Connecting up site hoist.

[odawire]

Hi, Ive been asked to connect up a site hoist and Ive never done one before, Its controlled by a 110v transformer, 5 KVA. Im calculating the cable size and am confused as to weather I should use 110 or 230 for the calculation, im pretty sure its 110 but I just wanna check as its a difference of about 25 amps, thanks in advance for any replies.

 

[guest119]

Is there not 2 calcs to do?
To the tranny 230/415 then to the hoist 110v.

Boydy

 

[telectrix]

are you calculating from supply to tranny or tranny to hoist?

 

[odawire]

Yea sorry, Im calculating from supply to tranny, it has its own set of cables from there, cheers

 

[Darkwood]

You ignore the secondary side then and calculate the primary demand .... when you transform down to 110v from 230v the voltage drops by half so the current will double (approx) on the 110v side. Whichever side has the highest voltage of a transformer will have the lowest current and this will be proportionate to the ratio of primary and secondary windings.

 

[odawire]

ah ok, so I calculate at 230 thanks, that'll save me a few quid, I was thinking that it was using 5KVA at 110v so would be drawing the higher current, thanks

 

[guest119]

You would be if it hadn't came with its own predetermined secondary (110v) cables.

Boydy

 

[DPG]

ah ok, so I calculate at 230 thanks, that'll save me a few quid, I was thinking that it was using 5KVA at 110v so would be drawing the higher current, thanks

Remember Ohms law - power stays the same from input to output (minus losses). And V x I gives power. Daz

 

[Darkwood]

A 5KVA 1ph 230v transformer would be approx 21-22amp primary ....

Your issue here is your protective device you use ...if a BS60898 then inrush is a big issue and to guarantee trip free use then you would need either a 63 (b) or a 32 (d) checking your zs complies so be cautious to cable sizing ... can we assume you already have overload protection in place for the loads?

 

[odawire]

A 5KVA 1ph 230v transformer would be approx 21-22amp primary ....

Your issue here is your protective device you use ...if a BS60898 then inrush is a big issue and to guarantee trip free use then you would need either a 63 (b) or a 32 (d) checking your zs complies so be cautious to cable sizing ... can we assume you already have overload protection in place for the loads?

I was thinking of a 32 c?

 

[telectrix]

so Zs needs to be max. 0.72. for a C32, you need to ensure that it's compliant, maybe up sizing the cable accordingly.

 

[Darkwood]

My recommendation comes from manufacturers guides they guarantee trip free on inrush ...some LV/LV transformers can peak inrush 20-25 times rated current although usually 10 -15 times if you fit a 32 (c) it may well give good service trip free but their again you may get occasional or frequent tripping its a hard call when you go outside the manufacturers guides... if you have a spare mcb that rating try it and explain to customer it may trip and if so you will order the correct size... this way you pre-rmpt the issue and don't loose face..... you could always fit other fuse types instead.

 

[odawire]

Yea, Its actually not far from the supply, without having zs for type d to hand sounds like it might need a 6mm on a 32a type d, cheers guys

 

[telectrix]

Zs for a 32A type D is 0.36. depending on your Zdb, you might have a struggle.

 

[Darkwood]

Yes following tels advice get the Zdb then you can calculate the cable resistance and estimate zs before you wire it ... a larger cable may be needed if supply is good enough to start with.... alternately you can protect with hrc fusing which is not so harsh regarding zs values ..... if you don't pre-empt this before hand you may find your installs doesn't comply so heads up ... If you have a TNCS system and a good reading you may get away with a 6mm but still calc it ...next cable size up should cover you if its too close to call.

 

[odawire]

Zs for a 32A type D is 0.36. depending on your Zdb, you might have a struggle.

Ah is it that low? ok, might have to think again, cheers

 

[odawire]

Yes following tels advice get the Zdb then you can calculate the cable resistance and estimate zs before you wire it ... a larger cable may be needed if supply is good enough to start with.... alternately you can protect with hrc fusing which is not so harsh regarding zs values ..... if you don't pre-empt this before hand you may find your installs doesn't comply so heads up ... If you have a TNCS system and a good reading you may get away with a 6mm but still calc it ...next cable size up should cover you if its too close to call.

ok, maybe ill look into the hrc fuses, thanks

 

[odawire]

Update: Board Zs is 0.39! So that's type D out the window, at the mo I think its gotta be 32 type c, I don't really want unwanted tripping, Darkwood, where did you find the information you mentioned earlier about the MF's?

 

[Darkwood]

Can the equipment be classed as a fixed load? ...?

 

[odawire]

Can the equipment be classed as a fixed load? ...?

Yes, I'd class it as fixed equipment