water heater and aircons sit on different phases and the load balances across all three.
Installed the 22kW unit. Three-phase Wall Case 3, with 6mA DC leakage detection built in.
The outcome
About ten weeks from application to completion, most of it waiting on utility approval — actual site work was two days.
Afterwards: at 22kW a 60kWh car goes from 20% to 80% in roughly 1.6 hours. Both cars charge in sequence overnight with room to spare, and running the aircon flat out while charging no longer trips anything.
Who this case is useful for
Three-phase is worth considering if any of these apply:
Two EVs on one bay
A 60A or smaller meter with a decent number of appliances in the house
You already need electrical work done for other reasons
But go in expecting this: with a three-phase upgrade, the time cost is harder to predict than the money cost, because it waits on the utility. If you want it, start the application early.
Want to know whether your supply is enough? WhatsApp us a photo of the meter and a list of your appliances, and we'll work it out.
你的物業與這個案例相似？Does your place look like this one?
傳送車位及電錶相片，我們會告知最接近哪一種情況、需時多久及費用多少。Send photos of your bay and meter. We'll tell you which case yours resembles, how long, and how much.
