Struggling with Calculating Energy Usage on 3ph

[lozarus]

Probably more of a maths problem than anything else / managing to confuse myself.

I have been asked to look at energy consumption of some temporary units. They're all electrically heated office style temporary buildings. Keeping it simple, at the origin of supply there's 3x 63a MCB's feeding distribution units which then feed clusters of the buildings.

I don't have access to monitoring equipment - this is only a rough and ready thing to give some indication using a clamp meter so proceeded as follows:

• Clamped L1, L2 and L3 individually on each 65a MCB
• Added these together to get total values for L1 L2 & L3
• I then converted to Watts using 230v as my nominal voltage
• Total W value added together

This is where I confused myself. I've used 230v as the nominal voltage because I've done this on a phase by phase process and ended up adding them all together

But it doesn't seem right? Am I missing something?

 

[newfutile]

If your running all single phase loads then your approach would seem to be correct.
is there not 3 phase meter to verify your calculations?

 

[lozarus]

If your running all single phase loads then your approach would seem to be correct.
is there not 3 phase meter to verify your calculations?

Thank you.

The site (it's a big complex with multiple buildings and an on-site sub) has one 3ph meter in its intake roomn measuring energy usage for the whole site. The temp buildings i'm looking at are just being fed from a existing 3ph board which in turn is fed from a fuse tray in the intake room - so sadly no dedicated metering for this. My only comparitor would be the energy usage for the entire site.

 

[DefyG]

Clamping is only going to give you a current reading at that particular moment in time, unless you've switched everything on (which may then be OTT), but you did say a 'rough and ready' idea!

 

[lozarus]

Clamping is only going to give you a current reading at that particular moment in time, unless you've switched everything on (which may then be OTT), but you did say a 'rough and ready' idea!

Indeed. I made htem aware of this and have told them I'm doing 2 days of sampling at predetermined times during the day to represent different usage. I know the heating times and routines throughout the day so have been able to plan my times with knowledge. They can then take an average of this I guess. It's just to forecaset some estimated costs - but it really is only rough and ready.

Should add I'm employed here full time and not selling my service - literally just doing said I'd do this to try and help out when asked about energy usage for these buildings.

 

[Julie.]

Your calculation is correct (ish).

Think of it this way, if each current was 100A at 230V you could calculate 100 x 230 (23kVA) x 3 = 69kVA
OR
100 x 400 x sqrt(3) = 69kVA
OR
100A (as measured on phase 1) x 230V (as measured on phase 1)
Plus
100A (as measured on phase 2) x 230V (as measured on phase 2)
Plus
100A (as measured on phase 3) x 230V (as measured on phase 3)

The latter would be the most accurate if you're measuring the voltage and current (ideally at the same time for all 6 measurements)

However!

You are missing the power factor in all the above examples like your question

If it's all resistive loads - not too much of an error, but where it's mixed loads, having capacitive, inductive and resistive loads, it's anyone's guess as to what an appropriate power factor would represent it!

 

[James]

If the readings are fairly stable, then your approach should give you figures that are consistent n the ball park.

 

[plugsandsparks]

I answered a similar question for the client, just looked at all the heaters (2KW per heater) and then asked for their half hour consumption figures from the energy supplier, looked at the times when there were closed up and then spotted the consumption of the heaters and external lighting, a bit of jiggery pokerry and gave them an estimate of the cost of heating 5 or 6 shipping containers/offices/welfare as the heating is left on overnight to dry out work clothes and offices are freezing first thing in the morning hence they are left on overnight.
TBH the cost of heating is directly proportional to what the thermostat is set at and the outside temp.