Inputting Lighting in Watts - allowed or not?

[Complex Potential]

This has been bugging me for a bit so I was wondering if someone could clarify for me please.

Are we actually allowed to use the "W" method of inputting lighting for EPCs and Part L2A 2013 DSM assessments if our lux level is less than the notional building? The option is unlocked in the software and para 83 of the NCM modelling guide appears to suggest this is fine.

However, para 82 of the NCM modelling guide seems to suggest that we should pro-rata the W/m2 figure if our lux level is less than that in the notional building and using the direct W input would appear to circumvent this.

Take the example of a 100 sqmt retail unit with a 300lux design figure. It might achieve 2.5W/m2/100lux which, when input via the lux method, would be multiplied by the 600lux as assumed by the notional building for sales areas giving a total installed power in the actual building of 2.5*6*100 = 1500W

However, entering the Wattage directly would allow me to manually calculate based on 300lux, giving 2.5*3*100 = 750W, suddenly halving my lighting power!

I'm not normally one to look a gift horse in the mouth but that seems like one helluva get out.

Am I going mad?

CP

[Terence]

I was speaking to our consultants regarding this, and we can all see where you are coming from with this one, but still think you need to pay attention to paragraph 82 and implement that guidance, before you follow paragraph 83.

Paragraph 82 on the 2013 NCM Modelling Guide states:

“The user sets the required general power density (W/m2) to acheieve the design illuminance in each zone provided that the design illuminance is equal to or greater than the activity’s NCM Lighting level in the Activity Database. Where the design illuminance is less than the NCM Activity lighting level, the general power density will be automatically pro-rated to the NCM activity lighting level.”

You can try and persuade your accreditation scheme that you should be allowed to though!

Regards,
Terence

[Complex Potential]

Thanks Terence

I’m inclined to agree although it’s typical of the BRE to lay down a rule and then provide a way around that rule in the very next paragraph! I suspect it would take a very eagle eyed auditor to spot that one though, never mind objecting to it.

Aside from input convenience, this basically means there's very little reason to use the W input method over the lux input method because it should (theoretically) never give you the better result. It's even more true under the 2010 NCM. It kind of makes me wonder why para 83 is in there at all.

CP

[VVladinovsky]

Hi CP,

I have just found this as I'm searching for some answer for luminous efficacy conversion into watts...might start a new thread.
In the past I have been inputting lighting values into to software in Watts mostly because I was given lighting layouts, luminaries schedules and design lux levels.
I always pro rate wattage figures against NB lux levels. By using your example, if the wattage in the room is 750 Watts at 300 lux but NB is using 600 lux then the entered value should be 1500 Watt (750 *600 lux /300 lux) - to make it an equal comparison with the NB lux level.
This method is stated much more clearly in the iSBEM Technical Manual 2010 p.73 under 'where a full lighting design has been carried out:' as opposed to the current NCM modelling guide 2013.

VV

[RossThompson87]

I'm searching for some answer for luminous efficacy conversion into watts

I'm not sure if you figured this bit out yet, but I found it very confusing.

The whole thing is based on equation 7 of the modelling guide:

Power Density per 100 lux = [(1.93 +0.07R + 0.063R^2)/ MF] * (60 / Your efficacy)

That last bit isn't mentioned. But I figured out that it must be there. R is the ratio of wall area to floor area. MF is the maintenance factor.

Then if you multiply out by the illuminance and floor area you should get the Wattage value that actually gets used to calculate the carbon emissions.

I hope this helps.

Ross