Three phase work, sized from your actual load rather than a guess.
Supplies, distribution, machinery connection and motor starting for Scottish businesses, from Aberdeen and Peterhead. We measure what your equipment really draws, design to it, and certify the result. Free survey, fixed price in writing.
Steven Watt holds City & Guilds 2391-52 inspection and testing, 2382-22 BS 7671, 2365-03 installations and 2921-34 EV charging. Registered in Scotland SC821028.
Three phase work covers the supply, the distribution and the equipment connection that single phase cannot carry. Machinery, large motors, workshop plant, commercial refrigeration, rapid EV charging and anything where the load has outgrown what one phase can deliver. Faithful Spark Electricians is a NICEIC Approved Contractor, enrolment 620239, TrustMark 4205123, registered in Scotland as SC821028 and insured to £2M.
This page is written for Scotland, where the supply side runs through SSEN, and we work from Aberdeen and Peterhead across the north east.
Most three phase pages tell you it is more efficient and to get in touch. This one shows you the capacity arithmetic, how a load assessment is actually done, what starting a motor does to your supply, and what phase imbalance costs you. Then we survey your premises free and price it in writing. Already know you need a board and a supply upgrade? That is covered on our three phase consumer unit page.
That ratio is the whole argument. Three phase is not a bigger fuse, it is a different shape of supply, and it is why a unit that keeps tripping at 100A single phase can run comfortably on the same 100A rating once it is three phase.
Do you actually need three phase?
Sometimes the honest answer is no, and finding that out costs you a survey rather than a supply upgrade. These are the cases we see.
Yes, almost certainly
- Equipment that is only made in three phase. Most machine tools, commercial ovens and bakery plant, larger compressors, lifts and industrial refrigeration.
- Motors above roughly 3kW. Single phase motors that size are inefficient, hard to start and hard on the supply.
- Maximum demand approaching 23 kVA. Once you are near the single phase ceiling, adding anything means an upgrade.
- Rapid EV charging, or several 22kW chargers on one site.
- A building being subdivided, where separate tenancies need separate distribution.
Probably not, and we will say so
- One new machine on an otherwise light load. A load assessment sometimes shows the existing supply copes with room to spare.
- Nuisance tripping. Frequently a circuit design, a faulty appliance or a discrimination problem rather than a capacity one. Upgrading the supply would not have fixed it.
- "The unit next door has it." Not a reason. What matters is your demand, not theirs.
- A single 7kW EV charger on a typical small commercial supply, which usually fits with load management.
We would rather charge you for a survey and tell you to keep your money than sell an upgrade you did not need.
What a survey settles. We measure the existing maximum demand rather than estimating it, check the incoming supply and main fuse rating, look at what you are adding, and tell you plainly which of those two columns you are in. The survey is free and the answer comes in writing. Call 07304 027013 or book a load survey.
Not sure whether your supply can take the new machine?
That is a measurement, not an opinion. Free load survey across Aberdeen, Peterhead and the north east, with the answer in writing.
How a load assessment is actually done
Adding up every nameplate on site gives you a number no building ever reaches. Maximum demand is what the installation actually draws at its busiest, and the gap between the two is where money gets wasted on supplies nobody needed.
1. Connected load
Every nameplate added together. Useful as a ceiling and almost never the right answer, because nothing in a real building runs everything at once.
2. Diversity
The realistic allowance for what runs simultaneously. A kitchen with six appliances never has all six at full draw; a workshop rarely starts every machine together. Applying diversity correctly is the difference between a sensible supply and an expensive one.
3. Measured maximum demand
Where possible we log the existing installation rather than calculate it, so the design starts from what the building really does. Calculation fills the gap for equipment not yet installed.
4. Headroom for what is coming
A supply sized exactly to today is a supply you upgrade again in three years. We ask what is planned, because adding capacity at design stage is a fraction of the cost of revisiting it.
5. Starting current, separately
Motors draw far more starting than running. A supply sized on running current alone will hold up until the biggest motor starts, then drop the voltage across the building. See the next section.
6. The metering consequence
Above 100 kVA maximum demand a site normally moves to half hourly metering, which changes your tariff and your billing. Worth knowing before you cross it rather than after.
Why this is the section competitors skip. We looked at the pages ranking for three phase work in the UK. Across six of them, none mentions load assessment, maximum demand, diversity or kVA at all. It is the single most important input to the design and the field treats it as invisible. Our survey starts here and it is free. Call 07304 027013.
