Sizing electrical cables etc?

[ondablade]

Hi guys, hoping for some Bob type expertise here. I'm buying wiring materials for my workshop next week, but am unsure about how cable size is defined.

I'd always heard cable size referred to in terms of it's cross sectional area in square millimetres - that is mm2. This being the cross sectional area of the conductor.

Digging around in supplier information this week i seem to be running up against just plain millimetres or mm. Is this a typo, or a convenient short form designation that really means mm2 or sq mm, or is it actually the diameter of a solid circular copper conductor? (e.g. they seem to refer to cables by wire gauge in the US)

If the latter, then how is the designation of size handled in the case of multi-core flexible cable?

Is there then a definitive (but concise and normal English readable) source of information/a table that says how to size a cable - not just for given amps, but also taking account of differing sorts of loads?

At a safe but basic level, in that with my shortish runs i don't mind buying cable that's maybe a bit larger/more expensive than the minimum, but safe in a worst case scenario - i know that precise sizing gets complicated if heat loss vs. insulation, trunking type and run distances is factored in to give very exact numbers.

Figuring out something resistive like an immersion heater is fine, but something like a motor that's largely (?) inductive means you get an inrush/surge of current on start up may be a bit more complicated. e.g. I've seen it said you need to size cable/other conductors like switches etc for the name plate full load current plus 25 or 30% for a single phase motor.

A final question. It makes sense that MCBs/breakers should be sized so that they are (a) able to handle the current drawn plus the vagaries of the type of load without nusiance trips, but (b) that the cable/conducting devices in the circuit must have some reserve capacity at this load level.

Is this something that can be presumed to be built into the device's nominal/label/catalogue load/amp ratings, or is it up to the electrician to factor this in on top of the nominal rating? e.g. are the methods used to determine the nominal rating of e.g. a 20A MCB, a 20A switch or socket and a 20A cable such that despite the nominal 20A rating in all cases that the conductors have the required extra margin in hand when handling a current that will (just) trip the MCB?

It looks like you have to ask for the correct type of MCB, but at the nominal amps/size - there are C type breakers that are designed to handle start up current surges from motors and the like without tripping....

 

[jasonB]

This is a reasonable calculator

http://www.tlc-direct.co.uk/Technical/C ... eDrop.html

 

[andycktm]

Thats a handy calculator Jason, and as a bonus it blows all the voltage drop rubbish out of the water to boot.
7 volts @100' :lol:

 

[Digit]

First question first Ian.
Yes, they just drop the (sq) prefix, so 2.5 is understood to be the CSA in sq mm. The US uses their equivalent of SWG. (Standard Wire Gauge).

If the latter, then how is the designation of size handled in the case of multi-core flexible cable?

If I understand you correctly you asking if the CSA is the same in a multi core cable, if so the answer is yes. A 3 core 2.5 will have three conductors of 2.5.

Is there then a definitive (but concise and normal English readable) source of information/a table that says how to size a cable - not just for given amps, but also taking account of differing sorts of loads?

Yes. But without an electrical background you might well be no wiser as in most cases the text may well use unfamiliar terms.

Figuring out something resistive like an immersion heater is fine, but something like a motor that's largely (?) inductive means you get an inrush/surge of current on start up may be a bit more complicated. e.g. I've seen it said you need to size cable/other conductors like switches etc for the name plate full load current plus 25 or 30% for a single phase motor.

A 'rule of thumb' is add 25% load then use the next size cable up.

Your last question can be a bit tricky. It all depends on how fast the motor, plus its load, spins up to working speed. The blade on a table saw for example can not accelerate faster than the motor's own rotor, but a large dia, heavy blade can add significantly to that time.
This can become important on large commercial machines but is very unlikely with the machines most of us use.
PTs will normally start without a type C but TSs usually do benefit from their usage.
Hope that helps.

Roy.

 

[ondablade]

Thank you very much guys, you've gone to a lot of trouble.

