Terminal Block Current Question

jackdoe · 27 Apr 2015

I understand that by using multiple wires, we could increase the surface area of the conductor and thus, allowing more current to pass from one point to another. Does that apply to terminal blocks as well? Here is an example:

Say if I had a terminal block where each contact is rated at 15A, and wish to pass 20A current through it. So if I were to use a cable rated at 20A to connect 2 separate contacts on the terminal block strip, will those 2 contacts then able to support 20A of current?

PrenticeBoyofDerry · 27 Apr 2015

The connectors are rated at the maximum current that should be carried by them, the cable size does not alter that. It is what it is, if a higher current rate is to be carried, a suitable rated connector is needed.

Unless your question is related to two totally separate circuits at 10A, being connected in to differing terminals on a connector strip?

Taylortwocities · 27 Apr 2015

You could share the load if you were using stranded conductors, but most <6mm² cable are solid conductors so your approach does not work.

bernardgreen · 27 Apr 2015

jackdoe said:
I understand that by using multiple wires, we could increase the surface area of the conductor and thus, allowing more current to pass from one point to another.

At mains frequency the skin effect is insignificant. ( skin effect = where the current tends to flow in the surface of the copper ). The increased surface area does dissipated more heat if the strands are in free air.

Terminal blocks can be connected in parallel but it is far better to use the correctly sized terminal block. Failure of one ( loose screw ) will put the entire current through the other which will then be overloaded.

There are two basic types of junction box connection.

[1] the conductors do not overlap each other and all current passes through the block

[2] the conductors do overlap each other and are compressed against each other by the clamp in the terminal block, most of the current passed cable to cable and only a fraction of the current passes through the block.

The first type are limited by the amount of metal in the block. The second type can carry higher currents based on conductor size with less metal in the block / clamp.

ericmark · 27 Apr 2015

This a terminal block

there are many factors one being the strain exerted on the block by the cable and also the size of link bars plus is it the screw or a plate pressing on the cable the one shown will take 0.5mm² 13mm² actually rated in AWG and rated current is 57A I would not think one would ever pass 57A through a 10mm cable although possible so it's really more down to cable size which will fit the block than the block as to current rating.

I suppose you could feed with three cables and draw on 20 cables so you could without passing more than 57 amp through any single cable pass far more through the block. As you go into technical data it states maximum load current 76A so I would assume that is the current the connecting bar will take?

These terminals are normally carrying only one wire each link bars are used if you want to connect say 24 cables so 12 terminals are connected together. Clearly this is not the case with a simple choc block. But to be able to feed with two or more cables it would need to be the type shown. But to be rated 15A the connectors would be very small if that type.

JohnW2 · 27 Apr 2015

jackdoe said:
Say if I had a terminal block where each contact is rated at 15A, and wish to pass 20A current through it. So if I were to use a cable rated at 20A to connect 2 separate contacts on the terminal block strip, will those 2 contacts then able to support 20A of current?

As has been said, the best approach is obviously to use a terminal block of adequate current rating - and they are so cheap that I can't really see why one wouldn't simply do that.

However, as has also been said, what you suggest is conceptually reasonable - although, as has been implied, unless the conductors are stranded (so that you can share strands between two terminals), you would have to install a 'link wire' between the two terminals to use them in parallel.

Although I shouldn't say it, in the situation you describe, I'm sure that the reality is that a "15A" connector block could happily take 20A!

Kind Regards, John

ricicle · 28 Apr 2015

It is not the surface area of the conductor that determines it's current carrying capacity but it's cross sectional area. Skin effect can be ignored in the most part for sizes smaller than 16mm² at 50Hz.

JohnW2 · 28 Apr 2015

ricicle said:
It is not the surface area of the conductor that determines it's current carrying capacity but it's cross sectional area. Skin effect can be ignored in the most part for sizes smaller than 16mm² at 50Hz.

Skin effect is certainly irrelevant at 50Hz, but the surface area (particularly the surface area: CSA ratio) is a very important determinant of CCC. As CSA increases, surface area becomes smaller in relation to CSA, so that heat dissipation from surface decreases in relation to heat production (which depends upon CSA) - hence CCC relatively decreases. Hence (Table 4D5 Method C figures):

4 x 1mm² conductors in parallel have a total CCC of 64A , but 1 x 4mm² has a CCC of only 37A
6 x 1mm² conductors in parallel have a total CCC of 96A, but 1 x 6mm² has a CCC of only 47A
10 x 1mm² conductors in parallel have a total CCC of 160A, but 1 x 10mm² has a CCC of only 64A
16 x 1mm² conductors in parallel have a total CCC of 256A, but 1 x 16mm² has a CCC of only 85A

(Although all the conductors (4mm² upwards) are stranded, the effective surface area for heat dissipation is that of the overall bundle of strands, so the stranding does not appreciably increase the CCC)

Kind Regards, John

ban-all-sheds · 28 Apr 2015

Might be more informative to compare (by extrapolation) the CCC of, for example, 7 x 0.57mm² with 4mm²

JohnW2 · 28 Apr 2015

ban-all-sheds said:
Might be more informative to compare (by extrapolation) the CCC of, for example, 7 x 0.57mm² with 4mm²

I think the comparisons I presented were adequate to illustrate my point - which was that CCC is not linearly related to CSA, presumably mainly because the surface area:CSA ratio decreases as CSA increases.

