Oi! Nigel Calder! NO...

[pvb]

I’ve just read Nigel Calder’s article on electrical safety in December’s YM, in which he warns us that our boats are in danger of spontaneously combusting due to our lousy wiring.

Now I’ve always thought that Calder’s books were fairly well-informed, sensible bits of writing. But I think he’s gone over the top with this article.

Why? Because he warns us that using stainless steel washers in bolted connections “puts a relatively high resistance in the circuit”. How high? According to Calder, it’s so incredibly high that it “seriously impairs the performance of the equipment” and, worse still, “creates a lot of heat - more than enough in many high-current circuits to start a fire”.

This is b*ll*cks.

I don’t know what sort of “high-current” circuit Nigel has in mind, but if we assume that few cruising yachts are likely to see more than around 300A flowing through a circuit (equal to the current taken by a fair-sized bowthruster or inverter), a typical half-inch stainless steel washer will introduce a resistance which causes a voltage drop of about 2 millivolts (0.002V) which, with 300A flowing, will cause a heating effect of 600 milliwatts, about the same as a Christmas tree light! And highly unlikely to start a fire (although Calder has “seen several fires that originated this way” - really?)

Call me sceptical, but I think Nigel should drop the scare-mongering and stick to the facts.

And wouldn’t it have been nice if YM had insisted that the article talked in terms of cross-sectional area for wire sizes (the way we buy them in Europe) rather than the impenetrable AWG (American Wire Gauge) numbers?

 

[Guest]

I read that article too, and raised my eyebrows when I read that particular item. It does seem to be a little scare-mongering.

I agree with you about the use of AWG, which does seem to be gaining a foothold (in marine sales at least). I did a little research some time ago and it seems that the AWG sizes seem to slot midway between the European CSA standard sizes.

I am not an expert on crimping. AWG cable sizes fall within the tolerances of European standard size crimp terminals but it is another annoying factor to have to watch out for.

 

[Guest]

About AWG

What's wrong with it? It's easier to comprehend than the cross sectional area, just as metres per second is a nonsensical measurement for wind speed (name one aspect of that which has any relevance to a human).

Of course we could just buy our boats based on the RCD rating and volume of seawater displaced - that would tie in nicely when you go to the boatshow to view a Beneteau A912.444 (a b*****d for the marketing departments too).

 

[steve_cronin]

Dear Humperdinck,

Got most of your post but these LCD TFT screens aren't all they are cracked up to be.

I got most of the post but one bit came out - "....A912.444 (a b*****d for the marketing departments too)."

Could the corrupted word be "BASTARD"? Perfectly appropriate good old fashioned English word and seems to fit.

Please advise

Regards & Seasons Greetings

Steve Cronin

 

[Guest]

Stainless-steel Earthing washers/studs

I hope Nigel is wrong otherwise most of the air traffic control centres in the UK could go up in flames! All their Earthing is done through stainless-steel stands and washers. For the reason that the increase in resistance caused by corrosion on copper brass Etc could cause a fire. And a course the hideous price of gold.
The resistance of a wire is calculated using its cross section area times the length times the coefficient of the material. Using any other wire cage management just makes life more difficult for the poor engineer.

-{)>

 

[Guest]

Right first time but

Well, you know how politically correct Scuttlebutt is these days - nobody will name the marina which rips them off or the boat with wobbly decks. Thought Bastard was maybe "not on" these days!

Seasons Greetings!

 

[Guest]

Re: About AWG

I can always visualise the size of a conductor, given it's cross sectional area. I would only be able to visualise the size of a conductor, given only a number, if I worked regularly with those conductors and numbers - which I don't. I didn't say there was anything wrong with the AWG system. My point in support of the original post on this subject was Why mix up the systems. Look what happened to the Mars lander.

 

[pvb]

\"Easier to comprehend\"????

What planet are you on? American Wire Gauge has no logical relation to wire size. You need to know the wire size to compute voltage drop, etc. And, where's the logic in using a bigger number for a smaller wire??

Of course, as a YM contibutor, you may feel obliged to defend other YM contributors.

 

[Guest]

Re: \"Easier to comprehend\"????

pvb.....you put it correctly!! I would love to hear the "BS" and rationale behind the AWG sizing. Seems utterly stupid to issue a large number size to a small sized wire, and a small number to a large sized wire or actually "cable" such a "2 AWG" which would be full sized battery cable.

I think the appropriate word for it would be "doofus".

btw....I'm an American and I think the AWG system is lousy. I can only surmise it was made up the way it was simply to be different than the UK or European methods, just as the inches vs. metric, dollars vs. pounds, etc.....

 

[bryantee]

just read the article. we have to wait for the mag here.If I read it right ,it refers to a s/s washer interposed between the copper cable terminal and the copper bus bar.normaly a s/s scew with outside s/s washer is used to connect said cables to bus bar and if its copper to copper connexion, can't see a problem? The set up with two bus bars and the isolater switch and fuse links is a neat solution.My boat has the fuse links hanging off the switch terminals.Good excuse to nip down to the boat for a quick safety check."just read this article Dear ,better just check!!!!!"

 

[howardclark]

Agree totally - scaremongering is just a way to get an article read.

PS I am trying to work out a Bow thruster installation and am having difficulty calculating the voltage drop - you sound like someone who may know how!
If the thruster draws 560amp (12v) and the the wire is 120 sq mm whats the drop per metre? ( I know I have to add the length of the positive & negative together)

ta
Howard

 

[Twister_Ken]

There is a rationale

Used to work in the textile industry long ago. The standard measure of fibre thickness is 'denier'. The finer the fibre the higher the number, just like AWG.

