Cable Sizing Help

scano.ie

Hey looking for some help as I am studying for repeat exam and the lecture's note seem to have stopped short of explaining how to answer this question and he is not responding to email or calls. Anyhow the question is

Eight, twin 58W fluorescent lamps are to be installed 40m away from a 230V distribution
board using PVC insulated single core copper cables in steel trunking. The circuit is run in the
trunking along with 5 other similar circuits and the ambient temperature is 35 o C. Select a
suitable size cable for the load.

If anyone could help with this question it would be greatly appreciated

2011

Here are a few tips:

1) Work out the total load in terms of kW.

2) As this is an inductive load and a power factor has not been provided you should state that you are assuming a power factor to show that you understand the concept.

3) Using the assumed power factor calculate kVA and from this the design current can be calculated.

4) Select a protective device (an MCB) that is higher than the design current. The question suggests that all 8 lights are on one circuit. In practice 8 twin fluorescent lights may trip a 6A MCB as this is an inductive load. I would select a 10A C type MCB for this to avoid nuisance tripping.

5) Have a look at ET101:2008. This will provide you with different factors that have to be taken into account to size the cable. This includes a grouping factor and high ambient temperature (both apply in your case). You will also find a table providing the CCC of PVC insulated single core copper cables with the correct installation method (steel trunking). The CCC will be reduced by the various factors (inc. grouping and temp). Remember that the revised CCC of the cable size selected must be greater than the protective device (MCB) rating.

6) Calculate volt drop & demonstrate that it is not excessive.

I am going to guess that the cable size will be 2.5mm sq. Most marks will be for showing why you selected a particular the cable size.

frankmul

Sometime a factor of 1.8 is used instead of power factor to calculate the true kW rating of discharge lighting. This takes into account power factor, control circuit loss, etc. it depends on the lecturer

.G.

If the OP is in phase 4 they won't be looking for things like KVA, PF or the fact it's an inductive load. If its a phase 6 question they might.

scano.ie

hey thanks guys for the help would anyone be able to give the formula's for it like for the load itll be 58*2*8=928W?

.G.

Are you in phase 4? I might have some stuff on it I could scan for you. PM me your email address.

2011

frankmul wrote: »

Sometime a factor of 1.8 is used instead of power factor to calculate the true kW rating of discharge lighting. This takes into account power factor, control circuit loss, etc. it depends on the lecturer

Yes, for phase 4 I think you are correct.

Same answer in terms of protective device: A 10A type C MCB

.G.

PF only gets talked about in these questions in phase 6. Unless it's changed in the last few years. It was straight forward work out the load in KW and find the current with I=P/U and go from there when I did phase 4. Only factors considered where VD,temp, grouping and installation method.

arse..biscuits

That doesn't sound like phase 4 or 6. More like an Electrical Design module????

2011

arse..biscuits wrote: »

That doesn't sound like phase 4 or 6. More like an Electrical Design module????

I am not familiar with the syllabus for any of the phases.
When do apprentices cover cable sizing and what sort of detail do they go into?

Back in my day (before the current system was in place) it was a 3 month block release each year for 4 years. I only did the standard block release for years one and two.

arse..biscuits

It's covered in Phase 4. They would do enough to cover that question in class but the past exam questions would be easier.

arse..biscuits

Here is a phase 4 question. It's not that different, but in every phase 4 question I've seen, the current is given, so no kW kVA or pf to worry about.

(1) An SWA cable supplying a 30A, 230V single-phase circuit is to be installed on a horizontally mounted cable tray. The following conditions apply to the installation:
a. The cable is in contact with three similar circuits
b. The length of the cable run is 35m.
c. The ambient temperature will not exceed 40C
d. The circuit is protected by a general-purpose (Type circuit breaker
Select the minimum size of cable for this circuit that will comply with the National Rules for Electrical Installations (Third Edition)

Paul_Mc1988

928 watts/230 volts=4.03 Amps=Ib> use this to now pick protective device
Table A61-C1 6 Amp MCB Type B = In
Table A52-E Installation method B1 or B2 > as we are using steel trunking
Table A52-H Grouping factor = .57 > as its your circuit + 5 others = 6 circuits
Table A52-G1 Temperature factor=.94> as its 35 degrees
Iz=(6/(.57*.94))=11.19amps > use this to now pick the cable size
Table A52-F9 Cable cross sectional area = 1.5mm2 > Now find volt drop per meter
of cable
Table A52-J1 Volt drop = 29 mV/ampere/meter
Volt Drop=((29*4.03*40))/1000=4.67 Volts > This is the volt drop as it does not
exceed 4% the cable is off suitable size