Sounds like it simplifies in the end (most of the time - which is why an electrician may be a good idea) - at least as I seem to be seeing it:

1. Stock component and cable ratings (amps) will grind it down so there's not that many sizing choices.
2. Go with the rules for lighting and 13A socket ring main circuits - just take care that the total loads don't exceed the limits set by the guidelines. 'Workshop Electrics' by Alex Weiss (Argus Books) was mentioned here before, it seems to cover these well.
3. On dedicated (radial) circuits for items with motors above 13A it sounds like the conductors (cables, flex, switches, sockets and plugs etc) can normally be sized as you say at the plated/nominal full load current plus 25% - rounding upwards when selecting stock cable sizes.
4. Select MCBs for these circuits to match or slightly exceed the nominal full load current of the device - the differing types have the cushion needed to prevent nusiance trips at that. It seems that the cable rating must exceed that of the MCB, but that stock sizes will determine options in this regard anyway.

I've an electrician lined up to make sure what i plan is OK up front, and to check and certify afterwards - so hopefully all will be fine.

 

[Digit]

Consider radial mains rather than a ring Ian.
A ring will normally be protected by a 32 A MCB, which does very little for most single phase machines.
An example. A 3 HP motor will normally run on a 13 A fuse as a 13 A fuse will not normally rupture till about a 17/20 A overload, which will have little or no effect on the 32 A MCB.
Thus a radial main with a 16 A MCB offers greater protection for such a motor.
Other radial mains can then be likewise gauged to other machine loads.
I use radial mains in my own shop with a bus bar supplying power to the bigger machines with a second radial main on the opposite side of the shop supplying power to the compressor, pillar drill, battery chargers and CD player. Thus all plugs are correctly fused as are the mains.
Ring mains were introduced as a means of reducing the import of expensive copper, not because it is the better method.

Roy.

 

[ondablade]

Thanks Roy, that makes a lot of sense. Ring mains definitely smack of scrimping.

One point that may make some difference in my case is that the 13A sockets will not be powering the floor machines - which makes it worse in some ways.

Prior to buying the Hammer kit which uses 3kW/4hp motors my Robland (with 3hp motors) was powered from the 13A sockets installed when i originally built the shop.

The plan is that these sockets (about 5no doubles) plus several new will be downgraded from powering the larger machines to leave it so that there is a double 13A at each machine - but only for use on task lighting and/or powering a secondary hand tool or other equipment. Plus a number of 13A sockets in the bench area as well for routers etc.

I'm not quite sure how the existing socket circuit(s) are wired, but if it can be done without major complications I'll re-wire them radially as you suggest. I have to install a second consumer unit anyway as the existing doesn't have enough channels, so it shouldn't be that big a deal to move them all on to that. Luckily the shop is wired for a welding socket, so the feed to the existing CU should be man enough for all of this. (this to be confirmed by my electrician)

Plan B if the ring main is left in would be as you say to make sure that plug fuses are all correctly sized, but it's not at all ideal.

The plan is to place an isolating switch in the feed to the new CU. Stuff on the existing CU (lights, alarms, charging sockets in the office etc) will stay powered 24hrs, but flipping the switch will power down all the machines, and all the sockets in the actual workshop.

The floor machines themselves will each be powered from a dedicated radial circuit protected by an MCB sized to the machine - each connected by a wall/ceiling drop mounted socket. I've in the case of these circuits been thinking in terms of standardising all their conductors at the same rating (32A as 3kW @ our 220V with a 25% allowance is up around 18A - so 16W sockets and wiring seem too small) for flexibility - so that within this limit anything can be powered from any socket.

PS I had some problems with arcing in standard 13A sockets while running the 3hp motors on my old Robland - even though even with factoring in the 25% allowance this should have been (only just) OK.

