Design Current and Type B/C MCBs

[RzB]

I'm converting my garage to a small woodworking workshop. I need to install a CU in the workshop and run a cable from the house CU. As it happens the house CU will be replaced at the same time. I know how to do the cable calc, but I'm struggling with two aspects. One is understanding what I use for Ib (Design Current), the other is to do with Type B / Type C MCBs.

Design current first.

----------------------- This is design current of the workshop. I'm not using diversity because I am very clear about what the max current will be and that it will occur. What I don't know, is what to do about motor startup currents. Do I have to take them into account for Ib? Some of the machines (Eg.. 3Kw Hammer Sawtable) have soft start so I imagine the startup currents are handled by the electronics in the machine. Others, like the

2.2Kw Axminster ADE4000 Dust Extractor don't have soft start. Axminster have told me that I should double the running current and add another 15% in cold weather.

MCBs

------- My thinking is that I may need Type C MCBs in the workshop CU. I am really gozzy on the differences/benefits/requirements re Type B and Type C. My only understanding (apart from the various graphs/charts in the Regs) is that the Type C is less sensitive to startup currents. How do I determine if Type C MCBs are needed? Also, if I have Type Cs in the workshop, does that mean that I must have a Type C for this circuit in the house CU. If so I think I might have to run a separate (larger) CPC to satisfy the Shock Protection requirements.

Thoughts?

Oh - and BTW - I know about Part P - before we go down that route!!! :-)

Many thanks, Roy

[Andy Wade]

Individual motor circuits for all but tiny motors I presume? Ib can be taken to the motor's full-load running current (FLC) as given on the rating plate. That's certainly OK for cable sizing. It's only when you get to really big motors (>10s of kW) that you need to worry about the effect of starting current on cables. That said, don't skimp on cable size, it's not a major cost and keeping the impedance down helps with starting.

Type B will trip "instantaneously" at between 3 and 5 times In (the nominal current). For type C it's 5 to 10 times and for type D 10 to 20.

As a starting points use type C for all motor circuits and type B for your lighting, 13 A sockets circuit and for any electric heating. If you experience motor-start tripping you can either increase the MCB size by one step, or go to type D, after checking that the cable is s/c fault protected (adiabatic equation). This assumes overload protection is provided by an electronic or thermal overload device in the motor starter, which should be the case.

The best option at the house end, discrimination wise) is an HRC fuse (cartridge fuse) in a separate switch-fuse unit. (Tee into the meter tails using a service connector block.)

Total design current? Cable type, size and length? Is house earthing PME or TN-S?

Wire your workshop in conduit and trunking so that it's easy to make mods. Changes will be inevitable over time. A good strategy is a ring of trunking (say 50 x 50 mm) round the walls at high level, with conduit drops to sockets, switches and motor starters, and upward runs to lighting.

13 A sockets should be on an RCD protected circuit(s).

[RzB]

Andy, many thanks for your help...

See inline comments..

Roy

Yes - I won't skimp - I would like to keep the Vd to a mimimum. What would you think is a good goal to aim for?

Yes - I hink so... not sure about the Dust Extractor but I will check with Axminster.

Hmmm - Lost me here... Are you saying that I shouldn't be taking the workshop circuit through the house CU? Why wouldn't this just be one of the circuits off an MCB on the house CU?

OK - I have a 6mm cable already in place and I intend to leave that there to drive the workshop lighting and the external approach lighting. Was thinking of having a separate CU in the garage for this...

Design current - ignoring startup currents - see...

is attached to house... and distance between house CU and workshop CU is about 25M.

House is PME. Cable running through loft so might get to 40degC ambient in the summer - yes/no? Also cavity wall insulation will enclose cable when passing through wall fo a couple of inches... No grouping.. Otherwise - cable just clipped to wall/rafters?

After calcs, I was considering using 16mm T & E from house to Workshop. But I'm not sure what CPC size comes with standard 16mm T&E?

Runs from workshop CU to machines are all less than 5M.

Yes - that is what I was thinking - except that I'll probably run the lighting circuits in the roofspace.

[RzB]

One point of course is that I won't be running 4kW of heaters when the roof space is at 40degC!!!! :-)

Roy

[Andy Wade]

The old 2.5% limit is a good guide.

It's better not to, especially as you're really looking at a 50 or 60/63 amp distribution circuit for the workshop. The problem with cascading MCBs can be lack of discrimination on s/c faults - both MCBs may trip. (This may not matter to you though, as you've got a separate feed for lighting.) You also need to consider whether the house CU and indeed the supply is OK for ~45 A of extra load. (Any electric showers or space heating in the house?)

CPC is 6 mm^2. Yes, 16 mm^2 for the cable sounds about right, it'll give you plenty of current rating in hand and keeps the voltage drop down to

[Christian McArdle]

Your design current doesn't include startup currents. You want to use Type B if at all possible, as they are safer. These can accept a startup current up to 3 times the design current, which would appear to be fine, as your worst case scenario is the 2.2kW at x2.3 current which will give 22A which is not going to trouble the wiring or a 32A Type B MCB, even if the ring is already nominally fully loaded, unless the startup current lasts for an unduly long period of time.

Christian.