What happens to your supply when a motor starts
A three phase motor started straight off the line draws several times its running current for a few seconds. On a supply with no headroom that shows up as lights dimming across the building, contactors dropping out, and in bad cases the main fuse going.
| Method | Starting current | Suits | The trade off |
|---|---|---|---|
| Direct on line | 6 to 8× | Small motors, light starting loads, where the supply has plenty of headroom. | Cheapest and simplest. Hardest on the supply and on the driven machine. |
| Star delta | about ⅓ | Larger motors that can start unloaded, such as fans and some pumps. | Reduces inrush to roughly a third of direct on line. There is a transition step, and torque drops during starting. |
| Soft starter | around 42% less | Conveyors, pumps and anything where a mechanical jolt causes wear or spillage. | Smooth ramp, kinder to belts and couplings. More cost and another item to maintain. |
| Variable speed drive | about 1.4× | Anything where the speed should vary with demand, especially fans and pumps. | By far the lowest inrush and often a large running saving too. Highest cost, and it introduces harmonics, covered below. |
What this means in practice. The difference between direct on line and a VSD on the same motor is roughly six times the starting current. That is frequently the difference between needing a supply upgrade and not needing one, which makes it a commercial decision rather than a purely technical one. We work out both routes and tell you what each costs, including the case where a drive is cheaper than the supply upgrade it avoids. Call 07304 027013 or book a survey.
Machinery arriving and a date to hit?
Send the equipment schedule. We size the supply and the distribution against it, and give you a fixed price and a programme before the plant lands.
Phase imbalance, and what it quietly costs you
Three phase only works properly when the load is spread evenly across all three. Add a single phase circuit here, a machine there, and over a few years one phase carries far more than the others. Nothing trips. The bill goes up and the motors run hot.
The number that surprises people: current imbalance typically runs six to ten times the voltage imbalance. A 2% voltage imbalance that looks trivial on a meter can mean 12 to 20% more current in one phase than another, which is heat in a cable that was sized for an even split. Derating figures above follow NEMA MG-1, the reference used across the industry for this.
What causes it
- Single phase loads added over time without anyone tracking which phase they landed on. This is the usual one.
- A shop or office fit out where all the new small power went onto whatever phase was convenient at the board.
- A failed or degraded connection on one phase, which shows as imbalance before it shows as a fault.
- Supply side imbalance from the network, which is not yours to fix but is worth identifying before you spend money chasing it.
What we do about it
Measuring it is quick. We log current on each phase under normal operating conditions, identify which circuits sit where, and produce a rebalancing schedule: which circuits to move to which phase, and what that does to the numbers.
Often the fix is a few hours at the board rather than anything structural, and it pays for itself in reduced heating and longer motor life. Where the imbalance turns out to be on the supply side we say so rather than sell you work that will not change it.
Phase balance checks are part of every three phase job we take on, and we will quote one on its own if that is all you need. Call 07304 027013.
Distribution, discrimination and the things that trip the wrong breaker
A three phase installation is a hierarchy: incomer, main board, sub mains, sub boards, final circuits. How that hierarchy is designed decides whether a fault takes out one machine or the whole building.
Discrimination and selectivity
When a fault happens, only the protective device nearest to it should operate. If a fault on one machine drops the main breaker, discrimination is wrong somewhere between them.
It is a design calculation using the device characteristics, not something you can judge by looking at ratings. Getting it wrong is why some buildings lose everything over one faulty appliance.
Sub mains
The cables feeding sub boards around a building. Sized for load, volt drop and fault level, not just for the breaker in front of them. Long runs in large units are usually a volt drop problem before they are a current one.
Assemblies to BS EN 61439
Commercial switchgear is built and verified to BS EN 61439, which is a different standard from the domestic consumer unit most people picture. It governs how the assembly is constructed, tested and documented.
Harmonics
Variable speed drives, LED drivers, UPS and switch mode power supplies all distort the waveform. The effect shows up as extra heating in cables and transformers, and as neutral current higher than you would expect from the phase currents.
On a building with a lot of drives it is worth measuring rather than assuming.
Power factor
Motors draw more current than the useful power they convert. Poor power factor means paying for capacity you do not use, and on larger supplies it can attract charges from the supplier.
Correction is sometimes worth it and sometimes not. We work the numbers before recommending it.
Labelling and records
Which phase each circuit sits on, what each way feeds, and what the design assumed. Without it the next person to add a load guesses, and the balance you paid to correct drifts straight back.
Working with SSEN on the supply
Where the incoming supply itself has to change, that work belongs to SSEN as the network operator for the north east. We handle the application and the coordination, but the timescale is theirs.
- What the application needs: the site address, a photo of the existing meter and cutout, the proposed maximum demand, and a description of what you are installing. Vague applications get slow answers.
- A survey usually follows within a few weeks, checking the existing service cable, the distance to the nearest three phase main and whether the network needs reinforcing.
- Cost varies enormously. A fuse change where the cable is already adequate can be minor. A new cable, an excavation or network reinforcement runs into thousands. We tell you which one you are looking at as soon as SSEN does.
- Above 100 kVA you normally move to half hourly metering, which changes your tariff.
Because the lead time sits outside our control, we start this conversation at survey rather than after the internal work is designed. A supply application running in parallel with the on site work is the difference between a smooth job and a stalled one. Consumer unit and board specifics are on our three phase consumer unit page.
