Current Carrying Capacity - 230 vs 14
Discussion
I've posted this here since this is a matter of physics! 
As I've always understood cables current and voltage rating are based on:
- The cross sectional area (or gauge in America) - this determines the current carrying capacity of the cable
- How the cable is installed - trunking, conduit, clipped etc, this is another factor taken into account when determining the current carrying capacity of the cable due to heatloss etc
- Insulation - the properties/thickness of the insulation determines the suitable voltage the cable can be used for.
From what I know I would say the cross sectional area of the cable is based on the current it is designed to take, the voltage rating or insulation have no bearing on this (however I'm unsure). This is where my questions begins:
I am changing the starter motor cables on my current car - the starter is rated at 16kW - based on a car having 14V (approx) coming off the battery this would lead me to believe this would be a load of around 115 amps - however this would be an inductive load so it's better to overspec.
This is where the confusion begins, many forums have recommended 50mm which would give me a capability of 198 amps based on the table in the link below as the cable will have some flexible conduit on it, however will be hanging pretty freely. In the case that a cable that size was hanging freely it could have a capability of 275.
https://www.cse-distributors.co.uk/cable/technical...
Of course the above table is the cables rated at 230VAC, rather than 14VDC that you would find in an automotive application, but the capability (current) is based on the cross sectional area?
If I was to look at this ebay ad, it lists the 50mm cable of being able to carry 345 amps - is this correct? Both cables are copper and have the same CSA however the one in the ebay advert is saying it can carry a much higher current than the IEE would recommend - is this down to voltage, surely not?
http://www.ebay.co.uk/itm/PVC-FLEXY-BLACK-RED-BATT...
I'm happy to put the 50mm cables and associated lugs on my starter, however I'd like to know the physics behind this or if it is one of those typical ebay discrepancies.
As I've always understood cables current and voltage rating are based on:
- The cross sectional area (or gauge in America) - this determines the current carrying capacity of the cable
- How the cable is installed - trunking, conduit, clipped etc, this is another factor taken into account when determining the current carrying capacity of the cable due to heatloss etc
- Insulation - the properties/thickness of the insulation determines the suitable voltage the cable can be used for.
From what I know I would say the cross sectional area of the cable is based on the current it is designed to take, the voltage rating or insulation have no bearing on this (however I'm unsure). This is where my questions begins:
I am changing the starter motor cables on my current car - the starter is rated at 16kW - based on a car having 14V (approx) coming off the battery this would lead me to believe this would be a load of around 115 amps - however this would be an inductive load so it's better to overspec.
This is where the confusion begins, many forums have recommended 50mm which would give me a capability of 198 amps based on the table in the link below as the cable will have some flexible conduit on it, however will be hanging pretty freely. In the case that a cable that size was hanging freely it could have a capability of 275.
https://www.cse-distributors.co.uk/cable/technical...
Of course the above table is the cables rated at 230VAC, rather than 14VDC that you would find in an automotive application, but the capability (current) is based on the cross sectional area?
If I was to look at this ebay ad, it lists the 50mm cable of being able to carry 345 amps - is this correct? Both cables are copper and have the same CSA however the one in the ebay advert is saying it can carry a much higher current than the IEE would recommend - is this down to voltage, surely not?
http://www.ebay.co.uk/itm/PVC-FLEXY-BLACK-RED-BATT...
I'm happy to put the 50mm cables and associated lugs on my starter, however I'd like to know the physics behind this or if it is one of those typical ebay discrepancies.
Some points:
But otherwise, your understanding is mostly correct. Add in the operating conditions which might affect the tabulated ratings, such as proximity of other cables or materials & ambient temperature. The principle of current ratings is the delta t between ambient & limit.
Data is typically established by test & extrapolation of test data.
Voltage has little to do with current ratings, other than the effect of needing a high dielectric quality.
