Voltage Drop Calculation

[clive2016]

Hi can someone out there put me straight please.


I want to fit a longer lead to a domestic tumble dryer, the mains voltage is 230 volt.

The connection rating of the dryer is 2600 watts, so I would say I’m looking at 11.304 amps current draw.

The new lead will be 2.5 metres long twin and earth pvc covered cable with a 3 pin plug on the end which will simply be plugged into a conventional wall socket outlet. ie it won’t be buried in the wall, running with other cables or covered by loft insulation .

Obviously, there is a maximum permissible for voltage drop as there is for any cable.

Can someone show me how to work this out, showing if the new cable would be acceptable for this application, Iv'e no doubt it will be acceptable but I would like to do the sums

My thanks to anyone who doesn't mind doing a little maths

Clive

 

[Nigel Goodwin]

You simply work it out using ohms law V=IxR

You can find the resistance per metre in the cable specifications.

However, nothing to work out, just do it - it's perfectly fine - you don't waste your time working out drops across mains cables, it's already long since been done for you in the specification of the cables.

I'm pretty sure no electrician would even think about working out the drop - if an electrican would even know ohms law

 

[rjenkinsgb]

Cables are rated by current handling capability. You just get a type that has a current rating above that of the load or appliance.

I'd use a 1.5mm^2 type; that's rated a bit higher than is essential, but the difference in cost is likely to be minimal from the 1.25mm^2 that would be the smallest possible one you could use.

Trying to work it on acceptable voltage drop is a really bad idea, in that you could well be setting yourself up for a fire!
eg. If the cable was reducing the load voltage by 10% the appliance would likely still work just the same - but the cable would be dissipating around 10% of the total load, over 200W of heat.

That's why reel-style extension cables are supposed to be completely unspooled when in use.
They can create enough heat in the cable for it to melt or catch fire eventually, if it's still all rolled on the spool so it cannot dissipate the small amount of heat it produces due to resistive losses.

 

[Nigel Goodwin]

Cables are rated by current handling capability. You just get a type that has a current rating above that of the load or appliance.

I'd use a 1.5mm^2 type; that's rated a bit higher than is essential, but the difference in cost is likely to be minimal from the 1.25mm^2 that would be the smallest possible one you could use.

I must admit, I thought he said he was using 2.5mm twin and earth - rereading it I see he's using 2.5m of cable length.

I would strongly suggest he uses 2.5mm twin and earth rather than thinner cable, as a tumble dryer is a high current load.

 

[ChrisP58]

In this case, the wire should be chosen based on the current rating of the circuit breaker feeding the device, not the needs of the device.

If there is a fault in the device, you want the circuit breaker to trip before the wire melts down.

 

[Diver300]

In this case, the wire should be chosen based on the current rating of the circuit breaker feeding the device, not the needs of the device.

If there is a fault in the device, you want the circuit breaker to trip before the wire melts down.

As he's putting a plug on the end, that's going to be a UK plug with a 13A fuse in it, so that's close to the current rating of the dryer.

"Twin and earth" implies the solid cored mains cable, not flex, and I think it would be better to use flex anywhere that can get moved. For an 11 A load, 1.5 mm2 flex would be the obvious cable to use:- https://www.tlc-direct.co.uk/Products/CA1dot5F3B.html

 

[Pommie]

As he's putting a plug on the end, that's going to be a UK plug with a 13A fuse in it, so that's close to the current rating of the dryer.

How do you know it's the U.K.? Many countries use 230V. Of topic slightly, the U.K. 13A plug is an outdated relic. I've memories of burnt plastic around the live pin due to the heat from the fuse. It was probably a good idea at the time but modern breakers make it redundant.

Mike.

 

[ChrisP58]

How do you know it's the U.K.? Many countries use 230V. Of topic slightly, the U.K. 13A plug is an outdated relic. I've memories of burnt plastic around the live pin due to the heat from the fuse. It was probably a good idea at the time but modern breakers make it redundant.

Mike.

The OP's profile says "From UK"

 

[rjenkinsgb]

It was probably a good idea at the time but modern breakers make it redundant.

The circuits are normally 30A rated rings or thereabouts - the plug fuses are essential to protect the appliance-side wiring and switches etc. from overload.

 

[clive2016]

Thank you for your answers guys


The tumble dryer came with a 1.5mm cable with moulded on 13 amp fused mains plug, which in turn is plugged into a socket outlet which is part of a ring main protected by 32amp miniature circuit breaker which is in a split consumer unit ie two rcd’s, the supply is TN-C-S.


I think I’ll just change the 1.5mm flex for a 2.5 metre long 1.5mm flex, I’m sure the voltage drop in that length of cable will be negligible.

I understand 1.5mm should carry up to 15 amps, the machine consumption is only 2600 watts, so it should be more than enough.

I was just interested in working out what the voltage drop would be for the application and was hoping someone would do the maths using the figures I'd given.

 

[clive2016]

Nigel

I understand 1.5mm but what's does ^2 type mean?
All Iv'e seen is its to do with charging electric cars.

 

[Pommie]

Nigel

I understand 1.5mm but what's does ^2 type mean?
All Iv'e seen is its to do with charging electric cars.

To the power of 2 or squared. I.E. 1.5 square millimeters.

Mike.

 

[rjenkinsgb]

but what's does ^2 type mean?

As pommie says, it's typable or programming shorthand for "squared", to the power of (^) two.