 

[PeterSk]

If you want to go for some serious overkill you could always wire ring circuits with 2.5mm2 cable but only protect the circuit with a 16A breaker. Then you should have plenty of spare capacity in the cabling but still have the protection afforded by a 16A breaker

 

[Digit]

Arcing in 13 a sockets is quite common on occasion Ian, usually not as a result of the load IME, but wear from repeatedly removing and replacing the plug. The best solution I find is to wire permanently, (if feasible) to fused spur outlets.

Roy.

 

[ondablade]

I guess the arcing made me a bit wary of sailing close to the wind on socket ratings Rob. Your point has me wondering if by talking of using 32A conductors throughout i'm not overdoing it.

The extra cost is quite significant at + 60-70% on cabling and sockets - it's plus £0.50/m on the cabling, and +£4 each on sockets and probably plugs as well. At that it'd add about £250 to the cost of the job.

Reading the Hammer manuals this morning they specify a 16A type C MCB for machine protection which makes sense - 3000W/220V = 13.6A

On the other hand adding the recommended +25% rule of thumb for cabling/conductors used with motors brings the figure for the conductors up to 17A.

Which means that 2.5mm2 cable should be fine given that according to the workshop wiring book it handles up to 20A in a radial circuit.

On the other hand a 16A socket is below this 17A, so the rule of thumb suggests a 32A socket if i have it right.

My thought thought when I posted above was that fitting a 32A socket on cabling capable of handling 20A is surely misleading for users, and probably a no no. Upgrading the cable to suit suggests using 4mm2, but maybe that's getting out of hand.

I wonder what good practice on this is?

PS another argument in favour of going for 4mm2 cable/32A socket is that it would be fine if something a little larger needed running from the sockets - but 3kW is pretty much the upper limit for single phase anyway.

 

[Digit]

I've never known the rule of thumb applied to 13 a plugs and sockets Ian. The usual reason for arcing is wear of the knives in the socket, look at the CSA of the pins in the plug against the CSA of any cable you can fit into it. It's no contest!
Fused spur outlets are normally cheaper than 13 A plug and socket pairs and without the wear problem.
I've known both P and S burn from wear in the socket in kitchens where there is a shortage of sockets for all the gadgets the lady of house was using, thus constantly inserting and removing the plugs.
I use a bus bar system in my shop fitted with switched 13 A outlets, it has been in daily use for over 7 yrs without a hitch.

Roy.

 

[cgw59]

Hi I have read the posts on this subject with great interest , as I have an electrical background, one thing that feel may have been over looked when talking about cable sizes , ratings etc, is that there are two types of cables that are typically used in wiring up workshops, 1. double insulated. & 2. single insulated. The first is normally used where there is no issue of possible cable damage, like in ordinary domestic house wiring it can be used under floor or clipped on the surface, 2. is for use where it will be enclosed in either conduit or trunking. The one thing that must be remembered when using conduit or trunking is that it derates the cables due to heat , that is why only single insulated cables are used inside conduit or trunking where they cannot get the benefit of natural cooling from the surrounding air.
cgw59
p.s.
Ring circuits were intruduced to cut down on the number seperate circuits and isolating devices needed for the modern buildings, and by doing so to allow for a smaller size of cable to be used over longer lengths.

 

[ondablade]

Hi CGW. Thanks for that. I guess it's detail like that that sits behind any rule of thumb approach, but boy does it get complicated :? My thinking has been to avoid getting too detailed on this, and to settle for paying a bit more for cable with a bit of spare capacity instead if I could.

Ha! Good thinking Batman (Roy) If i have it right what you are saying (although you illustrated it with with ref to 13A sockets) is that sockets in general probably have loads of spare capacity built into the sizing of their conductors - so that since there's as a result no heating issue there's no need to apply a safety factor of any sort to their rating as is the case with cabling.

Thinking about it that makes sense - they are rated by their unqualified actual current capacity in Amps, so it ought to be OK to take that at face value. i.e. to presume that any required factor of safety to cover the circumstances of use is already built in.

