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Hi Everyone,

I'm hoping someone might be able to help answer a question which i have long pondered over. Firstly i am not an electrician, please dont hold it against me, i'm just interested to seek answers when there is something i dont quite understand.

I work in the event production industry and often have cause to run temporary electrical installs. There is nothing complex about this its just pluging leads and boxes together. We always use qualified electricians if ther is ever any hard wiring or high power requirements to check everything is safe.

My question relates to three phase power supply's. I have aksed many people this over the years and have never got a consitant answer.

It is common place to provided a 32amp three phase supply by a venue, from the mains or from a generator (im aware of power factors and will come to this later) Typically we would plug this supply straight into a 32amp three phase distribution box. On the other side of this box we would take 3 x 32amp single phase supplies. Further along these would be split into 2 x 16amp supplies and then ultimately at the end point a 13amp plug for the end user.

In order to identify what power supply we need to request on site we would normally start by working out how many amps we need. most of our equipment tends to be single phase. lets say it amounts to 12kw (50amps x 240v). We would ask the venue to either supply a 63amp single phase supply or a 32amp three phase supply.

My question is how many amps are available on a 32amp three phase supply, logic suggests upto 96 so we could draw a maximum of 32amps per phase. A total of 23.04kw. However i have been told that three phase power isnt that simple and not all the power is available, In addition we normally work out requirement using a voltage of 240 but i understand three phase to be 415v. I'm not sure what bearing this has on things.

I appreciate the answer to this question might be different if the supply is mains or generator.

I have been trying to use this calculator:

Electrical 3-phase and single phase power,current and energy calculator, free online - https://power-calculation.com/electrical-power-current-calculator-3-single-phase.php

but dont really understand how it works, i think the sqrt of 3 is involved somewhere. Seemingly when i put in a power factor of 0.8 and 415v it seems to suggest the active power equates to 18401w which means around 76.6 usable amps for our single phase appliances. Is this right or is it less that it should bedue to the power factor which i allowed for the generator? i'll be honest im stabbing around in the dark with this.

If anyone could shed any light on this i would be super grateful.
 
I'm sitting waiting for some stuff to install so here goes....

1. When a 3-phase 32A circuit is broken down into three single-phase circuits, each line feeds one circuit, so each can take 32A under normal conditions. The total current available to single-phase appliances is 3x 32 = 96A. The neutral, which serves all three single-phase circuits, does not however have to carry 96A. The magic of 3-phase power is that the neutral current generally doesn't exceed the current in any one line due to the difference in phase (timing of waveforms) of the three lines. I can explain this with some trigonometry or you can simply trust that it is true. *See below for caveat.

2. The voltage of a single-phase circuit is quoted as the voltage between line and neutral, which is 230V in the UK. However, when describing a 3-phase circuit, the convention is to state the voltage between any two line wires which is 400V in the UK. The line-neutral voltage is still 230V, regardless of whether there is one phase or three. So, when a 400V 3-phase circuit (i.e. 400V line-line and 230V line-neutral) is broken down into three single-phase circuits, each has 230V line-neutral. The ratio of 400/230 is sqrt(3) and again its significance can be explained trigonometrically, but when all your equipment is 230V you won't need to consider sqrt(3) in your calculations and again you can ascribe it to magic.

3. Some types of AC-powered equipment pass more current through than is needed to deliver the power they consume. The usable power is therefore less than the product of current and voltage as it would be in a DC circuit. To distinguish between the two, we quote (actual current) x voltage in kVA, while we still use kW for (effective useful current) x voltage. Power factor is the ratio of the two, and indicates how much of the current flowing actually delivers power to the equipment. Different kinds of equipment have different power factors, e.g. heaters typically 1.0, motors 0.8, wirewound lamp ballasts nearer 0.5. A motor with a power factor of 0.8, that consumes 10A at 230V, will only receive 10 x 0.8 x 230 = 1840W of electrical input power from the supply instead of 2300W. The wiring still has to carry the total current of 10A, so the lower the power factor, the less power available from a supply of a given current rating.

Many people confuse p.f. with efficiency, which is (useful output power) over (input power.) If the motor were 75% efficient the output power would be 0.75 x 1840, since it only receives 1840W of electrical input power.