Copper cables are rated by "CSA" - Cross-sectional area, in square millimetres for common metric-size cables.

The copper area of the conductor directly relates to how much current they can handle (and the resistance per unit length).
The resistance is directly proportional, but the current rating does not increase as fast, because a larger cable cannot dissipate heat as well.

eg. See the heading of the first column in the info:

https://www.reconelectrical.co.uk/image/data/pdfs/Cable%20Current%20Carrying%20Capacities.pdf

 

[Diver300]

The voltage drop is negligible.

If you are interested, copper has a resistance of 16.78 nΩ·m

So 1 m of wire with a cross section of 1.5 mm2 (1.5 square mm) will have a resistance of 16.78E-9/1.5e-6 Ω or about 11.1 mΩ

The current is taken in the live and neutral wires, so the voltage drop is for 2 wires, and at 11 A the total will be 0.24 V/m

2.5 m of that will give a voltage drop of about 0.6 V, or about 0.25% of the supply voltage. The cable will generate around 6.6 W of heat.

The voltage drop will be a bit larger if the temperature is higher as the resistance goes up.

 

[Nigel Goodwin]

How do you know it's the U.K.? Many countries use 230V. Of topic slightly, the U.K. 13A plug is an outdated relic. I've memories of burnt plastic around the live pin due to the heat from the fuse. It was probably a good idea at the time but modern breakers make it redundant.

Still a good idea now, and probably one of the best plugs in the world?. If you were seeing burnt pins you were doing something pretty wrong As a plug it's designed to work with the UK infrastructure, which uses ring mains, so a plug fuse is an integral part of that infrastructure.

Usual causes for overheating would be loose screws in the plug, or loose fuse connections - on heavily loaded plugs, and at 3KW capability it's higher than most other plugs. Interestingly South Africa use the old UK 15A plugs, round pins no fuse.

 

[Pommie]

Usual causes for overheating would be loose screws in the plug, or loose fuse connections

Interestingly, I only ever saw it on the live pin and that's the one pin that has no screws! A loose fuse connection or a fuse running near it's rating, I assume, is more likely. These plugs were always on heaters, probably rated at or above 2kW. Interestingly, here in Australia they use spurs to their outlets, no fused plugs, plug connectors 6mm x 1.5mm, plugs in bathrooms etc. I thought it mad when I first got here but electrocution just doesn't happen. AND, biggest plus, they don't hurt like **** when you stand on them.

Mike.

 

[KeepItSimpleStupid]

Let's just say that in the US, voltage drop is used along with other factors. Some factors are:
a) FLA or Full Load Amps
b) continuous (an oven in a restaurant is a continuous load. In a home it's not)
c) Current carrying conductors in a raceway
d) % fill
e) Temperature

The basic equation is R=pL/A; the p is the greek letter Rho, p is a material property of say copper; L is 2x the length; A is the cross-sectional area of the
conductor. One might use a calculator or a table with ohms/1000 ft. You can still find the resistance of 2x the length,
You also need V=I*R

The general rule in the US is <3% drop. The device needs to draw less than or equal to 80% of the breaker rating.

There now is a tendency to now use 4-wire hookups for 240V. L1, L2, G and N where L1 and L2 are fused and broken with a switch. 3-wire dryers and 3-2ire water heaters were permissable, BUT not you can have power vented water heaters requireing 120 VAC and soves require a bit of 120 VAC and therefore need a neutral. New work now should also use 3 conductors and ground to a a witch. This allows the use of automation controllers requiring a neutral.

 

[Pommie]

I'm only used to single (230V) and 3 phase (415V) systems. I'm guessing the US uses 2 phases so they have 230V available.

Mike.

 

[Nigel Goodwin]

Interestingly, I only ever saw it on the live pin and that's the one pin that has no screws! A loose fuse connection or a fuse running near it's rating, I assume, is more likely. These plugs were always on heaters, probably rated at or above 2kW. Interestingly, here in Australia they use spurs to their outlets, no fused plugs, plug connectors 6mm x 1.5mm, plugs in bathrooms etc. I thought it mad when I first got here but electrocution just doesn't happen. AND, biggest plus, they don't hurt like **** when you stand on them.

Spurs can be used in the UK as well, and in fact you can run a spur off a ring main - but 'generally' you would have ring mains for the power sockets.

So how do people not get electrocuted in Oz then?, they even get electrocuted in the US with their low voltage mains.

 

[KeepItSimpleStupid]

I'm only used to single (230V) and 3 phase (415V) systems. I'm guessing the US uses 2 phases so they have 230V available.

Mike.

The US householed power is generally called 240/120 single-phase. In the details, it's really split phase. The primary of the transformer near the house is supplied with single phase HV. The secondary is a (L1)120-0-120(L2) Center-tapped transformer. The zero(0) becomes the Neutral. Neutral and the ground rod are connected at one point and this becomes the reference to the house.

There is 240 single phase from end to end and 120 single phase from each 0 to an end. Neutral carries the unbalanced current. L1 and L2 are 180 degrees out of phase. This is not two-phase power. The power source needs L! and L2 to have a breaker. The breakers are mechanically tied together, so if one trip, the other one does too.

The domestic outlets are 120V typically. We had a 240 outlet installed for a hotel type AC unit that was used in the house. Electric stoves, all electric water heaters, electric dryers, electric furnaces and central air is usually 240 V.