circuit for garage hoist

[davelittlewood]

Hi guys,

I've bought a hydraulic car lift (probably similar to the one in this post, ) but it has only a 1.5 kw single phase motor driving a gear pump.

The garage is supplied by a 6.0 sq mm Pirelli SWA cable to a CU and then a 16A type B for a double socket and another mcb for the lighting circuit.

Unfortunately the garage supply is a spur from a 32A type B protected ring supplying the kitchen (not my choice!, was like that when we moved in).

I thought that the garage had its own supply from the house cu but when I went to check the mcb rating I found no dedicated circuit for the garage and by testing found it's on the kitchen ring main.

I didn't do my homework regarding starting currents otherwise i would planned this better, and I don't think that the supply to the garage will cope, even if the garage wiring was modified to make use of the incoming 6.00 sq mm.

I see that type Bs can take 3 times the nominal rating for up to 5 seconds.

At 1.5kw that's a current of 6.25A but probably has a starting current of maybe 4 times nominal, so say 25A.

This is within the 5 second rating of the 16 A type B BUT is this really protecting the wiring in the kitchen ring main?

I think I'm going to have to get a new circuit put in just for the lift.

The incoming supply to the house is only 14m away from the garage along a gravel drive so can I get the tails split and feed the garage lift circuit directly and bypass the house cu?

I'm degree qualified (mech eng) and so I can easily do the homework to calculate the maximum starting currents (and some of the other guys at work are industrial electrical engineers) BUT Part P exists AND just because I can research stuff on line doesn't mean that I know what I'm doing on something like this (I can read about how to plaster a wall, but I'm no plasterer!)

Any help would be greatly received and if I do need to get a new circuit installed I like some local recommendations (Rugby in Warks).

Cheers,
Dave

 

[davelittlewood]

Sorry,

As the subject of the query is essentially the same I didn't think I was hi-jacking.

Won't do it again.

Hang on, did my post remove all other previous comments? I hope not!


Oopse, didn't intend to mess up the previous posts.

More info.
The motor is cap start, cap run with 300uF/40uF capacitors.
The rating plate gives the current as 8A (full speed) and the internal wires are 2.5 sq mm.

ta.

 

[Robin0577]

A possible solution, using the existing SWA - You must do, or have someone do the calculations to ensure this is ok!

Have you meter tails split and a switchfuse unit installed to feed the garage, with a 30 or 40A cartridge fuse. Fuse size will depend on the length, installation method and loop impedance of your SWA. Extend your SWA, either inside the house with T&E (subject to concealed cable rules!) or outside with more SWA to connect directly to the new switchfuse.

If the garage consumer unit does not already have 30mA RCD protection, install it.

Again, depending on loop impedance, you may be able to change the MCB in the garage for a 16A or even 20A C type, which should take the starting current of your lift.

Also, check what kind of earthing system you have in the house, and if you have any metallic service pipes rising in the garage. You may be required to drop the house earth and install a separate rod for the garage.

 

[davelittlewood]

Robin,

Thanks for the info.

There are no rising metal supplies.

If the existing kitchen ring->garage circuit is not sufficient then it would be easier to dig a new trench from the main switch to the garage and lay new SWA and have two supplies in the garage. One for the existing sockets/lighting and the new one for the lift.

I'd dig the new trench and drill the holes in the concrete to get the cable in and let the electrician do the rest.

Dave

 

[ban-all-sheds]

If the existing kitchen ring->garage circuit is not sufficient

It won't be.

Then it would be easier to dig a new trench from the main switch to the garage and lay new SWA and have two supplies in the garage. One for the existing sockets/lighting and the new one for the lift.

Why not use the new supply for everything?

RCBOs in the garage CU will ensure that you won't lose more than one final circuit through an earth fault, and it gets the garage sockets and lights off the house CU.

The marginal cost of putting in larger SWA will be relatively small.

I'd dig the new trench

Useful guidance here: http://www.tlc-direct.co.uk/Book/7.13.3.htm

and drill the holes in the concrete to get the cable in

Remember you'll need fairly gentle swept bends for SWA, particularly the larger sizes. Presumably you plan to have the cable emerge hard up against a wall?

 

[JohnD]

If it's rated at 8A, how much will the surge be, and how long?

 

[davelittlewood]

Ban,

Ta.

Regarding having two supplies: I've got no idea where the SWA is connected to the kitchen ring, hence it would be a pain to remove it, I think I'd need to take out part of the kitchen units to get access (or I guess I could just disconnect the supply between SWA termination box and the CU in the garage).

Before I start digging any trenches or cutting into the concrete I'd get the electrician to advise on the route and the size of the hole needed in the concrete.
I'd probably use a diamond hole cutter to create a large diameter (maybe 4") elbow hole by drilling from both ends (if that makes sense) and then re-fill with concrete after the cable has been laid.

