Just want to check my logic before I buy....

I need to add buy another MCB for the distribution board in the workshop for my lathe power supply.

Its a 3 phase supply, with Type B (dont know why as the board should be type C for industrial I know....) MCBs.

The motor is rated at 5hp (so 3750watts).

Amps = Power / 1.73xvoltagexPF

So thats about 6amps or there abouts.

My concern is over the start up current, would a 10A type B be enough to not trip out on start up, or should I go to a 15A?

Cheers,
David

Can't you put a meter through it and switch it on?

I'd say 10 would be fine though.

Regards

Rich

interesting. my 3hp compressor takes over 16amps starting up. they said to wire it into a 32amp circuit, and it kept tripping its internal circuit breaker at 16amps. turns out it should have been fitted with a 32amp one.

eta, thats single phase on my compressor mind, 3 phase is the difference isn't it?

[Edited on 4/7/09 by blakep82]

Type B MCB is motor rated

You're going to be looking at around 30A on startup, but that's for a very short time. MCB characteristics are related to the allowed surge for such loads and are type B = 2-5 times, C = 5-10 times surge and D = 10-50 times. You need to be looking at a 10A type C really.

Not sure about your sums - is that some 3-phase thing?

Otherwise, watts=volts*amps. So 3750 watts at 240 volts is 15.6 amps.

Power conversions for 3 phase are different to single phase. They take Power Factor into consideration also.

u2u sent mate

Thanks guys, I'll fit a 15A MCB and see how it goes.

Should be ample, even with the Type B MCBs as it should allow at least 30A start up current by the sounds of it.

Thanks again,
David