Maximum Load on Extension Lead

[jimjim999]

Hi All

Sorry to ask such a basic question [:I], but I can't find anything on the internet to explain it!

On my 4-way extension lead, I have the statement "Do not exceed the maximum load of 13AMPS".

Please can somebody explain to me how to work out if the appliances that I'm plugging in to the extension lead exceed this or not.

Again, sorry to ask such an easy question! []
Many thanks
JimJim

[LazyPup]

First of all JimJim, Please do not appologize for asking a basic question. While it may be basic for those of us who work with this information it is obviously not basic to you or many of the other readers of our forums. The only dumb question is the one you don't ask.

Somewhere on all electrical devices that actually consume electrical energy there is a manufacturers rating for the device.

Normally all devices that have a motor such as the vacuum sweeper, fans, leaf blowers, hand power tools etc, will list the actual AMP draw for that device.

Applinces that have heating elements such as kitchen appliances, hair driers, space heaters and flat irons will either have an AMP rating or it may have a WATTAGE rating.

The formula for Wattage is P=E x I
Where P= Watts
E= Electromotive force (Volts)
I= Inductive Force (AMPS)

If it has a WATTAGE rating, divide the rated WATTS by the voltage listed on the device and the answer will be the amp rating for that device.

Example ...a 1500 watt hand hair drier listed at 120V.

1500w / 120v = 12.5 Amps.

A 60watt light bulb rated at 120v.

60watt / 120v = .5amps

You could also compute the amperage from the Horsepower.

746Watts = 1 horsepower electrical.

Thus,if you have a 1/4HP motor rated for 120v you first determine the wattage by dividing the electrical horsepower constant by the horsepower fraction for the motor..

746Watts / 1/4HP = 746/ 4 = 186.5watts..

186.5watts / 120v = 1.55AMPs


Once you know the load for each device, you simply add the amp load for each device that is plugged into the circuit to find the total amp load on the circuit.

For safety one should not run extension cords at more that 80% of its rated MAXIMUM capacity.

Other safety considerations with extension cords:

An extension cord should always terminate in the same room as the source. Extension cords should not be run through doors or windows.

Extensioni cords should only be used as a temporary sorce of power. If you require a permanent source you should consider having an additional outlet installed at the point of the load.

Extension cords used on a jobsite, or to run power tools should always be plugged into a GFCI circuit or have a GFCI junction box on the source end.

[jimjim999]

Thanks for the info! [8)]

[jimjim999]

...Thanks LazyPup...for a second there, I thought you called me Mary! ...

I'm in the UK here, with 240v on the mains. I was searching the internet and other people with similar discussions suggest that for a 13amp fused extension lead it should be ok to plug stuff in as long as the total load (add up the power rating in watts) doesn't exceed 3000watts.

Does this sound about right? [?]

If so, then a monitor, computer, printer, etc shouldn't come near this.

I guess the lesson here is that you SHOULD NEVER plug things in like toasters, lawn mowers, more than 1 hairdryer, etc into extension leads!

[LazyPup]

I do appologize for having called you mary and you see i later corrected itl I was in a hurray and when you said many thanks i misread the word Many. I think i need to get my eyes examined again.


Now to your question:

Again from the formula P= E x I
Where P= watts
E = volts
I = Amps

We could then say..

Power watts = E (volts) x I (amps)

.E (vols) = P(watts) / I (amps)

Or

I (amps) = P(watts) / E(volts)

Since you know the wire is rated for 13 amps maximum and you know your line voltage is 240v you multiply the voltage x the amps to find maximum wattage, thus 13a x 240 v = 3120watts.

To insure a good margin of safety you shold then reduce the load to 80% of maximum thus you have:

3120watts x .80 = 2496watts.

You can exercise a bit of judgement here though. The 80% derating is very important if all the loads plugged into the cord are to be operated at the same time, however if you have two or three devices plugged into the cord but would only anticipate operating them one at a time, such as one man working with a circular saw, a jig saw and an electric sander, then you only need to insure that no single device exceeds the rating.

I am sure these formulas may be of great interest to our friends in the UK for you often holiday on the continent where line volages often vary country by country from 180 to 250v.

[HayZee518]

Just to add something to lazypup's discussion, in the UK and elswhere, wire size is NOT in AWG but in metric sizes. I have seen machinery made in Holland where each circuit's interconnect wiring is for whatever amps the control circuit is drawing and is fused accordingly. Realistically you can have umpteen sizes of wire other than the basic American version of 10, 12, 14, 8,6,4, 2, and "ought sizes of wires.