Watts to Amps

[NoSparks]

For single phase its easy Amps = Watts/Volts.

Is it as easy for 3 phase?

Amps = Watts/Volts/3 per phase.

or is there a root 2 in their somewhere?

I need to know to calc the voltage drop along the supply.

Thanks in advance for any offers.

 

[andyb]

P=Vx1.73xI (there is also a power factor to take into account)

So I=P/(Vx1.73)

 

[m4tty]

not being bitchy but its 1.732

 

[NoSparks]

Thanks. So from your formulea I need 10.2 amps per phase (call it 11). Now have I got this correct:

I'm allowed 6% voltage drop. 6% of 230 = 13.8volts.

at 11 amps = 1.2545 volts/amp

at 40 meters = 1.2545 / 40 = 0.0313636 volts / amp/ meter = 31 mV/A/M

and according to table 4D2B col 4 this can be achieved with 1.5mm cable.

1.5mm cable run using Reference Method C (clipped direct) can support 17.5 A. So I install a 16A 3 Phase type C MCB and off I go. Right?

Thanks for any comments.

 

[Jason Smith]

Thats far to close, a type c 16 will allow LOTS more than 17.5 amps through it ( ok i forgot how much more )for a short duration of time, if your cable max is 17.5......... I wouldnt sleep at night.

 

[SirKit Breaker]

Thats far to close, a type c 16 will allow LOTS more than 17.5 amps through it ( ok i forgot how much more )for a short duration of time, if your cable max is 17.5......... I wouldnt sleep at night.

MCB curves.

Type B 5 Times overload current rating for about 30 minutes.
Type C 10 times and type D 20 times.

Cheers............Howard

 

[andyb]

I'm allowed 6% voltage drop. 6% of 230 = 13.8volts.

Where do you get 6% from?

 

[scotsparky]

Thanks. So from your formulea I need 10.2 amps per phase (call it 11). Now have I got this correct:

I'm allowed 6% voltage drop. 6% of 230 = 13.8volts.

at 11 amps = 1.2545 volts/amp

at 40 meters = 1.2545 / 40 = 0.0313636 volts / amp/ meter = 31 mV/A/M

and according to table 4D2B col 4 this can be achieved with 1.5mm cable.

1.5mm cable run using Reference Method C (clipped direct) can support 17.5 A. So I install a 16A 3 Phase type C MCB and off I go. Right?

Thanks for any comments.

Double check your volt drop values

5% for power
3% for light

3% for lights because of the high frequency stuff being useed.

 

[WayneL]

Thanks. So from your formulea I need 10.2 amps per phase (call it 11). Now have I got this correct:

I'm allowed 6% voltage drop. 6% of 230 = 13.8volts.

at 11 amps = 1.2545 volts/amp

at 40 meters = 1.2545 / 40 = 0.0313636 volts / amp/ meter = 31 mV/A/M

and according to table 4D2B col 4 this can be achieved with 1.5mm cable.

1.5mm cable run using Reference Method C (clipped direct) can support 17.5 A. So I install a 16A 3 Phase type C MCB and off I go. Right?

Thanks for any comments.

You need to write in to the IET - You've put a whole new spin on cable selection.

I don't think you're supposed to select your cable based on volt-drop.

I thought it went something like this (briefly):

You take the design current and select your cable, taking into account the installation/reference method.
Then, having selected your cable, you select your protective device.
Then, take your design current and cable length and volt drop rating (mV/A/m) and calculate your volt drop - and compare this to what is allowed.

 

[Steve D]

Also, an alternative way to work it out is to divide by 3 then divide by 230.

Eg 12kW / 3 = 4kW 4kW / 230 = 17.39A

using the I=P/(Vx1.732) equation 12000/(400x1.732) = 17.32A


Ok, you don't get the identical answer, but it's as near as damnit.

However, I normally use the 1.732 method.

 

[gaffer]

Also, an alternative way to work it out is to divide by 3 then divide by 230.

Eg 12kW / 3 = 4kW 4kW / 230 = 17.39A

using the I=P/(Vx1.732) equation 12000/(400x1.732) = 17.32A


Ok, you don't get the identical answer, but it's as near as damnit.

However, I normally use the 1.732 method.

Close enough for me Steve! 4.0mm.

 

[NoSparks]

Well I came in to appologise for the way I expressed somthing earlier today. But since half the posts relevant to this topic have been lost I'm afraid I can't. Sorry about that.

 

[Guest123]

Well I came in to appologise for the way I expressed somthing earlier today. But since half the posts relevant to this topic have been lost I'm afraid I can't. Sorry about that.

There seems to be posts going missing everywhere at the moment.......I'll have a look into it.

 

[pevvers]

MCB curves.

Type B 5 Times overload current rating for about 30 minutes.
Type C 10 times and type D 20 times.

Cheers............Howard

Are you completely sure that these MCB's will hold with that kind of overcurrent? As far as I was aware, those overcurrents are for an instantaneous trip... Have a look at your trip curves at the back of your BRB.

 

[scotsparky]

sirkits origional post is correct it acounts for inrush current such as a motor starting ect or high frequency stuff. They will take the much larger inrush current for a longer period but not indeffinatly.