4. There is no difference in principle between a mains supply and a generator - voltages, currents, power factors all still apply. However, generators are more fussy about what kinds of load you connect, and may impose additional limitations of their own. There is also a complicated thing about power factor with generators, that is a cost saving / specsmanship exercise on the part of the manufacturers.

Consider a 23kVA, 230V, 100A genny, which when fully loaded with 100A at a typical power factor of 0.8, will have to supply 100 x 0.8 x 230 = 18.4kW. The manufacturer fits an engine just big enough to deliver 18.4kW, but still rates the generator at 23kVA because the alternator is capable of passing 100A through its windings. If your load power factor is 0.8 or less, you wouldn't know the difference. But connect a unity power factor load that absorbs the full 23kW from its 23kVA, and the engine will stagger under the load because they've 'cheated' and assumed your power factor won't exceed 0.8, hence fitted an undersize engine for this purpose.

* There are some types of load for which the magic spell is broken, and the neutral current exceeds the highest line current. This is due to current waveform distortion and typically applies to certain electronic loads. Generally, this situation can be ignored unless very extreme conditions apply.
 
I'm sitting waiting for some stuff to install so here goes....

1. When a 3-phase 32A circuit is broken down into three single-phase circuits, each line feeds one circuit, so each can take 32A under normal conditions. The total current available to single-phase appliances is 3x 32 = 96A. The neutral, which serves all three single-phase circuits, does not however have to carry 96A. The magic of 3-phase power is that the neutral current generally doesn't exceed the current in any one line due to the difference in phase (timing of waveforms) of the three lines. I can explain this with some trigonometry or you can simply trust that it is true. *See below for caveat.

2. The voltage of a single-phase circuit is quoted as the voltage between line and neutral, which is 230V in the UK. However, when describing a 3-phase circuit, the convention is to state the voltage between any two line wires which is 400V in the UK. The line-neutral voltage is still 230V, regardless of whether there is one phase or three. So, when a 400V 3-phase circuit (i.e. 400V line-line and 230V line-neutral) is broken down into three single-phase circuits, each has 230V line-neutral. The ratio of 400/230 is sqrt(3) and again its significance can be explained trigonometrically, but when all your equipment is 230V you won't need to consider sqrt(3) in your calculations and again you can ascribe it to magic.

3. Some types of AC-powered equipment pass more current through than is needed to deliver the power they consume. The usable power is therefore less than the product of current and voltage as it would be in a DC circuit. To distinguish between the two, we quote (actual current) x voltage in kVA, while we still use kW for (effective useful current) x voltage. Power factor is the ratio of the two, and indicates how much of the current flowing actually delivers power to the equipment. Different kinds of equipment have different power factors, e.g. heaters typically 1.0, motors 0.8, wirewound lamp ballasts nearer 0.5. A motor with a power factor of 0.8, that consumes 10A at 230V, will only receive 10 x 0.8 x 230 = 1840W of electrical input power from the supply instead of 2300W. The wiring still has to carry the total current of 10A, so the lower the power factor, the less power available from a supply of a given current rating.

Many people confuse p.f. with efficiency, which is (useful output power) over (input power.) If the motor were 75% efficient the output power would be 0.75 x 1840, since it only receives 1840W of electrical input power.

4. There is no difference in principle between a mains supply and a generator - voltages, currents, power factors all still apply. However, generators are more fussy about what kinds of load you connect, and may impose additional limitations of their own. There is also a complicated thing about power factor with generators, that is a cost saving / specsmanship exercise on the part of the manufacturers.

Consider a 23kVA, 230V, 100A genny, which when fully loaded with 100A at a typical power factor of 0.8, will have to supply 100 x 0.8 x 230 = 18.4kW. The manufacturer fits an engine just big enough to deliver 18.4kW, but still rates the generator at 23kVA because the alternator is capable of passing 100A through its windings. If your load power factor is 0.8 or less, you wouldn't know the difference. But connect a unity power factor load that absorbs the full 23kW from its 23kVA, and the engine will stagger under the load because they've 'cheated' and assumed your power factor won't exceed 0.8, hence fitted an undersize engine for this purpose.

* There are some types of load for which the magic spell is broken, and the neutral current exceeds the highest line current. This is due to current waveform distortion and typically applies to certain electronic loads. Generally, this situation can be ignored unless very extreme conditions apply.
Just pipped me to that answer lucien! o_O
 
I work in the event production industry and often have cause to run temporary electrical installs. There is nothing complex about this its just pluging leads and boxes together.

