Conversion in IES of specific lighting connected load

[Mirou]

Hi All,

Can any body please help on how IES VE converts the specific connected loads in (lighting power density per 100Lux) W/m2/100Lux to Luminaire lumens/Watts.
I am pretty sure that the correct method of entering the lighting loads when in Part L mode (DSM) mode is W/m2/100Lux. However, the equivalent Luminaire-Lumens/Watts given while while DSM input lighting mode is actually lot worse than the manufacturer data!It is very difficult to justify a fail to job to a client when clearly the manufacturer sheet say a completely different story! If however, only the Luminaire manufacturer data are available and no lighting design report (Lux plots for instance), then I would have no choice but to use those (Luminaire manufacturer), hence a better EPC and easier compliance in some situations.

Can you please explain how IES converts the specific connected loads given in the Lux plots (W/m2@100Lux) to Luminaire-Lumens/Watts? My initial thoughts are that the specific connected loads will take the zone's area concerned (m2) and the distance of the luminaire to the working plane in the iluminance calculations, whereas, the manufacturer Lumianire-photo metric data data simply don't, as it is a measure of the light intensity regardless of the distance and the area being illuminated. Obviously the design lux plots take this process into account, and when this is converted to an equivalent luminaire efficacy, it is actually less than the initial figures given by the manufacturer. Could please confirm this is actually correct or am I barking nonsense.

thanks in advance

[RossThompson87]

Hi,

This took me a while to get my head around too.

IES uses the quadratic equation on page 18 of the NCM modelling guide to convert between Lm/W and W/m2/100 lux:

Power density per 100 lux = 2.11 + 0.008R + 0.069R^2 x (55/efficacy)

So if you enter efficacy, IES calculates the W/m2/100 lux and this ultimately gets converted in Wattage to calculate the carbon emission.

Its a bit of a funny situation because you can get completely different results for the exact same lighting design depending on how you enter the values.

My understanding is that in January 2013, the revision to the NCM modelling guide made it allowable to enter efficacy for Part L and EPCs. It is now generally my preferred method.

It does however seem a shame that entering the efficacy data only considers the fittings and not the skill in the designer arranging them.

I hope this helps

Ross

[Complex Potential]

I think we have to be a little careful here.

According to the hierarchy in the EPC Convention Document, the first and preferred method should be to input the W/m2.100lux as per the actual design if the power consumption and associated lux levels are known, which they should be for a new build, detailed design. The use of lumens per circuit watt entries should only be used as a fallback in situations where such detailed design information is not available.



If I was auditing a new building model with a full design and the assessor had used the lumens method, I would flag it up because it either means the assessor is not following the official conventions or the lighting design does not include sufficient detail. In the case of an "as constructed", new build EPC you should not be having to guess on anything, and if you are, you should be refusing to lodge the certificate.

I personally see the lumens per circuit watt input as being more applicable to existing buildings where you may know the lamp types but not the associated lux levels.

My 2 cents.

CP

[Mirou]

Hi all

thanks for the feedback
I have established that indeed the correct input at least for new buildings is the power density per 100 lux unless we are dealing with an existing EPC or a new build where there are just no lux plots
The issue is that there is a very large discrepancy between the lumens inputs when used and the power densities in w/m2 at 100 lux. the NCM by means of the IES software gives a conversion from W/m2/100 lux to lumens/ watts, which is way different from the lumens manufacturers data. Is this because the geometry and reflectance are taken into account whereas manufacturers data simply don't
Any help on this would be welcome

[Complex Potential]

Hi Mirou

A manufacturer could not quote you a W/m2/100lux figure unless they had done a sample design for your particular floor geometry. If they are quoting such figures in general terms then I would treat them with caution.

When converting from lamp lumens/watt to W/m2/100lux the NCM assumes that those lamps have been "prefectly" distributed throughout the space to achieve maximum efficiency. In reality this does not happen due to most designs being relatively poor and so calculated figures often differ.

The fact that a discrepency exists is a non issue because you use one method or the other, not both.

When doing EPC and Part L models please keep in mind that the NCM is a compliance tool only and is not even attempting to predict the true consumption of your building, nor does it pretend to.

[Mirou]

Hi CP

thanks for the feedback,
The issue is that it is now clear that using the LL/cW instead of the power densities is more favourable in achieving compliance, which put the lighting design at a disadvantage as Ross mentioned before, so I was looking at how this is being formulated by the software or more crudely which one is correct the manufacturer data or the software conversion. I think it is probably the first one (manufacturer given efficacies). If I an allow myself to speculate, the software will generate an assumed approximation of LL/cW equivalent to the figure of W/m2/100Lux inputted using the equation 7 page 20 of the NCM modelling guide.

It is also interesting and worth mentioning that since Jan 2013 (amendment to the NCM), close 76 states clearly that the "general lighting CAN be defined explicitly (by means of full design, i.e. W/m2@100Lux OR by inference". So in other words it is giving you the option to choose as long as the Wattage is reported later in the software and the data are clearly noted on the records. I am sure this will be iron out in the next coming version of compliance as every one now would favour the inference method, particularly that this is now allowed? but would make the more realistic connected loads (w/m2/100Lux) ignored

thanks all for the input

Mirou

[Complex Potential]

Interesting points. I think the inference method is allowed in terms of building regulations compliance since I doubt a BCO would take issue with it. However, an LCEA carrying out an as built EPC assessment would still be expected to follow the guidance in the EPC Conventions document which means they would be taking a risk by using inference if full data was available.

Potentially an LCEA (Low Carbon Energy Assessor) found in breach of the conventions during an audit risks losing their licence at best and possible criminal proceedings if it is determined they were willfully trying to mislead potential buyers by artificially improving a rating.

[RossThompson87]

Reading the documentation again it does look like the efficacy method is OK for 'building regulations compliance' but the EPC conventions advice against it.

Sadly it looks like we may end up in a situation where to give clients the best value 2 models are required.

One with Lm/W that can potentially satisfy planning targets, BREEAM and building regs.

A second with W/m2/100 lux for the EPC.

I also don't fully understand why IES has a different interface for SBEM and DSM lighting input. Having stepped through the calculations they both seem to follow the equations in the SBEM technical manual.

The IES DSM interface doesn't quite match the EPC conventions leaving them up to a degree of interpretation.

I'm tempted to try and get permission from my accreditation body to use efficacy to avoid this 2 model scenario