Three phase work we take on
All of it certified under NICEIC enrolment 620239, all quoted in writing after a free survey.
Load assessment and design
Measured maximum demand, diversity applied properly, and a written recommendation on whether you need three phase at all.
Supply upgrades
SSEN application and coordination, plus all the on site work either side of it.
Boards and distribution
Main boards, sub boards, sub mains and discrimination designed as a hierarchy rather than assembled piece by piece.
Machinery connection
Isolators, local control, starting method selection and commissioning, coordinated with the machinery supplier.
Motor starters and drives
Direct on line, star delta, soft starters and VSDs, specified against what the supply can take and what the load needs.
Phase balancing
Current logging, a rebalancing schedule and the work to carry it out. Quoted on its own if that is all you need.
Testing and certification
Electrical Installation Certificates for new work, Minor Works for alterations, periodic inspection on the cycle. Commercial EICR from £300 + VAT.
Three phase EV charging
OZEV authorised installer, reference EVHS/WCS14254. Workplace chargers from £800 + VAT.
On pricing. Three phase work is not priced from a web page, because the two figures that matter most are your existing supply capacity and what SSEN say about changing it. The two numbers we can publish honestly are a commercial EICR from £300 + VAT and a workplace EV charger from £800 + VAT. Everything else is a fixed price in writing after a free survey, with variations agreed before they happen.
Get your three phase work priced properly
Free survey across Aberdeen, Peterhead and the north east. Measured load, a written recommendation, and a fixed price. NICEIC Approved Contractor 620239.
Three phase work ends in a certificate somebody else relies on
An insurer, a machinery supplier, a landlord or an incoming tenant will all read it. Every number below is on a public register.
Three phase questions we get asked
If yours is not here, call 07304 027013 and ask it.
How much more power does three phase actually give me?
Roughly three times, for the same fuse rating. A 100A single phase supply delivers about 23 kVA (230V times 100A). A 100A three phase supply delivers about 69 kVA (the square root of three, times 400V, times 100A). That is why a unit tripping at 100A single phase often runs comfortably on the same 100A rating once it is three phase. It is a different shape of supply, not simply a bigger one.
How do I know whether my existing supply is enough?
By measuring it rather than guessing. We log the existing maximum demand under normal operating conditions, apply diversity to whatever you are adding, and compare that against the supply capacity and main fuse rating. Adding up nameplates gives a number no building ever reaches and usually leads to buying more supply than you need. The survey is free and the answer comes in writing.
Why does the whole building dim when a machine starts?
Starting current. A three phase motor started direct on line draws roughly six to eight times its running current for a few seconds. On a supply with little headroom that voltage dip is visible across the building and can drop out contactors and controls. The fix is sometimes a supply upgrade, but often it is a different starting method: a soft starter or a variable speed drive cuts that inrush dramatically, and a drive can be cheaper than the supply upgrade it avoids.
What is phase imbalance and should I care?
It means the three phases are not carrying equal load, which happens naturally as single phase circuits get added over the years. It matters because current imbalance typically runs six to ten times the voltage imbalance, so a 2% voltage imbalance can mean 12 to 20% more current in one phase. That is extra heat in cables sized for an even split, and derating on any motors fed from it. Measuring it takes very little time and the correction is often a few hours at the board.
How long does a three phase supply upgrade take?
The on site work is usually the quick part. The variable is SSEN: the application, their survey, and whether the existing service cable and local network can take the change or need reinforcing. That can run from a few weeks to several months. We start the application at survey stage rather than after designing the internal work, so the two run in parallel.
Do we need a building warrant for three phase work in Scotland?
Usually not for the electrical work itself. A building warrant from the local council is triggered by changes to the structure, the means of escape or the compartmentation of a building, which comes up more with fit outs than with supply work. Part P does not apply in Scotland at all, and there is no Building Control notification here either, because that is the English process. We tell you at survey which side of the line your job falls.
Can you work with our machinery supplier?
Yes, and on most installs we have to. We take the electrical specification from them, confirm the supply can carry it, agree who terminates what, and commission together. What we ask for is the equipment schedule early, because the starting method and the supply sizing both depend on it and both have lead times.
Send us the equipment schedule
Call Steven on 07304 027013, or send what you are installing and the site address. We measure the existing supply, tell you plainly whether you need three phase, and price it in writing. The survey is free and there is no travel charge in AB42 or AB43.
Faithful Spark Electricians, 17 Thomson Road, Peterhead, AB42 3FJ. Registered in Scotland SC821028. NICEIC Approved Contractor 620239. TrustMark 4205123. OZEV EVHS/WCS14254. Motor starting and derating figures are standard engineering references, NEMA MG-1 for imbalance derating; the design for any specific installation follows a site survey. Prices exclude VAT and were correct at September 2026.