I get where you're coming from, however "just plugging leads and boxes together" without careful planning will create less than ideal circumstances, similar to those who "just connect lots of extension reels and 4-way blocks together" and hope for the best.

You need to be familiar with BS7909, Code of practice for temporary electrical systems for entertainment and related purposes. From what you've described, your events require in excess of 6kVA, therefore come under the "large/complex" category, irrespective whether you or I think that running a temporary 32/3 or 63/3 is actually large, or complex!

The completed temporary system should be inspected and tested with a completion certificate and schedule of test results recorded at circuit level.

There are circumstances unique to events, which place great importance on load balancing when using local generator power. For example, think of a live stage - the band finishes their song and the lampie hits the audience blinders, pulling a sudden high demand on the LX circuits, at the same time that the sound phase has gone from a steady load to virtually nothing. The larger the generator, the more tolerant it will be, but this is what can cause a genset to stall. Google the Tom Jones Alnwick fiasco from a few years back and watch the video, you'll see exactly this happen.

If you are powering concessions, you need to be mindful that power interruptions are likely to cause a breach of contract with the promoter/venue. Therefore you really want each concession on their own RCBO for selectivity. Concs will bring along equipment in various grades of condition, ranging from downright dangerous through poor to acceptable. Caterers are the worst, as their stuff is high current draw, they always have extra items that aren't on your advanced schedule and they've usually got a widowmaker or two amongst their own kit which will make an appearance if you give them the opportunity.
 
My thoughts are that some buildings may only have 63A 3 phase
at cutout for whole building , using all 32A + a little more
will soon cause overheating at cutout.
As the building may need more than the remaining 31A on some phases to function.
(ie They need to upgrade their cutout -But for commercial it may cost)
 
Hi Everyone,

I'm hoping someone might be able to help answer a question which i have long pondered over. Firstly i am not an electrician, please dont hold it against me, i'm just interested to seek answers when there is something i dont quite understand.

I work in the event production industry and often have cause to run temporary electrical installs. There is nothing complex about this its just pluging leads and boxes together. We always use qualified electricians if ther is ever any hard wiring or high power requirements to check everything is safe.

My question relates to three phase power supply's. I have aksed many people this over the years and have never got a consitant answer.

It is common place to provided a 32amp three phase supply by a venue, from the mains or from a generator (im aware of power factors and will come to this later) Typically we would plug this supply straight into a 32amp three phase distribution box. On the other side of this box we would take 3 x 32amp single phase supplies. Further along these would be split into 2 x 16amp supplies and then ultimately at the end point a 13amp plug for the end user.

In order to identify what power supply we need to request on site we would normally start by working out how many amps we need. most of our equipment tends to be single phase. lets say it amounts to 12kw (50amps x 240v). We would ask the venue to either supply a 63amp single phase supply or a 32amp three phase supply.

My question is how many amps are available on a 32amp three phase supply, logic suggests upto 96 so we could draw a maximum of 32amps per phase. A total of 23.04kw. However i have been told that three phase power isnt that simple and not all the power is available, In addition we normally work out requirement using a voltage of 240 but i understand three phase to be 415v. I'm not sure what bearing this has on things.

I appreciate the answer to this question might be different if the supply is mains or generator.

I have been trying to use this calculator:

Electrical 3-phase and single phase power,current and energy calculator, free online - https://power-calculation.com/electrical-power-current-calculator-3-single-phase.php

but dont really understand how it works, i think the sqrt of 3 is involved somewhere. Seemingly when i put in a power factor of 0.8 and 415v it seems to suggest the active power equates to 18401w which means around 76.6 usable amps for our single phase appliances. Is this right or is it less that it should bedue to the power factor which i allowed for the generator? i'll be honest im stabbing around in the dark with this.

If anyone could shed any light on this i would be super grateful.
Three Phase Current - Simple Calculation - https://myelectrical.com/notes/entryid/8/three-phase-power-simple-calculations
May be of use to you
 
Thank you Pete,

This is both very interesting and helpful.

@TonyMitchell, you will be pleased to know of interest i have purchased a copy of BS7909 and am having a leisurely read. Thanks for the advice everyone. We always used qualified electrictions on all our electrical installations above 13amp domestic plugs so dont worry. Info provided is only becasue i enjoy learning how things work.
 

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