As for what you suggest, it would only be possible if one did a lot of research and/or 'reverse engineering' in order to determine how the extrapolation could be done. 7 separate 0.57mm² conductors in parallel would undoubtedly have a much larger CCC than the 7 x 0.57mm² bundled strands in 4mm² cable but, on the basis of what I currently know, any attempt to determine the CCC of a 0.57mm² conductor would be little more than a guess. If one naively (and obviously incorrectly) took it to be 0.57 times the CCC of 1mm² (i.e. about 9.1A), then 7 (separate) x 0.57mm² in parallel would have a CCC of about 64A (as compared with 37A for standard 4mm²). However, one would suspect that 9.1A is probably an underestimate of the CCC of 0.57mm², so the true answer (for 7 x separate 0.57mm²) is probably greater than 64A.

Kind Regards, John

ricicle · 28 Apr 2015

My inference was that we do not initially choose conductors based upon their surface area only by their csa.

JohnW2 · 28 Apr 2015

ricicle said:
My inference was that we do not initially choose conductors based upon their surface area only by their csa.

Fair enough (and that's obviously true) - but it's not what you wrote

Kind Regards, John

ban-all-sheds · 28 Apr 2015

JohnW2 said:
As for what you suggest, it would only be possible if one did a lot of research and/or 'reverse engineering' in order to determine how the extrapolation could be done.

I'd be happy with a curve of the known ratings for single conductors, and a bit of Mk I Eyeball interpolation (apologies for saying extrapolation earlier) to determine intermediate values.

7 separate 0.57mm² conductors in parallel would undoubtedly have a much larger CCC than the 7 x 0.57mm² bundled strands in 4mm² cable but, on the basis of what I currently know, any attempt to determine the CCC of a 0.57mm² conductor would be little more than a guess.

I think it would be more than a guess.

JohnW2 · 28 Apr 2015

ban-all-sheds said:
JohnW2 said:

As for what you suggest, it would only be possible if one did a lot of research and/or 'reverse engineering' in order to determine how the extrapolation could be done.

Click to expand...

I'd be happy with a curve of the known ratings for single conductors, and a bit of Mk I Eyeball interpolation (apologies for saying extrapolation earlier) to determine intermediate values.

If it were interpolation, I would be much more comfortable - but you were surely right first time, in saying that extrapolation would be required. If we're talking about BS7671 tables, AFAIAA (and I suppose reasonably, given the minimum cable sizes per BS7671) none of them go below 1mm², so one would have to extrapolate to get a CCC for 0.57mm², wouldn't one?

ban-all-sheds said:
... on the basis of what I currently know, any attempt to determine the CCC of a 0.57mm² conductor would be little more than a guess.

Click to expand...

I think it would be more than a guess.

OK, a bit more than a guess. I've already given you what I regard as a 'lower bound' ... Given the 'surface area' business, I feel sure that the 'linear extrapolation' which gives the CCC of 0.57mm² as ~9.1A is an underestimate, so the overall CCC of 7 separate 0.57mm² conductors in parallel is going to be in excess of 64A (c.f. 37A for 4mm²).

I'll see if the curve for 1mm² - 16mm² can be reasonably modelled (or rationalised!), so that the estimate can be refined a bit.

Kind Regards, John

plugwash · 28 Apr 2015

ban-all-sheds said:
7 separate 0.57mm² conductors in parallel would undoubtedly have a much larger CCC than the 7 x 0.57mm² bundled strands in 4mm² cable but, on the basis of what I currently know, any attempt to determine the CCC of a 0.57mm² conductor would be little more than a guess.

Click to expand...

I think it would be more than a guess.

As I posted on a previous thread the accepted current ratings for small flex are weird, based on the heat dissipation argument one would expect sublinear increases with size but we actually see superlinear increases. I don't think accepted current ratings are tabulated for fixed wiring cables below 1mm.

When I tried to analyse things from first principles I got a power law with a power of 0.75. Comparing with BS7671 current ratings showed that the BS7671 ratings were more sublinear than that. I didn't go as far as trying to fit a power law to the BS7671 ratings but I would be interested in the results if anyone does try.

//www.diynot.com/diy/threads/2kw-fan-heaters-wired-with-2-core-075mm-flex.409828/page-2#3175368
//www.diynot.com/diy/threads/5a-bs-1363-plug.423426/page-6#3290853

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Terminal Block Current Question

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