- the flex conduit will act as an insulator
- the CU conductor in the in the eBay cable is "flexible" as opposed to "stranded" as the CSE table
- the insulation on the eBay cable will be 105°C (or possibly higher) as opposed to 70°C for standard PVC or the 90°C of the CSE cable
But otherwise, your understanding is mostly correct. Add in the operating conditions which might affect the tabulated ratings, such as proximity of other cables or materials & ambient temperature. The principle of current ratings is the delta t between ambient & limit.
Data is typically established by test & extrapolation of test data.
Voltage has little to do with current ratings, other than the effect of needing a high dielectric quality.
Cheers for the reply Smiler. 
I'm aware the conduit acts as a thermal insulator and therefore lowers the current carrying capacity. However I do feel a little silly as I didn't realise the difference in strands etc.
The cable will mostly be hanging free, apart from the small bit of flexible conduit used for mechanical protection so the 50mm will be good for the application.
Again, cheers!
I'm aware the conduit acts as a thermal insulator and therefore lowers the current carrying capacity. However I do feel a little silly as I didn't realise the difference in strands etc.
The cable will mostly be hanging free, apart from the small bit of flexible conduit used for mechanical protection so the 50mm will be good for the application.
Again, cheers!
The other issue you need to consider is voltage drop & given the short period of time the starter is cranking it might well be more important than the current rating. You've only got 14v to begin with so you want to lose as little as possible. The larger cable will have a lower resistance & therefore drop less voltage.
Don't use 14v, use 8v!
The battery won't be anywhere near to 14v during cranking.
If you are keeping the loom the same length, then use the same CSA as the original loom. If you are moving say the battery to the boot and hence increasing the length of the high current loom, then you should, if possible, go up a wire gauge. Work out the resistance of the existing loom (you should be able to look up the mOhm per meter for the wire gauge used, and try to ensure you don't end up with any more resistance than that value)
For starting applications, the insulation of the jacketing is irrelevant because we are talking about intermittent operation. You cannot run a starter continuously, either because the battery will go flat, or the starter itself will over heat.
The battery won't be anywhere near to 14v during cranking.
If you are keeping the loom the same length, then use the same CSA as the original loom. If you are moving say the battery to the boot and hence increasing the length of the high current loom, then you should, if possible, go up a wire gauge. Work out the resistance of the existing loom (you should be able to look up the mOhm per meter for the wire gauge used, and try to ensure you don't end up with any more resistance than that value)
For starting applications, the insulation of the jacketing is irrelevant because we are talking about intermittent operation. You cannot run a starter continuously, either because the battery will go flat, or the starter itself will over heat.
Mr Pointy said:
The other issue you need to consider is voltage drop & given the short period of time the starter is cranking it might well be more important than the current rating. You've only got 14v to begin with so you want to lose as little as possible. The larger cable will have a lower resistance & therefore drop less voltage.
Looked into this also, I'm familiar with volt drop, impedance etc 
Max_Torque said:
Don't use 14v, use 8v!
The battery won't be anywhere near to 14v during cranking.
If you are keeping the loom the same length, then use the same CSA as the original loom. If you are moving say the battery to the boot and hence increasing the length of the high current loom, then you should, if possible, go up a wire gauge. Work out the resistance of the existing loom (you should be able to look up the mOhm per meter for the wire gauge used, and try to ensure you don't end up with any more resistance than that value)
For starting applications, the insulation of the jacketing is irrelevant because we are talking about intermittent operation. You cannot run a starter continuously, either because the battery will go flat, or the starter itself will over heat.
I'll use 14 The battery won't be anywhere near to 14v during cranking.
If you are keeping the loom the same length, then use the same CSA as the original loom. If you are moving say the battery to the boot and hence increasing the length of the high current loom, then you should, if possible, go up a wire gauge. Work out the resistance of the existing loom (you should be able to look up the mOhm per meter for the wire gauge used, and try to ensure you don't end up with any more resistance than that value)
For starting applications, the insulation of the jacketing is irrelevant because we are talking about intermittent operation. You cannot run a starter continuously, either because the battery will go flat, or the starter itself will over heat.
the battery may only supply 8v when cranking however this would then mean that the current would be lower. I'd rather overspec than underspec. Also, the insulation is relevant because the cable is directly connected to the battery therefore always live, just with no current flowing through it. Without the insulation my beloved Honda could be a smouldering mess.