Cables on the other hand are sold in terms of conductor cross section dimensions, and as CGW has just said are subject to all sorts of subsequent insulation and installation vagaries which by increasing or reducing cooling influence their safe current carrying capacity.

Which means that the specifier has to take account of the specific type as well as the installation in deciding which to use - if only by applying a rule of thumb. The TLC calculator Jason linked above takes this a step further by requiring that you specify both the situation and the type of cable.

Pardon my being so pedantic (it'd be easy to take a flier), but I'm keen to understand it properly.

What's emerged from the mist of the discussion seems to be as follows (but bear in mind that this is a strictly amateur view):

1. Select items like plugs, sockets and junction boxes that are rated in terms of their actual current carrying capacity in amps by comparing that number to the actual steady state full load/plated/labelled/full load current (FLC) the powered device(s) will draw - they (plugs etc) have slack designed in to accommodate start up surges and the like.

Just select the standard plug, socket or whatever that at minimum equals or next exceeds the full load Amps it may be required to carry in any foreseeable normal use.

2. Select breakers/MCBs and fuses to protect the powered device and the circuit in the same way (by their plated FLC Amps rating), but go for variants at these sizes (e.g. C type breakers on motors) as required to maximise protection while avoiding nusiance trips caused by transitory start up currents etc.

Maker's manuals will often specify the size and type of MCB required to protect their machines. Hammer for example specifies a 16A type C MCB on those of their 3kW machines I've looked at. (my saw, spindle moulder and planer)

The difference here is that to do its job the MCB should not be over rated any more than the minimum forced by the stock sizes that are available - double checking that any over sizing above the nominal full load current as a result of this is not going to cause an unacceptable risk, or allow the possibility of overloading anything else in the circuit. (cabling, connectors, powered devices etc)

3. Size cabling with reference to the installation circumstances and type, using e.g. a reliable calculation tool or stock recommendations from reliable guides/manuals to make sure that when these are taken into account that the size (sq mm) will have a current carrying capability at the absolute minimum the comfortable equal of that the breaker/MCB is sized for. (but preferably greater)

In the case of motors (inductive loads, and subject to short term current surges) it seems that one widely used rule of thumb is to in addition add 25% to the actual steady state full load/plated/(FLC) current/amps the breaker is sized for, and to select the cable to handle this 125% full load current.

The current/Amps rating of the cable and other other conductors must always exceed or at minimum comfortably equal that of the MCB/breaker/fuse.

It makes sense in a workshop not to skimp on cabling - that is to size the cabling to safely handle any reasonably foreseeable future increase in MCB size/load. e.g as would be required by changing to a more powerful machine with a greater power/current draw

As above this is dangerous territory - please cross check the above with somebody competent. This is just a summary of where we may have ended up in this discussion.

 

[Digit]

That's about it Ian. Actually I am/was qualified and the 'rule of thumb' approach was something that electricians/engineers learnt from experience to save having to do all the theoretical calculations that would otherwise be involved.
One word of warning though, the 'rule' breaks down when using very long conductors, but unless you are fortunate enough to have a ginormous workshop it won't bother you!
I haven't used conduit in my shop, simply routed the cabling on the surface either high or very low where impacts are extremely unlikely, this enables me to modify the circuitry if required

Roy.

 

[ondablade]

Ta Roy, you've been a big help. The drops in my shop are in PVC conduit, so i'll probably go with that for the visuals.

I'm a mechanical engineer, but it's so long since I was trained in electrics (they didn't anyway teach that much by way of practical applications) plus my practical experience is limited to DIY) that it's taken a bit to get under some of the thinking.

I imagine that these days designers of larger installations may not do all that much by way of first principles sizing of stuff either - chances are it's handled by CAD packages....

 

[Digit]

I must admit I did all the hard way at first, but apart from factory installations, where you could have long runs, back in the days before bus bars, I never got a very different answer to the rule.
One tip you might find useful if you use separate conductors. Put one more conductor in than you need, it can then be used to pull additional lines through the ducting if required at a later date.

Roy.