Load Calculations

[aMoon]

When calculating load calculation for sub-DBs and Main DBs breakers, as well as the final load at the end (let's say in Amps). how do you do it? Do you include a safety factor (to account for spikes) and a diversity factor (to account for the design factors - e.g. lights are only going to be used at night) ?

 

[Spoon]

It all depends on the design. Each design is calculated depending on may factors. Its hard to just put one down.
Do you have more info or are you just asking out of curiosity?

 

[aMoon]

A little bit of curiosity, but I'm just wondering what other methods people use. I'm new to the firm and I've had some experience in the Consulting Elec Eng business, but not enough (imo). I'm currently looking for new ways to do these calculations, since I was told at a meeting that I over-compensate most of the time.

In an example, how would you calculate the total load of a small house with a stove, an air conditioner, lights, plugs, oven and a geyser? I would try to split the load equally among 3-phases (since the house gets 3-phase power), but then I sometimes get confused as to how I should calculate the load for breakers, wire thickness, and total load (in Amps) at the end.

 

[Engineer54]

Marvo, is our man in South Africa, he'll be the best to advise you on how things are done over there!! lol!!

 

[aMoon]

hehe. I've been speaking to Marv all morning about MEP. He's been a great help.

 

[Spoon]

I work in Industrial Engineering but if you search on this site you will finds lots on what diversity they use on houses. Most of the houses here (UK) are on single phase and not 3 phase.

 

[aMoon]

Oh wow. Ok, that's very interesting to hear. I can't imagine doing single phase design .

 

[Engineer54]

Oh wow. Ok, that's very interesting to hear. I can't imagine doing single phase design .

You don't seem to be too clever at 3 phase design either!! lol!!
Sorry aMoon, i just couldn't resist!!:devil:

 

[aMoon]

Lol. I'll take that ... That's why I'm looking at getting help. For some reason I can't wrap my head around some of these concepts. I'm qualifed as a Mechatronics Engineer, so I deal mostly with Control Engineering & Instrumentation, and DSP/DIP.

You don't seem to be too clever at 3 phase design either!! lol!! Sorry aMoon, i just couldn't resist!!:devil:

 

[Marvo]

.......In an example, how would you calculate the total load of a small house with a stove, an air conditioner, lights, plugs, oven and a geyser? I would try to split the load equally among 3-phases (since the house gets 3-phase power), but then I sometimes get confused as to how I should calculate the load for breakers, wire thickness, and total load (in Amps) at the end.

Small and average sized houses in South Africa are generally single phase 60Amp incoming supply. There are some small houses with 3-phase supply but it's not the norm.

If it's a 3-phase DB then hopefully geysers (immersion heaters or hot water cylinders) will be spread across the phases if there's more than one. Same with air-con, heating, plug points and lighting. If the load is fairly balanced then you can't really apply diversity without using common sense, for example it's reasonable to assume the air-con and the underfloor heating loads won't be simultaneous but other diversity considerations would be best taken on a case by case basis rather than a blanket formula being applied.

Geysers (HWC's) are remotely shed in some areas via a ripple relay at the DB, in other areas they're not. This might affect peak load calcs. Also geyser timers are commonplace nowadays and can be use to prevent multiple cylinders running simultaneously as well as saving overall energy consumption.

I prefer to think of diversity as applying common sense to peak loads rather that as a 'one size fits all' figure to every installation.

 

[aMoon]

I agree with what you say about diversity --- It's never one size fits all. I'm currently busy with a school and they wanted 2 aircons each in 6 different classrooms. I was a bit confused when trying to use the phases to balance the loads. Since they were quite big aircons, when I did my calculation normally, it vastly increased the load on the entire system.
How do you calculate whether you have enough power coming in? (I know in most cases, with respect to a school, you always have enough Power). Is it a simple power calculation? and how do you do it if you have it on 3 Phases? --- Not sure if my question is silly, or if I'm just not thinking about it correctly.

Small and average sized houses in South Africa are generally single phase 60Amp incoming supply. There are some small houses with 3-phase supply but it's not the norm.

If it's a 3-phase DB then hopefully geysers (immersion heaters or hot water cylinders) will be spread across the phases if there's more than one. Same with air-con, heating, plug points and lighting. If the load is fairly balanced then you can't really apply diversity without using common sense, for example it's reasonable to assume the air-con and the underfloor heating loads won't be simultaneous but other diversity considerations would be best taken on a case by case basis rather than a blanket formula being applied.

Geysers (HWC's) are remotely shed in some areas via a ripple relay at the DB, in other areas they're not. This might affect peak load calcs. Also geyser timers are commonplace nowadays and can be use to prevent multiple cylinders running simultaneously as well as saving overall energy consumption.

I prefer to think of diversity as applying common sense to peak loads rather that as a 'one size fits all' figure to every installation.

- - - Updated - - -

I agree with what you say about diversity --- It's never one size fits all. I'm currently busy with a school and they wanted 2 aircons each in 6 different classrooms. I was a bit confused when trying to use the phases to balance the loads. Since they were quite big aircons, when I did my calculation normally, it vastly increased the load on the entire system.
How do you calculate whether you have enough power coming in? (I know in most cases, with respect to a school, you always have enough Power). Is it a simple power calculation? and how do you do it if you have it on 3 Phases? --- Not sure if my question is silly, or if I'm just not thinking about it correctly.

Small and average sized houses in South Africa are generally single phase 60Amp incoming supply. There are some small houses with 3-phase supply but it's not the norm.

If it's a 3-phase DB then hopefully geysers (immersion heaters or hot water cylinders) will be spread across the phases if there's more than one. Same with air-con, heating, plug points and lighting. If the load is fairly balanced then you can't really apply diversity without using common sense, for example it's reasonable to assume the air-con and the underfloor heating loads won't be simultaneous but other diversity considerations would be best taken on a case by case basis rather than a blanket formula being applied.

Geysers (HWC's) are remotely shed in some areas via a ripple relay at the DB, in other areas they're not. This might affect peak load calcs. Also geyser timers are commonplace nowadays and can be use to prevent multiple cylinders running simultaneously as well as saving overall energy consumption.

I prefer to think of diversity as applying common sense to peak loads rather that as a 'one size fits all' figure to every installation.

 

[Marvo]

There's very little actual design involved here, first prize would be to install 3-phase air-con units then the main load would be balanced. If they're single phase units then just install the 2 units in each classroom on separate phases and make sure you end up with 4 units on each phase overall.

You can't apply any diversity to the air-con load because it's a reasonable assumption all the units would be or could be running simultaneously. The most likely peak air-con load will be the sum of the loads of all the individual units.