You de-rate the current carrying capacity for constant high temps, or when run over long distance (volt drop per metre at the design current) etc and if combined in a loom or in free air etc.
You just need a suitable flexible (multi core) cable capable of carrying 200 amps or so short duration. You'll possibly struggle to terminate and bend solid solid cables hence multi strand flexible may be used, but you may be over thinking this a bit. Speak to a good car parts /audio supplier and see what they recommend.
Found this on the net if it helps for typical
Current carrying capacity
16 sq.mm, 7.5mm OD, 110 amps
25 sq.mm, 9.8mm OD, 170 amps
40 sq.mm, 11.8mm OD, 300 amps
60 sq.mm, 14.5mm OD, 415 amps
http://www.vehicle-wiring-products.eu/product.php/...
Make sure all your connections are perfect as that's where you get added resistance etc
You just need a suitable flexible (multi core) cable capable of carrying 200 amps or so short duration. You'll possibly struggle to terminate and bend solid solid cables hence multi strand flexible may be used, but you may be over thinking this a bit. Speak to a good car parts /audio supplier and see what they recommend.
Found this on the net if it helps for typical
Current carrying capacity
16 sq.mm, 7.5mm OD, 110 amps
25 sq.mm, 9.8mm OD, 170 amps
40 sq.mm, 11.8mm OD, 300 amps
60 sq.mm, 14.5mm OD, 415 amps
http://www.vehicle-wiring-products.eu/product.php/...
Make sure all your connections are perfect as that's where you get added resistance etc
200Plus Club said:
You de-rate the current carrying capacity for constant high temps, or when run over long distance (volt drop per metre at the design current) etc and if combined in a loom or in free air etc.
You just need a suitable flexible (multi core) cable capable of carrying 200 amps or so short duration. You'll possibly struggle to terminate and bend solid solid cables hence multi strand flexible may be used, but you may be over thinking this a bit. Speak to a good car parts /audio supplier and see what they recommend.
Found this on the net if it helps for typical
Current carrying capacity
16 sq.mm, 7.5mm OD, 110 amps
25 sq.mm, 9.8mm OD, 170 amps
40 sq.mm, 11.8mm OD, 300 amps
60 sq.mm, 14.5mm OD, 415 amps
http://www.vehicle-wiring-products.eu/product.php/...
Make sure all your connections are perfect as that's where you get added resistance etc
Thank you. I think I did overthink it a bit, I was also interested in the physics side of it all - electrics is my qualified area, just not on vehicles so I was trying to weigh up the differences and although a lot of my suspicions were correct I didn't take in to account factors such as the type of conductor used (silly, really). Don't worry, I value good connections, where there is resistance, there is heat You just need a suitable flexible (multi core) cable capable of carrying 200 amps or so short duration. You'll possibly struggle to terminate and bend solid solid cables hence multi strand flexible may be used, but you may be over thinking this a bit. Speak to a good car parts /audio supplier and see what they recommend.
Found this on the net if it helps for typical
Current carrying capacity
16 sq.mm, 7.5mm OD, 110 amps
25 sq.mm, 9.8mm OD, 170 amps
40 sq.mm, 11.8mm OD, 300 amps
60 sq.mm, 14.5mm OD, 415 amps
http://www.vehicle-wiring-products.eu/product.php/...
Make sure all your connections are perfect as that's where you get added resistance etc
I appreciate the advice, thanks! Yes volts drop is king and given it's not expensive cable over short lengths then bigger is usually better.
I had to flash a starter motor at 18 and then 24v to get a brand new 6 litre v8 engine to turn over fast enough to fire once :-)
The 11kv cables we jointed and tested on Tuesday are carrying just 60amp per phase but I can guarantee a dodgy joint will result eventually in a very big bang lol.
