Home |
Search |
Today's Posts |
|
UK diy (uk.d-i-y) For the discussion of all topics related to diy (do-it-yourself) in the UK. All levels of experience and proficency are welcome to join in to ask questions or offer solutions. |
Reply |
|
LinkBack | Thread Tools | Display Modes |
|
#1
Posted to uk.d-i-y
|
|||
|
|||
Design Current and Type B/C MCBs
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 |
#2
Posted to uk.d-i-y
|
|||
|
|||
Design Current and Type B/C MCBs
RzB wrote:
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. 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. 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. 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. How do I determine if Type C MCBs are needed? 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. 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. 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.) If so I think I might have to run a separate (larger) CPC to satisfy the Shock Protection requirements. Total design current? Cable type, size and length? Is house earthing PME or TN-S? Thoughts? 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). -- Andy |
#3
Posted to uk.d-i-y
|
|||
|
|||
Design Current and Type B/C MCBs
Andy,
many thanks for your help... See inline comments.. Roy "Andy Wade" wrote in message ... RzB wrote: 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. 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. 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? 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. 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. How do I determine if Type C MCBs are needed? 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. Yes - I hink so... not sure about the Dust Extractor but I will check with Axminster. 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. 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.) 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? If so I think I might have to run a separate (larger) CPC to satisfy the Shock Protection requirements. Total design current? Cable type, size and length? Is house earthing PME or TN-S? 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... http://www.gillandroy.com/diy/workshop/electrics Workshop 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. Thoughts? 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). Yes - that is what I was thinking - except that I'll probably run the lighting circuits in the roofspace. -- Andy |
#4
Posted to uk.d-i-y
|
|||
|
|||
Design Current and Type B/C MCBs
"RzB" wrote in message ... Andy, many thanks for your help... See inline comments.. Roy "Andy Wade" wrote in message ... RzB wrote: 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. 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. 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? 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. 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. How do I determine if Type C MCBs are needed? 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. Yes - I hink so... not sure about the Dust Extractor but I will check with Axminster. 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. 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.) 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? If so I think I might have to run a separate (larger) CPC to satisfy the Shock Protection requirements. Total design current? Cable type, size and length? Is house earthing PME or TN-S? 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... http://www.gillandroy.com/diy/workshop/electrics Workshop 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. Thoughts? 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). Yes - that is what I was thinking - except that I'll probably run the lighting circuits in the roofspace. -- Andy One point of course is that I won't be running 4kW of heaters when the roof space is at 40degC!!!! :-) Roy |
#5
Posted to uk.d-i-y
|
|||
|
|||
Design Current and Type B/C MCBs
RzB wrote:
"RzB" wrote in message ... 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? The old 2.5% limit is a good guide. 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.) 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? 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?) Design current - ignoring startup currents - see... http://www.gillandroy.com/diy/workshop/electrics Workshop 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? 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 2%. A separate parallel 10 or 16 mm^2 CPC would be prudent, although I haven't done the calculations to see whether it is strictly necessary - this depends on your choice of overcurrent device at the house end in any case. -- Andy |
#6
Posted to uk.d-i-y
|
|||
|
|||
Design Current and Type B/C MCBs
"Andy Wade" wrote in message
... RzB wrote: "RzB" wrote in message ... ---- snip 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?) 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 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 2%. A separate parallel 10 or 16 mm^2 CPC would be prudent, although I haven't done the calculations to see whether it is strictly necessary - this depends on your choice of overcurrent device at the house end in any case. 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 |
#7
Posted to uk.d-i-y
|
|||
|
|||
Design Current and Type B/C MCBs
Andy,
I have re-thought the workshop loading and done some cable calcs and put them on... http://www.gillandroy.com/diy/workshop/electrics It 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... http://www.tlc-direct.co.uk/Products/WY108.html Many thanks for your help, Roy |
#8
Posted to uk.d-i-y
|
|||
|
|||
Design Current and Type B/C MCBs
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. 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. |
#9
Posted to uk.d-i-y
|
|||
|
|||
Design Current and Type B/C MCBs
"Christian McArdle" wrote in message
... 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. 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. 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 |
#10
Posted to uk.d-i-y
|
|||
|
|||
Design Current and Type B/C MCBs
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. 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. |
Reply |
Thread Tools | Search this Thread |
Display Modes | |
|
|
Similar Threads | ||||
Thread | Forum | |||
Tub drain: Standard lever type vs pop up type | Home Repair | |||
Electric hob prewired with 13A plug. | UK diy | |||
My shed & garage electricity supplies & earthing! | UK diy | |||
15A Circuit Breaker on a Power Tap | Electronics | |||
Lighting circuits and MCB's tripping | UK diy |