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Wiring

Load capacities

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Lynne09/02/2018 21:56:58
53 forum posts
23 photos

I am fitting a new NVR switch to a Naerok mill. The motor is probably 1/2 hp,.

but cannot be sure as it is not plated. Have noticed that the wiring to the motor, from the switch has 37 may be 38 strands of .007" dia. The wiring from the mains to the switch has 30 strands of .007" dia. I have tried to establish what the load carrying capacity of the cables,by going on line, but I cannot make a lot of sense from what I read. I feel sure that the electrical members on here can enlighten me. regards Lynne

ps. I did look to see if there was a topic for electrical, but could only find electronics.

duncan webster09/02/2018 22:25:50
avatar
1594 forum posts
18 photos

Are you sure you've measured that correctly? Your sizes for the 30 strand stuff give an equivalent single core diameter of only 1mm, area 0.75 sq.mm
This might be big enough (not my expertise) but it sounds pretty small to me, the lighting circuit in my house has bigger wire

 

 

Edited By duncan webster on 09/02/2018 22:26:25

Dave Daniels09/02/2018 22:28:20
64 forum posts

Sure you've counted them right ?

A 'standard' stranding for some US stuff is 33 wires / 32 AWG ( 0.008" dia. ).

This is 0.75mm^2 cable.

Other options are available .. smiley

D.

Emgee09/02/2018 23:56:25
820 forum posts
173 photos

Before metrifacation UK flexible cables were .007" diameter strands so I guess you miscounted the 40 strands
(40/007),

From memory the cable is suitable for your 1/2HP motor, provided of course it hasn't perished or if rubber sheathed and tinned wires there may be some degradation of the insulation as it is attacked by the sulphur content in the rubber covering.

Emgee

Edited By Emgee on 09/02/2018 23:57:05

Muzzer10/02/2018 00:33:41
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2793 forum posts
441 photos

Loads of types of multistrand wire, even in the UK. Over there they have loads more. It's difficult to define a "standard" wire size.

If you are bothered (and can't do the sums), see if it gets warm when you run it hard. Assuming it doesn't smoke or actually fuse, the bottom line is whether it gets too hot. Ultimately the max temperature is what is used to determine the max continuous current for wiring and connectors. Even really crappy wire will be rated at 85C max temp yet the pain threshold is only 50-55C or so. Obviously you'd want to check the wire temp with the power off.....

Murray

XD 35110/02/2018 05:19:57
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961 forum posts
38 photos

What is the main power lead rated for ?

If the mains lead is plugged into a 10 amp rated socket then use wire rated for 10 amp or higher , if it is 15 amp then match it or better it and you won't have a problem . Running cable of a higher rating ( amperage ) won't hurt anything proveded the terminals will accept larger dia wire - don't trim conductors down to fit in smaller terminals its a big no no !

Billy Bean10/02/2018 07:49:26
174 forum posts
1 photos
Conductor Size
Current
Maximum power (Watts)
0.5 mm2
3 amps
Up to 720 Watts
0.75 mm2
6 amps
Up to 1440 Watts
1.0 mm2
10 amps
Up to 2400 Watts
1.25 mm2
13 amps
Up to 3120 Watts
1.5 mm2
15 amps
Up to 3600 Watts
Billy Bean10/02/2018 07:56:24
174 forum posts
1 photos

From chart, 0.75 is adequate.

Geoff Theasby10/02/2018 08:36:16
549 forum posts
14 photos

As a rough rule of thumb, 6 x wire diameter in mm = currrent in amps.

So, for 1 mm copper, 6 x 1 = 6 amps.

Geoff

not done it yet10/02/2018 13:50:32
1961 forum posts
11 photos

Unfortunately many references to the area of the conductor are written as linear measurements. Ie. they refere to 2.5mm when they actually mean 2.5mm^2.

Cooker conductors, which are 4mm^2 in area, have diameter of about 2.2mm and can safely conduct 45Amps.

Area is proportional to the square of the diameter, so a linear calculation only holds good over a very limited range (even though most of the electrons travel in the outer layers of the conductor - as like charges repel [think here of a Faraday cage]).