2011

Paul_Mc1988 wrote: »

928 watts/230 volts=4.03 Amps=Ib> use this to now pick protective device
Table A61-C1 6 Amp MCB Type B = In
Table A52-E Installation method B1 or B2 > as we are using steel trunking
Table A52-H Grouping factor = .57 > as its your circuit + 5 others = 6 circuits
Table A52-G1 Temperature factor=.94> as its 35 degrees
Iz=(6/(.57*.94))=11.19amps > use this to now pick the cable size
Table A52-F9 Cable cross sectional area = 1.5mm2 > Now find volt drop per meter
of cable
Table A52-J1 Volt drop = 29 mV/ampere/meter
Volt Drop=((29*4.03*40))/1000=4.67 Volts > This is the volt drop as it does not
exceed 4% the cable is off suitable size

I think you would be better off with a 10 amp C type MCB.

.G.

In real life probably would be but for the sake of the question it's merely trying to find out if you know how to size a cable so 6a B is fine.

mikeyjames9

superg wrote: »

In real life probably would be but for the sake of the question it's merely trying to find out if you know how to size a cable so 6a B is fine.

in real world would be a C10

that 928 is the wattage of the tubes only

doesn't include efficiency or pf

Paul_Mc1988

2011 wrote: »

I think you would be better off with a 10 amp C type MCB.

Yes in real life i would do the same but as this is an exam type question it should be answered as such :-)

mikeyjames9 wrote: »

in real world would be a C10

that 928 is the wattage of the tubes only

doesn't include efficiency or pf

This is the most basic form of this type of question. As they go on Pf and fault loop impedance is added into the questioning process. Again like i said above these questions must be answered in a very particular fashion in order to get the correct answer with regards to the ETCI rules.

2011

Paul_Mc1988 wrote: »

Yes in real life i would do the same but as this is an exam type question it should be answered as such :-)

Surley in an exam candidate should answer a question to the best of thier ability?

I take your point that allowances would be made on the basis that the apprentice is phase 4, but I think it is good to explain how this works in real life.

.G.

mikeyjames9 wrote: »

in real world would be a C10

that 928 is the wattage of the tubes only

doesn't include efficiency or pf

Neither of which are part of the question. I agreed real world would be C10. However this is about a question in an exam. Phase 4 exam questions leave no ambiguity. If it's phase 4 they aren't even part of the cable sizing syllabus. PF and transformer efficiency was phase 6. You won't get any more marks for including things you don't need to. Again, assuming it's a phase 4 question, the answer will either right, partially right and showing the correct process but mis calculating etc or just wrong and is 100% focusing on testing your ability to size a cable using the process you learned in phase 4. You'll get separate questions testing your knowledge of other things. They aren't always phrased very well mind you.

The way Paul answered this one would get 100% of the available marks for that question.

2011

superg wrote: »

Neither of which are part of the question. I agreed real world would be C10. However this is about a question in an exam.

.....and the purpose of going to college and doing exams is to learn how to work as an electrician in the real world. Although this might not be part of the syllabus at this early stage if a student has additional knowledge it would be best to use it. By not using information already learnt the student is just reinforcing how not to do things.

The way Paul answered this one would get 100% of the available marks for that question.

I agree, as would the answer with a 10 A C type MBC once the reason for selecting it is clearly explained. This can easily be explained in one sentence without getting technical.

Paul_Mc1988

2011 wrote: »

.....and the purpose of going to college and doing exams is to learn how to work as an electrician in the real world. Although this might not be part of the syllabus at this early stage if a student has additional knowledge it would be best to use it. By not using information already learnt the student is just reinforcing how not to do things.



I agree, as would the answer with a 10 A C type MBC once the reason for selecting it is clearly explained. This can easily be explained in one sentence without getting technical.

This is the problem with the apprenticeship process there is no allotment for ambiguity and the lecturers want you to answer the question in almost a robotic fashion without thinking for yourself. 2011 in any application like the question posed i would also use a 10 A C type MCB for real life applications, but i was just trying to help the OP by sticking to the answer a lecturer would want.

