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Downsizing Neutrals BS7616

03/24/2015 2:31 AM

section 524.3 permits downsizing of neutral if (among other conditions) the neutral is protected against overcurrents appropriate to its crossection area. so lets consider a very common cable: 3c x 300mm2 + 1c x 185mm2. 431.2.1 says the overcurrent detection on the neutral shall cause the line conductors to be disconnected, but not necessarily the neutral. whats the most common way to do this? for example, a CT on the neutral trips a relay controlling a 3-pole breaker for the line conductors? or how would you do it?

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#1

Re: downsizing neutrals BS7616

03/24/2015 3:01 AM

yes I meant BS7671 17th edition 2008. sorry.

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#2

Re: downsizing neutrals BS7616

03/24/2015 3:39 AM

I'd do it just as you said.

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#3

Re: Downsizing Neutrals BS7616

03/24/2015 8:04 AM

I'm not sure where you are headed with this, but there are a lot of arguments for why downsizing neutrals is a bad idea. The potential cost in equipment damage far exceeds the cost of the properly sized conductor. If you are just transmitting power, use a delta instead of a wye. If you are breaking out power in a wye, you have to realize that an unbalanced load is very likely at one time or another. Is it really a good idea to add breakers to force a balanced load or non-harmonic loading? I seriously doubt it has an advantage to anyone but the contractor who is pulling the wire.

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#6
In reply to #3

Re: Downsizing Neutrals BS7616

03/25/2015 12:08 AM

You and Crabtree are both correct.

A smaller neutral conductor can add up to big savings at the sizes OP quoted.

Reduced neutral sizing in this size range is not for short runs nor is it uncommon.

Have a look at what is strung on the poles down your street .... US doesn't reticulate utility power with 3 phase aerial bundled cabling but where it's used it's obvious.

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#9
In reply to #3

Re: Downsizing Neutrals BS7616

03/25/2015 6:14 AM

GA

Further to your excellent and to my mind very accurate post, the likelihood of dropping all three phases appears to me to be much (3x?) higher than normal. As ALL phases must be dropped if an overload is detected on the neutral. Or they must be somehow "cascaded" to drop one after the other till the neutral current is at normal or less....

Therefore, extra interruptions are basically "programmed in", whereas in a normal phase to neutral environment, only the overloaded phase would be dropped, leaving the other two in full working order...

I also see no "pluses" from such a system,only minuses.....

Has anyone here had actual long term experience of such a method of saving a dollar or two on the cable costs?

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#4

Re: Downsizing Neutrals BS7616

03/24/2015 9:37 AM

More often than not downsizing the neutral in a circuit causes more issues than the savings can justify.

Not only is there the risk of damage to the neutral wire from unbalance but also in most cases there is excess noise created in the system which can be detrimental to any/all electronic devices including lighting, computers, security monitoring, and communications.

It is acceptable to use an individual in-line neutral monitoring CT for protection or you could use a "zero-crossing" window inductive type CT mounted around the neutral conductor.

Either application would be used to trip the source breaker for circuit protection.

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#5

Re: Downsizing Neutrals BS7616

03/24/2015 1:55 PM

There's a new term for this.

It's called light-weighting.

Don't do either.

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#7

Re: Downsizing Neutrals BS7616

03/25/2015 1:11 AM

reduced neutrals are standard use in this part of the world and conform to

British Standard and to IEC. (Not NEC afaik)

ive seen it sold as 3.5c x 300mm2. for ex., here http://www.kei-ind.com/images/pdf/lt-cables/3.5core-aluminium-pvc-armoured.pdf

so i am asking about proper ways to terminate it, besides the one way i suggested, bec. its usually terminated wrong for ex., on a 4 pole breaker, or the line conductors on a 3p breaker and the neutral on the neutral bus. in fact, i have never seen it terminated according to code.

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#8
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Re: Downsizing Neutrals BS7616

03/25/2015 2:54 AM

Directly connected without an interrupt device direct to a bus bar would be to code if the protection device for its associated phase cables was sized according to the neutral's current carrying capacity. The larger phase conductors would be sized for voltage drop not the single phase Imax.

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#10

Re: Downsizing Neutrals BS7616

03/25/2015 8:13 AM

Wal, good answer, thank you Sir!

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#11

Re: Downsizing Neutrals BS7616

03/25/2015 4:21 PM

It all depends on your voltage application and only for Equipment Ground fault protection. On service voltages less than 600V (Not under 150v to Ground) and 1000A or more main service disconnect and solidly grounded neutrals. Consult NEC (Arts. 230-95 & 240-13). Usually by calculations, since the at normal full load current flow in 3PH-WYe circuits, the neutral carries only two thirds (2/3) of the phase to phase conductors current. Thus, in theory and by code, the neutral may be sized accordingly. The only reason to use the same size conductors is because of contractor's convenience or because during short circuit conditions it is convenient to size the neutral the same size as the phase conductors. In fused disconnects there is the possibility of single and two phase operation where the neutral current might be higher. Only for mains (not equipment) at or above 480/277V shall the neutral be required to use a ground fault interrupting device. Of course, on delta derived systems this is not the case. In any case, or if in doubt, consult the 2014-NEC for additional or particular rules on all applications. In emergency circuits and separately derived system not connected to ground, it is not required.

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1capybara (3); Andy Germany (1); Crabtree (1); jbourass (1); lyn (1); NotUrOrdinaryJoe (1); SHOCKHISCAN (1); Wal (2)

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