I put in a supply to my garage in my previous house (before Part P arrived!) and I buried that according to regs

Dave

 

[davelittlewood]

John,

Yeah, that's the question. I dunno!!

The motor is a 4 pole, ML90L4 at 1.5kW.

I can't find the data sheet for the exact motor (Dajin?, never heard of 'em) but this is close:

http://www.mikrospin.co.uk/downloads/108/1-phase-motors-technical-datasheet.ashx.

This gives the nominal current as 9.6A for a 220V supply and the starting current as 55A, where as the rating plate on mine suggests 8A but then the capacitors are not quite right.
Mine has 300uF/40uF where as the one in the pdf has 200uF/35uF.

Talking to the electrical engineers at work (we make power stations, so please leave your lights on and buy more electricity!!) the starting current could be 4 to 5 times nominal as a rough guess.

 

[JohnD]

so it would probably not trip a 32A type B MCB

I wonder how long a 13A cartidge fuse can carry a surge?

 

[bernardgreen]

As well as supplying the start up current while the motor accelerates to speed the supply has to cope with the switch on current when the voltage is applied to the winding. The switch on current is set by applied voltage and the resistance of the windings.

This transient current lasts until the opposing magnetic field is generated in the motor and the impedance of the windings becomes effective in controlling the current. Normally this transient switch on current is very short duration but can be much higher than the start up ( or stalled ) current.

You may need ( in fact you should ) notify the DNO that you are intending to use a motor greater than 1 Kw as the starting current of the motor can and often does cause voltage dips to other houses in the area.

 

[JohnD]

that's interesting

I've seen electronic controls on motors, I think to manage their speeds. Is that a "soft start motor" and would it reduce the surges?

 

[Iggifer]

that's interesting

I've seen electronic controls on motors, I think to manage their speeds. Is that a "soft start motor" and would it reduce the surges?

these are used a lot on big HVAC / extract / refrigeration units too.

The fans start up noticeably slower with these 'soft starts' fitted, some of them have some very interesting readouts on the display too

 

[SimonH2]

Hang on, did my post remove all other previous comments? I hope not!

Probably the Mod split your post off into it's own thread.

I've seen electronic controls on motors, I think to manage their speeds. Is that a "soft start motor" and would it reduce the surges?

Whether that's an option depends a lot on the load. For a fan it's not a great problem since the mechanical load increases gently with speed and so you can allow it to "wind up" under lower power.

For your hydraulic lift, it's quite likely that within a fraction of a turn you'll be up to full hydraulic pressure (and hence torque requirement) - which could be substantial if you've a car on the lift. Thus if you tried to start under lower power (ie a soft start), the motor would stall as it wouldn't have the torque to drive the load.
It may well be possible to modify the system to add a mechanical or hydraulic soft start - such as adding a small accumulator or a dump valve to reduce the hydraulic pressure while the motor is winding up. But you are into modifying the system and it's controls for that.

Unfortunately I suspect the manufacturer won't be very helpful (assuming it's new). My experience is that they tend to just specify ridiculously oversized supplies to get around the problem - I wouldn't be surprised if the manual specifies a 30A supply as a minimum. It avoids them having to consider how the best design the system - they can just make it SEP (someone else's problem).

To give an example of this, I was discussing supplies with a technical rep from a well known UPS manufacturer. At work we are limited to a 63A supply (that's the capacity of the submain from meter room to our unit) unless we shell out money to replace (upgrade) cabling. At present we have about 30A of load running, and if we were to increase significantly then I'd start worrying about the extra load from battery recharging after an outage. Eg, if we got up to (say) 45A load, and allow 10-15A for battery charging (about what I've measured during tests), then that's getting uncomfortably close to the submain fuse rating (and we have some office loads to include as well). I was specifically asking the rep about whether their units can limit the total supply current (ie sacrifice battery recharge time).
His answer was quite simple, for a UPS to supply our load they would not guarantee it with anything less than a 100A supply (*ie oversized by a factor in excess of 2), and if we went bigger then a bigger supply still would be required. He just could not see that there is anything wrong in ignoring real world constraints that customers have to work under.
So far as I can tell, this is a universal way of working - ignore the fact that real world customers have real world constraints that they may not be able to change (economically).

 

[davelittlewood]

Hang on, did my post remove all other previous comments? I hope not!

Probably the Mod split your post off into it's own thread.

Ah good, I've just joined the forum, not a good way to start!

I've found a guy on Pistonheads who's installed one of the same lists recently so I'll see what cable and connections he's used.

 

[JohnD]

I'd be tempted to try it with a 13A fused plug and see what happens.

Speculation and guesswork will only get us this far.