IVR

[Chri5]

On calcs do you use 230v or 240v when working out load factors.

Reason I ask is that I referred to my DISQ notes and it suggests 230v and yet more seasoned posters seem to use 240v

Davy you know who I mean

 

[C&GStudent]

I use 230 all the time.

As the voltage is closer to 240 you know the current will always be a little bit less than you calculated. Which is nice!

 

[davy_owen_88]

I explained why I use 240v for some calcs yesterday, quickly followed by adam giving a better explanation (as someone always does ). In situations where you are pretty close to the limit of a cable/MCB then calculating the load at 230V might push you past the limit, when in the real world you would be ok.

Most loads assumed to be purely resistive such as showers give their ratings for both 240v and 230v. The highest rating obviously belonging to the 240v rating.

9.5kW @ 240v = 39.58A
9.5kW @ 230v = 41.30A
When the real rating at 230v is = 8.7kW so 37.82A

Nothing wrong with installing higher rated cables and higher rated MCB's but for this guys case, 40A should do.

The 'headline' rating on shower boxes is generally at 240v, at least on the ones I've seen, it says something like "9.5kw@240v, 8.7kw@230v" I suppose strictly speaking you should use the 230 rating and calc using 230 (as you would do if you only had a 230v rating), but that just seems a tad daft when the voltage is 240v in the real world, using the headline (240v) rating and using 230 in your calculation won't do any harm though, just gives you a design current figure a tad bigger than the real world one and leaving breakers with some 'headroom' is good practice because they can run cooler, etc (I believe the NEC across the pond calls for the design current to be no more than 80% of the nominal current of the breaker for reasons upon this line of thought)

EDIT: So after all that, I'd say use the rating @240v and use 240 when you find the design current

 

[bernardgreen]

I use 230 all the time.

As the voltage is closer to 240 you know the current will always be a little bit less than you calculated. Which is nice!

WRONG

The only constant is the resistance ( other then a slight variation with temperature )

Therefore if more volts then more amps and more watts.

If an item is rated as 2 Kwatts at 230 volts and is a resistive load
then the resistance is 26.45 ohms

W = V² / R which is (230 * 230) / 2000 = 26.45 ))

So the resistance is 26.45 ohms

Current when 230 volts is applied will be 230 / 26.45 = 8.69 amps
Current when 240 volts is applied will be 240 / 26.45 = 9.07 amps

 

[C&GStudent]

I use 230 all the time.

As the voltage is closer to 240 you know the current will always be a little bit less than you calculated. Which is nice!

WRONG

The only constant is the resistance ( other then a slight variation with temperature )

Therefore if more volts then more amps and more watts.

If an item is rated as 2 Kwatts at 230 volts and is a resistive load
then the resistance is 26.45 ohms

W = V² / R which is (230 * 230) / 2000 = 26.45 ))

So the resistance is 26.45 ohms

Current when 230 volts is applied will be 230 / 26.45 = 8.69 amps
Current when 240 volts is applied will be 240 / 26.45 = 9.07 amps

Nice.

 

[EddieCurrent]

Hello,
On a recent course I was informed that 230 is to be used when doing all calc's , except PFC , why I dont know .....
Ill informed ???

 

[ricicle]

Hello,
On a recent course I was informed that 230 is to be used when doing all calc's , except PFC , why I dont know .....
Ill informed ???

Using 240V for the PFC will give you the highest possible single phase PFC (or highest phase to phase when calculating)

It is an interesting subject this because if you look in the regs for max Zs for MCBs the table header says it is based on a Uo of 230V but if you look at the figures they are based on 240V (type B 5X, type C 10X, type D 20X)
If the Uo was in reality lower than 240V then less fault current would flow for a given Zs and perhaps the MCB would not trip instantaneously (still maybe quicker than 0.4 sec)

 

[EddieCurrent]

Another 'glitch' in regs !!!. ???

Like the csa of the earthing conductor on a TT system, in one chart it says min of 6mm, then in another a 2.5 is permitted !!!!
As if the 'bible' was not confusing without these tings !!!

 

[Spark123]

It is an interesting subject this because if you look in the regs for max Zs for MCBs the table header says it is based on a Uo of 230V but if you look at the figures they are based on 240V (type B 5X, type C 10X, type D 20X)
If the Uo was in reality lower than 240V then less fault current would flow for a given Zs and perhaps the MCB would not trip instantaneously (still maybe quicker than 0.4 sec)

There is a short explination in Appendix 3 which says something along the lines of max Zs = Uoc / Ia.
Uoc is the open circuit voltage of the supply transformer
Ia is the current causing operation of protective device in specified time
Uoc is presumed to be 240v where the nominal supply voltage is 230v (Uo)

 

[dingbat]

And all those nice and easy in-your-head calculations for Zsmax wil go out of the window in the 17th edition when Uoc becomes 230. (Even though it will remain 240 in real life for at least the rest of mine!)

 

[scousespark]

The nominal voltage in the UK is 230/400 (used to be 240/415) and brings us in line with Europe. The calculation for load is Power/Nominal Voltage.

Therefore for SP, Ib = Power/230

I have 3 questions to add

First, why would we use 240 to calculate Ib then use this when we calculate voltdrop and compare this with 4% of 230V?

Second, why use the declared voltage on an appliance, when this is not the nominal voltage in this country now? I know we still really have 240V, but this is not the stated value now.

Third, why complicate things further by looking at resistive loads to drop the current further? The original post dealt with a breaker for a domestic shower, not a complicated industrial install.

Is this just a case of over complication? I'm not sure there is a definite answer here, and have raised this on th IEE forum and got similar confusion - but more replies lean towards using 230.

This is an interesting thread, seeing as it concerns something which seems so simple.

 

[bernardgreen]

Third, why complicate things further by looking at resistive loads to drop the current further? The original post dealt with a breaker for a domestic shower, not a complicated industrial install.

The domestic shower element is a resistive load.

Therefore Ohms law of V = RI applies.

The confusion arises when the wattage is quoted and then assumed to be constant whatever voltage is applied to the resistor.

IF the wattage was to be constant then the resistance would have to alter with a change of applied voltage.

Nothing industrial about it.

 

[scousespark]

Morning Bernard.

I was not implying that resistive loads only apply in industry.
This question followed on from an earlier one, in which a customer had a 45A breaker for a 9.5Kw shower. He'd bought a new board with a 40A breaker and asked if this was okay.

The answer was agreed that it's fine, but Davy and I quoted different Nominal Voltages (I uses 230 - Davy uses 240). The fact the shower was a resistive load was mentioned and is a factor to take into account on existing installations, but I would not do so for a new circuit - and in this case I believe it's 230V. I know in the real world it's still 240 and will remain so for quite a while. When we harmonised there wasn't a massive replacement programme for DNO transformers

 

[dingbat]

Shower manufacturers generally give power ratings for both voltages. For example, a 7.8/8.5kW shower is 7.8kW at 230V and 8.5kW at 240V. If you work out the current for each combination, then use that to find the resistance you will see that it really is the same element!

For cable calculations you should always use the nominal 230V, but if you are given the power rating at that voltage, why not use it? Otherwise, given a power rating without a reference voltage, you should assume 230V, as this will err on the side of safety.

 

[EddieCurrent]

Hello All,

So should you use the voltage you have measured at the particular installation in these calc's ???

Just a thought.