[RzB]

---- snip

Ahh,... Now I just hadn't even thought about the total current coming in to the house. That would have popped up when I looked at diversity for the house as part on the new CU work. Hmmm... What happens if the calcs come out over 100A? Do I just ask the suppliers for "more please" ! Hmm... We have a shower - only used very occasionally by visitors so probably not a big deal. But it will probably blow the diversity calcs. Hmm...

------- snip

Yes - I need to redo the calcs but I believe the 6mm CPC will be OK if I have a Type B in the house. I think I would still run a separate CPC though. However I can see that your suggestion of a fuse is preferable. I need to redo the calcs for this. I would imagine 6mm would not be enough for this.

Many thanks, Roy

[RzB]

Christian,

I was expecting to provide separate circuits to each of the static machines in the workshop. So there won't be any ring to worry about. The workshop sockets ring will be protected by an RCBO.

Many thanks for your help, Roy

[Christian McArdle]

I would say that a fixed 2.2kW motor appliance with total inrush current of

22A will be fine on a 16A Type B MCB, which won't instantaneous trip until at least 48A. Obviously if it takes an age to spin up then it could blow the thermal curve, although at only 6A over the 16A rating, this is unlikely.

Christian.

[RzB]

Andy,

I have re-thought the workshop loading and done some cable calcs and put them on...

looks like a 16mm T&E with 6mm CPC will satisfy the regs with whatever protection is back at the house. However, I think I will put in a separate

10mm CPC (belt & braces) and have a fused switched by the house CU as you suggested.

I'm not clear about the sort of Fuse to use ? BS 88, BS1361 ? Or what? Or will that get determined by the switch-fuse unit.

Do I take it this is the sort of thing you are suggesting...

thanks for your help, Roy

[Andy Wade]

see (with whitespace removed) is:

Either, and yes (obviously) it will be determined by the type of fuse carrier in the switch-fuse.

fed through a 108M (the metalclad version) fitted with a 45 A fuse. You could also look at things like the MEM Excel range, which take BS 88 fuses, but they're much more expensive for no real advantage.

[RzB]

Yes - Servers got moved today... sorry

Should be OK now..

Many thanks, Roy

[RzB]

Hmm... now seems broken again! D'oh...

[RzB]

OK - Should be OK now.... !!!

[Andy Wade]

Yep. My comments on your calcs are:

1. Current rating: You've used It >= Ib/(Ca*Ci). Since overload protection is required here this should be In/(Ca*Ci) - i.e. based on the device rating, not the design current. Moreover in your case the Ca factor (for loft temperature) and Ci factor don't apply simultaneously (the cable thro' the wall won't be at 40 deg. and the cable in the loft will be well-supported away from thermal insulation). With Ib = 41.3 A and your intention of using the Wylex 108, we can select the 45 A BS

1361 fuse, hence It >= 45 A / 0.81 = 56 A (10 mm^2 still OK). Or with a 60 A fuse we get It >= 74 A (16 mm^2 required).

1. Shock protection: your calcs will be on the conservative side because you've assumed 70 deg. conductor temperature (the 1.2 factor). In fact the greatest part of the cable's length will be running well under its rated current, so the loop resistance to be added to Ze is correspondingly less.

2. CPC thermal compliance and disconnection time, with 6 mm^2 CPC and 50 A Type C MCB: fault current at 479 A [*] is less than 10*In, therefore 5 seconds disconnection time is not necessarily achieved. Design is non-compliant, go no further (or retry with larger CPC). For the Type B yes, it's OK to use 0.1 s (compliant), ideally use manufacturer's published I^2*t data, which can be more favourable.

[*] The fault current will fall as the cable heats up, so you have to err on the pessimistic side here when anything is marginal. Design based on the use of measured values of Ze is discouraged, since this value is outside your control. In principle the DNO could alter their network, resulting in a higher value.

Incidentally, are there any metal service pipes or other extraneous-conductive-parts entering the workshop? If so, you may need local bonding to the earth bar in your dis-board - see Regulation

413-02-13 (a challenging read).

[RzB]

Andy,

Apols for delay - have been away for the weekend...

See inline comments..

Many thanks for your VERY helpful feedback...

I think I will be going with 16mm T&E + additional CPC - 10mm ? Through a separate 45A or perhaps 60A Fused switch.

BTW - If, after doing the house diversity calcs, it comes out greater than 100A, what is the likly response from the supplier?

Many thanks, Roy

Yes - daft typo... formula sasy In then I use Ib... D'oh...

My understanding was to use both - really don't understand the rationalle behind this. However I think the calcs come out OK with 16mm^2 whatever factors are used!

Yes - OK.

Yes - understand...

Yes - no need to do adiabatic calc if the 5sec time is not met on the time const graph table.

Yes - I understand.

Yes - I understand.

Yes - good point...

Not at the moment but there will be. Thanks for the reminder re bonding.

[Andy Wade]

I wouldn't worry too much, and I doubt that they will. From all you've said I'd expect any overloads to be of fairly short duration (i.e. minutes not hours) and not likely to do any great harm. (I'm assuming here that you do have a 100 A main fuse, not a 60 or 80 A one.) Make sure that nothing around the service position is getting excessively hot

- fishy smells are sure sign of problems.

[RzB]

Andy,

OK - Yes 100A.

Many, many thanks for your excellent help.

Roy