There is a rational reason for this - the trouble is that 30 years on, I can't remember what it is!

 

[HaraldS]

Re: There is a rationale

I read somewhere that the system came from the 1st wire gauge that wires were pulled through, being named number 1, and then they worked their way to finer wires by running it through more stages, in between tempering it again. Halfing the cross section took abot 3 additional steps and that's about where it settled. So from AWG 2 to AWG 5 it halfs the cross section, from 5 to 8, the same happens and from 8 to 11 again and so on.
It's quite confusing though and I have made myself a little spreadsheet that calculates all the nice things, voltage drop for various loads and lengths and maximum fusing for every size.

 

[HaraldS]

82mV

 

[pvb]

Easiest way to work out voltage drop...

An easy rule of thumb for calculating voltage drop is my TOT rule. “TOT” stands for “Treble One Twenty”, and is all you need to remember. The 3 ones and the twenty work as follows:-

* 1 amp going 1 metre along a 1 sq mm wire gives a 20 mV drop.

That’s your basic figure, and all you need do now is change the 20mV figure in proportion to the other changes. So, for example:-

* an increase in current increases the 20mV proportionally
* an increase in length increases the 20mV proportionally
* but an increase in wire size decreases the 20mV proportionally.

Now before the electronics experts start, I did say that the TOT rule is only a rule of thumb. It’s about 90% accurate, which I reckon is close enough for leisure boats, and it’s more accurate with smaller wire sizes. But, and this is important, it’s easy to remember, and easy to work out (which is why I believe it’s easier to think in millivolts, as there’s less chance of confusion with the decimal point). I hope you find it useful.

In your case, the bowthruster will draw 560 amps, increasing the 20mV to a 11200mV drop. But you’re planning to use 120 sq mm cable, which reduces the voltage drop to about 93mV per metre (11200 divided by 120). Multiply by the total length of cable (positive and negative) to get the total voltage drop. If, for example, you had 12 metres of wire in the circuit (6 metres to the bowthruster and 6 metres back), this would increase the drop by a factor of 12, bringing it to 1116mV (ie 1.1V). That’s about the maximum drop you can tolerate with a 12volt thruster, so if your cable’s any longer you’ll need to look for a different solution.

Had you thought of getting a 24volt thruster, which will allow you to use longer or thinner cables? There’s a very easy way to create a 24volt supply in a 12volt boat.

 

[Guest]

Circular mils !!!

Take AWG 14 and then AWG 12 : The circular mils are 3702 and 5833. So to work out what the 'ampacity' is, you have to calculate the cross sectional area by using the definition "a circular mil is the area of a circle pf diameter one mil, or 0.001 inch."

It's much easier having an IEE wiring code that specifies capacity by mm^2 net cross sectional area so that you can calculate wire impedance by just typing in the area rather than having to calculate the area by referring to the AWG min circular-mils for the AWG size!

That's not the point. What YM Editorial (IMHO) should have done was to have had somewhere in the copy, a translation of AWG to IEE sizes so that the graphics could link EASILY with the text. (Yes Nigel Calder did do the cross sectional areas translations in the text .... but there was no understandable correlation between the graphics and the text, ie sometimes he referred to AWG and sometimes he referred to, for example, "2/0" - which we 'al'l know refers to stranding characteristics.)

BTW, there ws no mention of whether or why boat wiring should be either Type II or Type III stranding - And that very important too.

Is there an ISO on this ... ????

 

[pvb]

Er, sorry, you\'re mistaken...

Seems you’re a bit confused too! Especially when you say that Calder “sometimes referred to AWG and sometimes he referred to, for example, “2/0” - which we all know refers to stranding characteristics”. 2/0 is actually an AWG wire gauge and has nothing to do with stranding.

The AWG system uses progressively smaller numbers for progressively bigger wire. When it reaches 1 AWG (approx 40 sq mm), it then goes to 0 AWG (approx 50 sq mm), 00 AWG (approx 62 sq mm), 000 AWG (approx 81 sq mm) and 0000 AWG (approx 103 sq mm). Because these multiple-zero codes are potentially confusing, the Americans tend to use the format “x/0”, where “x” denotes the number of zeros in the AWG code. So, 2/0 represents 00 AWG which, as Calder’s text mentioned, is closest to the standard metric wire size 70 sq mm.

Your oblique reference to Type 2 and Type 3 stranding perhaps needs a bit of explanation, because most people outside the USA won’t have heard of it. Ideally, boat wiring should be made up of a large number of very thin conductors, which makes it flexible and very resistant to fatigue-failure caused by vibration. In the USA, this is called “Type 3 stranding”.

In the UK, few suppliers will know what Type 3 stranding is. However, the term “flexible” is often used to denote Type 3 wire. Larger sizes such as 70 sq mm will typically have 400-500 very fine conductors, and will feel quite flexible to handle. In contrast, Type 2 wire of the same size will require some effort to bend and will have fewer than 100 thicker conductors, each nearly 1mm diameter. We should always try to use the most flexible wire available in boat applications.

 

[davidhand]

Re: About AWG

I thought in England they measured wire in a thing called SWG.

 

[Guest]

Yes, I was mistaken

Had I continued down the table and over the page I would have found the '2/0', 3/0' and '4/0' sizes. Tucked away elsewhere is an AWG/ISO comparison of conductor cross-section areas in mm^2.

 

[Guest]

Re: About AWG

Used to work in a wire business. In practise, we used to use mm dia above about 1.6mm, and swg below. No better reason than it was easier to talk without mistakes about 32 swg than the decimal metric or imperial equivalent.

odd thing was, we would happily make and sell a metric tonne of cut wire 16swg diameter and 12 inch long, never thinking about the contradictions in that.