I had to flash a starter motor at 18 and then 24v to get a brand new 6 litre v8 engine to turn over fast enough to fire once :-)
The 11kv cables we jointed and tested on Tuesday are carrying just 60amp per phase but I can guarantee a dodgy joint will result eventually in a very big bang lol.
200Plus Club said:
Yes volts drop is king and given it's not expensive cable over short lengths then bigger is usually better.
I had to flash a starter motor at 18 and then 24v to get a brand new 6 litre v8 engine to turn over fast enough to fire once :-)
The 11kv cables we jointed and tested on Tuesday are carrying just 60amp per phase but I can guarantee a dodgy joint will result eventually in a very big bang lol.
Always overrate I had to flash a starter motor at 18 and then 24v to get a brand new 6 litre v8 engine to turn over fast enough to fire once :-)
The 11kv cables we jointed and tested on Tuesday are carrying just 60amp per phase but I can guarantee a dodgy joint will result eventually in a very big bang lol.
Do you work in power transmission? I've heard all sorts of stuff from DNO's etcNot always overate no that's expensive and wasteful lol. I was only referring to this particular case of a short length of battery-starter cable.
I don't work for a DNO but I am an authorised person on HV and LV and powered up a new 11kv substation on Tuesday. Interesting day, spiked and jointed an existing ring cable that runs alongside another 11kv feed so we had to get the right cable 100% before spiking it :-)
I've worked with APs from the DNO who have blown substations up on occasion and seen colleagues burnt or killed unfortunately.
One of my friends is on big boys toys, 400kv installation maintenance :-)
I don't work for a DNO but I am an authorised person on HV and LV and powered up a new 11kv substation on Tuesday. Interesting day, spiked and jointed an existing ring cable that runs alongside another 11kv feed so we had to get the right cable 100% before spiking it :-)
I've worked with APs from the DNO who have blown substations up on occasion and seen colleagues burnt or killed unfortunately.
One of my friends is on big boys toys, 400kv installation maintenance :-)
TehRin said:
Max_Torque said:
Don't use 14v, use 8v!
The battery won't be anywhere near to 14v during cranking.
If you are keeping the loom the same length, then use the same CSA as the original loom. If you are moving say the battery to the boot and hence increasing the length of the high current loom, then you should, if possible, go up a wire gauge. Work out the resistance of the existing loom (you should be able to look up the mOhm per meter for the wire gauge used, and try to ensure you don't end up with any more resistance than that value)
For starting applications, the insulation of the jacketing is irrelevant because we are talking about intermittent operation. You cannot run a starter continuously, either because the battery will go flat, or the starter itself will over heat.
I'll use 14 The battery won't be anywhere near to 14v during cranking.
If you are keeping the loom the same length, then use the same CSA as the original loom. If you are moving say the battery to the boot and hence increasing the length of the high current loom, then you should, if possible, go up a wire gauge. Work out the resistance of the existing loom (you should be able to look up the mOhm per meter for the wire gauge used, and try to ensure you don't end up with any more resistance than that value)
For starting applications, the insulation of the jacketing is irrelevant because we are talking about intermittent operation. You cannot run a starter continuously, either because the battery will go flat, or the starter itself will over heat.
the battery may only supply 8v when cranking however this would then mean that the current would be lower. I'd rather overspec than underspec. Also, the insulation is relevant because the cable is directly connected to the battery therefore always live, just with no current flowing through it. Without the insulation my beloved Honda could be a smouldering mess.
The DC resistance of the starter motor is tiny, in the milliohms, as it has to be able to pull a huge current from a very low voltage. Once it's spinning, most of the supply voltage is used to oppose the motors Back EMF, not to drive current through it. But on a cold, static engine, the inrush current is enormous (actually the internal resistance of the battery itself become the limiting factor!).
Regarding the insulation, i mean't from a thermal perspective. Obviously you need electrical insulation between the loom and the chassis! (Although at these low voltages that insulation can really be pretty anything). For a continuous load, you need to consider the thermal impedance of the insulation, as the heat generated from the resistive losses in the copper have to escape out through the insulation (for a characteristically "long" wire). But for starting applications you have a non repetitive, intermittent duty, and the thermal inertia of the cable and insulation themselves will be sufficient to limit peak conductor temperature.