.G.

2011 wrote: »

.....and the purpose of going to college and doing exams is to learn how to work as an electrician in the real world. Although this might not be part of the syllabus at this early stage if a student has additional knowledge it would be best to use it. By not using information already learnt the student is just reinforcing how not to do things.

Christ.

If someone is in phase 4 they haven't already learned this knowledge therefore can't use it. You learn the knowledge required in stages. This is also why the question doesn't ask them about it. This is what I have been painstakingly trying to get across. Futile it seems.

People argue the smallest of things in this place lately, it's argumentative for the sake of it. Quite a few threads have descended into this rubbish lately, people going back and forth arguing some small point that bears no relation to the question the OP posed.

mikeyjames9

superg wrote: »

Christ.

If someone is in phase 4 they haven't already learned this knowledge therefore can't use it. You learn the knowledge required in stages. This is also why the question doesn't ask them about it. This is what I have been painstakingly trying to get across. Futile it seems.

People argue the smallest of things in this place lately, it's argumentative for the sake of it. Quite a few threads have descended into this rubbish lately, people going back and forth arguing some small point that bears no relation to the question the OP posed.

ok well maybe they should stick with incandescent lightbulbs to avoid any potential confusion with apprentices


as it was standard back in the day to use multipliers of 1.8 and 2

for design current and switchgear respectively

although those figures are high with modern fluorescent lighting

.G.

mikeyjames9 wrote: »

ok well maybe they should stick with incandescent lightbulbs to avoid any potential confusion with apprentices


as it was standard back in the day to use multipliers of 1.8 and 2

for design current and switchgear respectively

although those figures are high with modern fluorescent lighting

Like I said, questions aren't always posed very well and confusion can be cleared up with a question to the examiner on what is required and what is not. Also in my full apprenticeship under the current system I never came across the multipliers you speak of so I don't think they apply them anymore. Only factor mentioned was derating switches by 0.8 when using inductive loads. I was just trying to keep things simple for the OP as he's clearly had difficulty passing this exam once already, keeping things as "need to know" as possible would be more helpful, IMO of course. Even with a 20% increase in the quoted wattage a 6A C would still be usable but none of that matters because that wasn't asked.

arse..biscuits

A couple of points.

First off, I doubt it's phase 4 as the next repeat is in December and it's not a phase 4 question. Could the OP clarify please???

Also, lecturers do allow for the ambiguity of some questions. The lecturers don't write the questions and are often unhappy with them. I have often given full marks for completely different answers because the question was ambiguous. For example, when sizing trunking for a number a three phase circuits, however there is no mention of whether or not there is a neutral. I give full marks either way.

mikeyjames9

superg wrote: »

Like I said, questions aren't always posed very well and confusion can be cleared up with a question to the examiner on what is required and what is not. Also in my full apprenticeship under the current system I never came across the multipliers you speak of so I don't think they apply them anymore. Only factor mentioned was derating switches by 0.8 when using inductive loads. I was just trying to keep things simple for the OP as he's clearly had difficulty passing this exam once already, keeping things as "need to know" as possible would be more helpful, IMO of course. Even with a 20% increase in the quoted wattage a 6A C would still be usable but none of that matters because that wasn't asked.

the old multipliers are dated now with modern lighting efficiency

old figures below:

http://www.tlc-direct.co.uk/Book/6.2.1.htm
"For example, the steady state current demand of a 240 V circuit supplying ten 65 W fluorescent lamps would be:

I =10 x 65 x 1.8 A/240= 4.88A

Switches for circuits feeding discharge lamps must be rated at twice the current they are required to carry, unless they have been specially constructed to withstand the severe arcing resulting from the switching of such inductive and capacitive loads."

frankmul

mikeyjames9 wrote: »

Switches for circuits feeding discharge lamps must be rated at twice the current they are required to carry, unless they have been specially constructed to withstand the severe arcing resulting from the switching of such inductive and capacitive loads.

Rule 537.5.2.5 state that switches must be deflated by a factor of 0.8 unless designed for discharge lighting

mikeyjames9

frankmul wrote: »

Rule 537.5.2.5 state that switches must be deflated by a factor of 0.8 unless designed for discharge lighting

yes the quote from TLC is dated, but it was used back in the day for apprentices