what size mcb on the secondary of a 7.5kva transformer

[magpiemorgans]

I am installing a 7.5kva transformer 240v / 110v single phase .
The primary side of the transformer is fed from a 32A type D mcb and wired in 3 core 10mm swa from a board next to the transformer .
The secondary side is wired in 10mm 3 core swa and feeds a 12 way enclosure next to the transformer.
There is one circuit in the enclosure at present , a radial feeding a 110v 16A commando socket which is protected by a 16A DP mcb and is about 5 meters from the enclosure .
My question is what size mcb should I install in the enclosure as a main switch ?
I was thinking of maybe a 125A DP mcb or a 63A DP RCCB ?
could anybody offer any advice please .

 

[Knobhead]

I/P = 31A
O/P = 68A
I’m sure you can work it out from there.

 

[TF and the wire]

as Tony has worked out, the transformer is able to supply 68amps if required. So your protection should be appropriate to that, the closest being a 63amp MCB.

A 125amp MCB would be no good as the tranny would burn out due to overload before the MCB would trip. Also the 63a rccb would provide earth leakage protection only. You need to protect it from overloads, either due to a fault or by too high a load applied. You could get a combined RCCd and MCB known as a RCBO.

 

[Knobhead]

The transformer O/L protection is usually just the supply side. So fit a RCCB as your isolation in the board and as you say the out going feeds are individually protected.

BUT I’d have a look at the size of the cable J

 

[davelerave]

i fitted a couple of similar barebones 400/110 trafos.

fitted DP mcb protecting trafo in the sec DB's.

- had to oversize the D type on the primary due to the 'magnetising current'

although you're close enough here with the D32 on the primary

-earth fault protection(if needed) is usually 10ma or less on 110v-biggest they make in 110v 10mA is 16amp rcd/rcbo

so it's usually 16a 110v rcbo's or DP mcb's

 

[mozzo]

should power factor correction be applied to calculate the correct current given we are talking KVA ? I am a bit rusty as it has been over 20 years since doing power factor correction on inductive, capacitive and transformer loads.

 

[davelerave]

you just work off kva


for your FLA and overload protection

 

[geoffa]

i'd say yes as kVA rating is only apparent power not true power.
What's the pf correction value?

 

[davelerave]

no- your trafo 'continuous rated' kva is all you need


if you were calculating loads connected to trafo -you might need to work back from KW to KVA

 

[Knobhead]

Where the hell did KVA…..KVAr Come from?

 

[magpiemorgans]

thanks for all the advice guys , I fitted an rccb on the board fed from the 110v transformer also installed a 32A rotary isolator on the supply side feeding the transformer , all cabling was in 10mm and the outgoing circuit from the board was 2.5mm . the only change was I installed a 20A DP type c in the board as there was a 16A DP on the socket just to give it discrimination .
The 32A mcb did trip twice but then held in and working ok now suppose it was the initial in-rush and now the tranny magnetized its working ok .

 

[Knobhead]

Sorry? You installed a 20A MCCB in the board. What happens when more ways are used? Given the output capability of the transformer an 80A would be more in order.
The input MCCB will give overload protection the output MCCB short circuit protection for the board.

 

[davelerave]

the next person to add a circuit will have to change it

 

[topquark]

I'd echo the comment on the cable sizing as well, 10mm? Max design load works out at just over 68.18A!

 

[magpiemorgans]

There is a 32amp type d mcb feeding the transformer via a rotary isolator both cabled in 10mm swa , then on the secondary side of the transformer to the new board cabled in 10mm swa with a 63a dp rccb main switch , then a 20A dp type c mcb feeding a radial cabled in 2.5mm swa , on the end of that is a twin 110v commando skt with built in 16A dp mcb's which feed a computer workstation in a factory

 

[Knobhead]

It’s beginning to make a bit more sense now.