200Plus Club said:
Not always overate no that's expensive and wasteful lol. I was only referring to this particular case of a short length of battery-starter cable.
I don't work for a DNO but I am an authorised person on HV and LV and powered up a new 11kv substation on Tuesday. Interesting day, spiked and jointed an existing ring cable that runs alongside another 11kv feed so we had to get the right cable 100% before spiking it :-)
I've worked with APs from the DNO who have blown substations up on occasion and seen colleagues burnt or killed unfortunately.
One of my friends is on big boys toys, 400kv installation maintenance :-)
I don't work for a DNO but I am an authorised person on HV and LV and powered up a new 11kv substation on Tuesday. Interesting day, spiked and jointed an existing ring cable that runs alongside another 11kv feed so we had to get the right cable 100% before spiking it :-)
I've worked with APs from the DNO who have blown substations up on occasion and seen colleagues burnt or killed unfortunately.
One of my friends is on big boys toys, 400kv installation maintenance :-)
Boring (Ex) 690 lv here lol.
For the uninitiated
"High voltage is used for electric power transmission to reduce the energy lost in the resistance of the wires. For a given quantity of power transmitted, doubling the voltage will deliver the same power at only half the current. Since the power lost as heat in the wires is proportional to the wires' resistance as a share of the total resistance, and doubling voltage allows for the quadrupling of non-transmission resistance without losing power, doubling the voltage reduces the line losses per unit of electrical power delivered by approximately a factor of 4. While power lost in transmission can also be reduced by increasing the conductor size, larger conductors are heavier and more expensive."
HV is a lot more interesting than LV work for us but a lot less common, and you tend to have to spend a full day writing permits and sequences out for the job to run smoothly on the day itself. The only actual difference in the switchgear and methodology over LV is the switchgear all has an on/off/earth function. You can have cables in one of 3 states. You only ever spike or joint with a cable identified and earthed. You also pressure test generally at 5.5kv or 11kv depending on if it's old or newer cable.
The switchgear tends to look mind blowing but once you get your head around the functions they all work very similarly. To keep our ticket we have to be active annually with functional switching and 3yearly sit a weeks refresher.
LV permits on complex substations can actually be more time consuming lol, but we have to get 2 signatures per HV permit so a second person has to check your work and sign it's ok to go ahead. Saves blowing oneself and others up lol
The switchgear tends to look mind blowing but once you get your head around the functions they all work very similarly. To keep our ticket we have to be active annually with functional switching and 3yearly sit a weeks refresher.
LV permits on complex substations can actually be more time consuming lol, but we have to get 2 signatures per HV permit so a second person has to check your work and sign it's ok to go ahead. Saves blowing oneself and others up lol
200Plus Club said:
HV is a lot more interesting than LV work for us but a lot less common, and you tend to have to spend a full day writing permits and sequences out for the job to run smoothly on the day itself. The only actual difference in the switchgear and methodology over LV is the switchgear all has an on/off/earth function. You can have cables in one of 3 states. You only ever spike or joint with a cable identified and earthed. You also pressure test generally at 5.5kv or 11kv depending on if it's old or newer cable.
The switchgear tends to look mind blowing but once you get your head around the functions they all work very similarly. To keep our ticket we have to be active annually with functional switching and 3yearly sit a weeks refresher.
LV permits on complex substations can actually be more time consuming lol, but we have to get 2 signatures per HV permit so a second person has to check your work and sign it's ok to go ahead. Saves blowing oneself and others up lol
It's all there for very good reasons.The switchgear tends to look mind blowing but once you get your head around the functions they all work very similarly. To keep our ticket we have to be active annually with functional switching and 3yearly sit a weeks refresher.
LV permits on complex substations can actually be more time consuming lol, but we have to get 2 signatures per HV permit so a second person has to check your work and sign it's ok to go ahead. Saves blowing oneself and others up lol
Stay safe and keep your keys in your pocket
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