Lynne10/02/2018 14:39:40
53 forum posts
23 photos

Thanks to all who have responded. Have never seen smoke, nor have I noticed the cabling getting hot. Had miscounted the number of strands of the larger cable, mainly because some had severed close to the sleeve. I have a stock of 40 strand cable, so I will upgrade the 30 strand cable to this. Regards Lynne

john fletcher 110/02/2018 15:12:49
415 forum posts

Hello Lynne, did you get a manual for your RDM 350 I have an exploded view of the machine, would you like copy ? John

john fletcher 110/02/2018 15:30:16
415 forum posts

Further to my posting above to Lynne. If you go to More Latest Post, then insert in Key Word Naerok and then open

An Other's thread, you find how you can down load an exploded view of the machine. Shortly I will be send russ details of all the "mods" I have done to my 350 in the past 25 plus years, perhaps you might also like a copy. John

not done it yet10/02/2018 18:59:10
1961 forum posts
11 photos
Posted by Lynne Have never seen smoke, nor have I noticed the cabling getting hot

You probably would not. Cable would get warm, only. Then it would thermally run away as the resistance would increase with temperature, ... causing a greater power loss in the cable, ... causing a more rapid increase in conductor resistance and quite suddenly you have real smoke and possibly a fire.

Martin Connelly10/02/2018 20:20:03
avatar
682 forum posts
71 photos

Thermal runaway occurs with resistances with a negative coefficient of resistance versus temperature. Most resistances have a positive coefficient which is why incandescent lamps do not suffer thermal runaway. Cables will increase their resistance when heated and current will decrease.

Martin C

not done it yet10/02/2018 22:40:52
1961 forum posts
11 photos

Are you suggesting I am wrong?

The current through the conductors will be generally used for doing work at the load, so the voltage apportioned across the lead will be vey low. Increasing the resistance of the lead compared to the load will transfer more voltage drop across the lead. Watts developed in the lead will be VI, so will increase. Heat produced in the lead will increase and current will not appreciably decrease for the load and lead. Then around the ever decreasing cylcle until ‘poof’.

If your argument were true, a too small lead would never cause a fire. Unfortunately history has a different story to tell!

Gordon Tarling11/02/2018 11:09:00
158 forum posts
4 photos

NDIY - I think you need to review Ohm's law. If the resistance of the lead increases, then the voltage dropped across it will increase and the current through the load will DECREASE. Martin C is right.

not done it yet11/02/2018 12:39:13
1961 forum posts
11 photos

I think you need to look at the whole circuit resistance, not only that of the lead. I do know Ohm’s Law. Do remember that the lead conductors are insulated - thermally, as well as electrically. I^2 will not halve, and the the resistance of the circuit will not double, but the resistance of the flex conductors will mean more thermal conversion in the lead. Do remember that Newton’s Law of cooling only holds for force cooled objects. Think about it a little.

I used to show a video clip of an electric fire, with a too small flex attached, to GCSE students. I hoped it was sufficiently dramatic to demonstrate that leads with too small conductors were not safe and that fuses wrre fitted to protect the flex, not the electrical item (no, necessarily, the operator).

So that makes two of you that think that using too-small flex wires will limit the current to the load sifficiently to avoid a fire? Perhaps you needed to join my GCSE classes of twenty years ago.

Billy Bean11/02/2018 13:40:33
174 forum posts
1 photos

Just a shame that sarcasm finds its way into forum posts to spoil it for others.

Alan Vos11/02/2018 13:58:22
71 forum posts
3 photos

It isn't thermal runaway. All (normal) conductors heat up to the temperature where electrical power in and thermal power out reach equilibrium (provided they don't melt or vaporise first). For a given conductor, the higher the electrical power in, the higher that temperature. In the GCSE scenario given by NDIY, that temperature is high enough to start a fire.

As NDIY said, when the lead heats up, Current does go down, but voltage dropped across the lead goes up, the result is that VI for the lead goes up. You have to apply Ohm's law to the whole system.

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