Workshop electrics

Hi. I'm currently planning a new brick garage/workshop and am drafting a kind of specification for the purpose of obtaining quotations. I have a Myford lathe, pedestal drill, Dore Westbury mill, 150A MIG welder, 3HP air compressor with sundry grinders, polishers. The building will be block partitioned into a "clean" and "gritty" room.

My spec currently says "32A armoured underground cable to new garage with RCD/MCB consumer unit supplying 32A power ring, 5A lighting and non RCD protected feed to burglar alarm"

Will this do?

Many thanks

I would be inclined to increase the size of the supply cable to say 6mm or even 10mm, nothing worse than finding in the future you need more power than a 2.5mm cable can supply, also distance has to be considerations. Worth checking **LINK**

I have a 10mm cable from the house consumer unit to the garage consumer unit, as the garage/workshop is attached it would not have been a problem to change the cable as it runs through loft of my bungalow. I selected 10mm as it allows for some flexibility.

Split garage consumer unit should do the trick to allow for non RCD supply, I would suggest you allow some spares in the consumer unit to future proof it, well hopefully.

I have a ring main (32amp) around the garage/workshop ( 7 x 4.5 meters) with 16 double sockets, lighting (6amp) and a separate 32 plug socket for high load items like welders, thinking 250 amp welder.

Bob

If you intentend to use any VFD inverters these can be a bit of a nuisance on RCD protected circuits. It may be worth having spur outlets to supply any devices of this nature not protected by the RCD.

Mike

Edited By Michael Poole on 30/01/2015 10:41:46

For things like starting the compressor, voltage drop can be at least as important as the current carrying capacity of the cable (this becomes more important as cable length increases. Hopefully the Electrician can advise on suitable cable size (also take note of future possible expansion as Bob suggests).

If likely to use inverters - your spec should mention this, standard domestic RCDs are prone to nuisance trips with inverters. Also consider where the supply will originate (to avoid a workshop fault tripping whole house out).

Thanks Bob. I'll specify 10mm2 cable to the garage rather than 32A.

Good points David and Michael- you now have me slightly worried! The new workshop is part of a master plan that involves a substantial house extension with its own new consumer unit. If the garage supply is protected by the house RCD, how would I supply a VFD drive in the garage that wasn't supplied via the house CU RCD and the garage Consumer Unit RCD? I thought all new consumer units only had a the facility to supply low power to smoke and burglar alarms that bypassed the RCD.

Thanks

I think you should discuss your specification with an electrician who is fully up to date with the regulations and capable of electrical design, a wireman or even someone who has the testing qualification may not be fit to advise.

Mike

I had the electrics in our mid 60's bungalow completely redone as the original layout was not ideal, originally both gas and electric meters were in the kitchen which when the bungalow was built the garage so having them there was not a problem as a garage. When the original garage was converted in the kitchen in the mid 70's the meters were left so the gas meter ended up at the back of a kitchen corner cupboard and the electric meter and consumer unit in another tall kitchen unit. When we purchased the place neither suited the new kitchen layout never mind the fact you had to remove every thing to read the meters, so they were moved to the outside wall. The consumer unit was now positioned across the other side of the bungalow but needed to have an isolator and sub main to the consumer unit. I opted for RCBO's, rather than a split consumer unit and a single RCD, this means I could supply the garage from a standard type C MCB, but if you have a split house consumer unit just supply the garage off the non RCD side.

Personal not had a problem with my VFD or anything else causing a false trip, touch wood, but it would be easy enough to add a dedicated supply if it became needed as the garage consumer unit is 12way.

Bob

Supply to my garage consumer unit comes from upstream of main house consumer unit - when garage RCD trips, it doesn't affect the house supply. A good electrician should be able to sort all this out.

People would generally use a time delay RCD in the house so that the workshop one would trip first.

Last time I did the wiring regulations these weren't permitted, it might have changed since though as things were getting difficult.

I have the RCD tripping issues when using my pillar drill with a Parker (Eurotherm) VFD installed. The Telemecanique VFD on my lathe has never caused a problem. Both have an EMC filter installed. My property has a 'TT' earthing system, as the power is brought in on overheads with no direct link back to the substation for the earth, other than that provided by an earth electrode in the ground. As such, the incoming supply is wired through a standard 30mA 80A RCBO, and the entire house goes into darkness when the supply is tripped by the VFD.

I am looking to split the incoming feed, have a time delayed 100mA RCBO installed to protect the installation between incomer and workshop, then install separate 30mA RCDs in the workshop for the sockets and machines. The domestic side of the installation will still be protected by the existing RCBO.

My installation also has 10mm sq SWA cable running between the current consumer unit (via a breakout/junction box) and the workshop. Having undetrtaken a 16th edition course on system design, verification and testing in the past, when sizing the cable there are a number of factors to take into account, not just the current carrying capacity of the cable. These are (but not limited to):

* How is the cable installed. What effect will bundling/installation within walls have on the temperature rise of the conductor and in turn the cable insulation

* What is the voltage drop over the length of the cable run (live AND neutral!) and is it within limits at expected current loading

* What is the earth loop impedance? Will this impedance allow a fault at the far end of the installation trip the protection device within the required time? Will the current let-through cause unacceptable temperature rise in the cable?

Often after working through these calculations, the cable size has to be a lot larger than first expected!

Regards,

Anthony

My house in Cambridge also had a TT (overhead) supply. This relies on an RCD to protect against a ground fault, as there is no protective earth provided with the feed. Unfortunately the original wiring relied on the plumbing for the "protective" earth. I say "unfortunate", as both the gas and electricity came in through plastic feeders. You could get a fine little tingle from the bathroom taps and light a neon (>90V, then) due to the fact that the ground was effectively floating and defined simply by whatever was plugged in at the time.

I fitted a proper 1m copper coated ground spike and replaced the incomer RCD (130mA originally??) which had ceased to function properly. Thereafter I used a similar scheme to what you describe - modern consumer units tend to have dual circuits anyway and I added a 3rd circuit for the garage with a consumer unit there.

Not sure about playing games with time delays. You may require type C MCBs if you have welders, isolation transformers or on-line motor starters, whereas most domestic MCBs are type B to support lights and appliances.

Most VFDs have a soft start circuit to limit the inrush current when connected to mains but this can still be quite a surge. It's more likely to be the inrush current than the EMC filter that's tripping your RCD, as the earth leakage current due to the EMC filter is limited (by legislation) to quite a bit less than RCD levels. I had similar issues with my lathe and drill VFDs and the garage consumer unit with its own RCD was the cure. Sounds like a good plan!

Murray

Hi Otley

Anthony has given some good advice, there are many factors to consider when sizing a cable for a sub-main to another consumer unit.

Consult a qualified person for a system design and installation after deciding exactly what sockets/lights you want.

Be safe in your new workshop.

Emgee

An RCD is fitted to detect earth faults and an MCB to detect overcurrent. As indicated previously type C MCB are used in motor circuit.

.

I have an RCD across the mains and run two VFD's without tripping issues.

VFD's are Mitsubushi of this type **LINK**

Nick

Many thanks for all your helpful replies - I think I am now in a much better situation to have a semi intelligent conversation with an electrics contractor!

Any industrial demolition going on around you? Worth checking the scrap yards for heavy cross section armoured cables for the supply feed. Mine had been used inside and was far cheaper than "New" and was/is in perfect condition.

Regards Ian.

Good advice indeed Michael. It's a little worrying how many different answers you can get from qualified electricians!