3ph help and then some!

Initforthemoney · Aug 15, 2023

Hi guys,

So, a friend of mine has a three phase 100a supply and wants to install 2 x three phase 20a fork lift chargers and a three phase car charger.

Now, he used to have a guy that did all his three phase stuff and I just did the odds and ends there but the guy has now retired and moved away so is pretty much out of contact, so he has asked me if I would look into doing it but, since qualifying a million years ago, my three phase knowledge can be read as non existent.

So, there is a three phase 100/phase supply pme which is around 60m from the workshop where the car and forklift chargers will be installed.

Now, the fork lifts will be on charge overnight and the car charger will be during the day with regards to loading.

So, with that, what sub main size should I run the 60m? I am thinking 16mm 4c swa, fed from an 80a switched fuse at the supply end, into a three phase db in the workshop and from there 2 x 20a three phase mcbs to forklift charger rotary isolators fed with 4mm 4 core swa, and a 32a three phase mcb to a 32a rotary Isolator fed with 6mm 4c swa?

Does this sound about right?

Would appreciate the help as he is an old friend and would like to help him out!

Darkwood · Aug 16, 2023

A few things to note, be very careful here, forklift chargers are pretty high on inrush currents, manufacturers guidance is a must for protective fuse/device rating and type or a lot of experience to make that call which reflects on cable size to meet Earth Loop Impedance requirements, also maybe worth pointing out forklifts shouldn't be left overnight to charge although many companies do, many insurers and H&S are clamping down on this as its a common cause of fires.
It may be wise to pass this one on, it's a hard call to give advice on just a little info provided, often the age, make and model of chargers can require different approaches to the supply circuit, I have often followed many a problematic install by others which have simply been designed wrong, requiring me to install from fresh.

pc1966 · Aug 16, 2023

A quick check on one of the on-line calculators for 80A at 5% drop has 25mm.

But as above, you really need to check what each of the units needs in terms of protection (both steady state current rating like "20A", and B/C/D curve for inrush) and if ADS can be met on MCB alone, or if RCD protection is needed for a fault. Indeed as RCD protection might well be needed anyway.

Car charges are a whole extra ball-game, might need CT for demand curtailment, etc. I really can't speak from experience here.

EDIT: The basic cable calculator is here:

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Darkwood · Aug 16, 2023

I hazard a guess you may have used online calculators for cable rating, it is likely these are 90c values but given your gear will only be rated for 70c you would need to use the regs tables for 70c rated and the associated values thus as others have expressed you need to size up and do the calcs for volts drop.
For the rating and sizing of the circuits you need to work out kva rating if the TXs in the forklift chargers then look up the recomended mcb rating from the mcb manufacturerer, this is the first job as this could reflect the sizing requirements of the cable, also a prospective short circuit current would be a starting point to know if the install may need upsizing to limit eli resistance, this is why I often discourage those with little experience in these areas from taking the jobs on.

Initforthemoney · Aug 19, 2023

Thanks guys, really appreciate the comments.

In truth, he has already had a four electricians in to quote etc, but one came back at 10k+(!) and the other two have gone incommunicado and cannot get any response from them. One other just didn't bother to show up.

The chargers and forklifts are brand new and all they have asked for is a 20A three phase socket and they install and set them up.

The car charger will only be used occasionally and will not be used as a general charger for employees or visitors cars.

This is the car charger he has already to install.

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Looking at the cable calc link above, it does suggest 16mm but will happily go to 25mm 4c, and potentially an 80a 3phase MCB upfront.

As for rcd protection of the chargers, the supplier gave no indication of them being needed, although i could substitute the upfront 80A mcb for a four pole Type F RCD?

I am going to see if i can get some proper specs of the chargers but the supplier is not really that helpful.

All i have so far is 20.81A input and 14.40kva.

westward10 · Aug 19, 2023

20A TP socket outlets are not a standard size. You need to confirm what the forklift chargers demand as your options are sockets rated at 16A or 32A, the 32A could be protected with 20A protective devices. As for RCDs use sockets which incorporate the RCD because they will need it regardless of what the forklift manufacturer states.

Initforthemoney · Aug 21, 2023

westward10 said:
20A TP socket outlets are not a standard size. You need to confirm what the forklift chargers demand as your options are sockets rated at 16A or 32A, the 32A could be protected with 20A protective devices. As for RCDs use sockets which incorporate the RCD because they will need it regardless of what the forklift manufacturer states.

Thanks for that. I’ve never seen one!

I’ll go for the 32a as you suggested and use a 20a mcb in the db.

James · Aug 21, 2023

If they are new FLT Chargers, ask what plug will be supplied with them.
it may be 4 pin or 5 pin. 16A or 32A
you don't want to install sockets for 1 of the 4 possible options and find out that "YOU GOT IT WRONG"
they may even come with an unterminated flex or even no flex, just some terminals to connect to.

you may even prefer not to put a plug and socket in the circuit and possibly avoid the need for an rcd.

Initforthemoney · Aug 21, 2023

Thanks James,

They did specially ask for just the socket so I am assuming they will connect according to what’s there.

I was thinking that the 32a 3ph+n socket fully wired and they can then wire their plug to suit whatever connections they need.

It’s been really difficult to get specific information from them for some reason!