Advice on 3Phase Hob

[smokeyc]

Hi there

I have not done a huge amount of work on 3phase.

I am installing a Induction Hob as part of a new Kitchen refurb- to be specific the hob is NEFF T59FSRX2.
It is a 11,100W with the ability to power limit and support single or three phase connection.

NEFF have not provided great support, so I've made an assumption that the power is distributed evenly amongst the phases.
I think it draws roughly 15A per phase, 19A when you add margin into it.
Forgive ignorance as I say the manuals are not overly informative for electrical demand or even how it selects between single and three phase.

The install I am doing will be in separate conduits surrounded by 50mm -100mm rockwool. (50mm wall, 100mm ceilings) for a 25m run.

So I have chosen 10mm singles and a protective device rating of 20A.

Does this sound correct?

 

[nicebutdim]

From NEFF:

Connected load: 11.1 KW

4.6kW power output (20A); current management options for 16A, 13A or 10A connection: limit the maximum power if needed (depends on fuse protection of electric installation).

Is 10mm what you have calculated for a 20A load per phase?

 

[ERFspark]

Any chance you could put your detailed design breakdown on here? I’m interested to see how you came up with it.

 

[smokeyc]

Ok sorry I have redone it; this is what I've got.

11000/(1.73x415x0.8)
11000/575 = 19.13A
19.13x1.2 = 23A

Table 4.1.2a 6mm2/2.5 with a max cable length 50m
Then in AppendixF ref method A 6mm2 allows for 31A and 10mm2 allows for 42A.

So I've aired on caution with 10mm cables but need a 32A protective device.

I think without the power factor it came to 15A and safety margin as 19A.

4.6kW power output (20A); current management options for 16A, 13A or 10A connection: limit the maximum power if needed (depends on fuse protection of electric installation).

nicebutdim I've seen that in the manual but haven't been able to work out if there is an option on the hob to select between single and three phase. I get so confused with that statement you mentioned as I am unsure if that is single or three phase. If you limited it to 20A (3phase) then surely that is 13.8kW in 3phase? That's higher than its rating? Which makes me think they are referring to just single phase. Or is the 20A setting just saying its maximum? And therefore all it is saying is the need for a 20A 3phase MCB?

 

[nicebutdim]

I'm inclined to agree the manufacturer's instructions are clear as mud. Have you tried speaking to them - they're bound to have a technical dept that could clarify this?

 

[smokeyc]

I'm inclined to agree the manufacturer's instructions are clear as mud. Have you tried speaking to them - they're bound to have a technical dept that could clarify this?

Thank goodness for that; they just simply say refer to the manual!
I've been emailing but I may call tomorrow.

In fact looking at my calculation in the last post again, I am going to change my mind.
The quick install shows the three phase with 220-240 all to neutral (shared neutral).
This now makes me think it doesn't know whether its single or three phase. (Just a hunch).

Looking at there power and there breakers for 10A, 13A, 20A; they have no safety factor accounted for. That must give a lot of errors. So I am assuming at 11000 - they think the breaker for Single Phase would be 47.82A, where it should be 59.78A with a 25% safety factor.

So I am now at the point where I need to choose 20A or a 32A 3phase MCBs.

I took a look at the Fisher and Paykel CI926DTB4 which has a far better installation instructions; it says a max current of 48A @ 11kW 220-240V or 3x16A @ 11000kW 220-240V. Though it states max current draw not breaker size.

I think I have made a decision of 10mm cable with a 20A 3phase MCB in conduit Reference Method A.
I am pretty sure 6mm would do but honestly the cost difference not worth hesitating and 10mm is used for ref101 as a 'Standard Circuit'. So with the insulation I can rest fairly easy

 

[davesparks]

The quick install shows the three phase with 220-240 all to neutral (shared neutral).
This now makes me think it doesn't know whether its single or three phase. (Just a hunch).

Of course it doesn't 'know' how many phases it's connected to, its just a hob. You'll fit shorting links for single phase or else connect to 3 phase.

Looking at there power and there breakers for 10A, 13A, 20A; they have no safety factor accounted for. That must give a lot of errors. So I am assuming at 11000 - they think the breaker for Single Phase would be 47.82A, where it should be 59.78A with a 25% safety factor.

Where on earth does this 25% safety factor come from? That's certainly not from bs7671. Artificially increasing the design current by 25% is not going to improve safety!

An 11kW hob would be fine on a 32A supply, or if you wanted to ignore diversity a 45A supply, why on earth would you put it on a 63A supply?

11kW 3 phase cooking appliance would need, worst case, a 20A 3 phase supply allowing for a little imbalance. If the load is perfectly balanced then a 16A supply.

 

[davesparks]

Ok sorry I have redone it; this is what I've got.

11000/(1.73x415x0.8)
11000/575 = 19.13A
19.13x1.2 = 23A

Why are you multiplying by 0.8?
Why are you multiplying by 1.2?

Table 4.1.2a 6mm2/2.5 with a max cable length 50m
Then in AppendixF ref method A 6mm2 allows for 31A and 10mm2 allows for 42A.

What book are you referring to here? There is no table 4.1.2a in BS7671 and the appendixes are numbered, not lettered!

You've missed out a few steps in your cable calculation too, you've made no mention of rating factors!

 

[James]

I am off to bed now,

it’s a bit disappointing to see so many posts knocking this guys work so far.

I hope by the morning that someone has helped with the calculations and probably come up with 4mm minimum or 6mm to be on the safe side if there are significant de rating factors.

 

[davesparks]

it’s a bit disappointing to see so many posts knocking this guys work so far.

Nobody has 'knocked their work' just asked questions.

 

[smokeyc]

Hi Dave

As stated this is my first 3phase induction install.
Your comments seem a bit unkind and unfair, I am after all asking for help and advice.

I also mentioned I was using Standard Circuits which have assumed conditions and don't need volt drop calculations for the given length, the appendix's are from the Onsite Guide for Part P Building Regs. You mentioned breaker sizes for single phase and asked about 0.8; 0.8 is a worst case Power Factor used in a 3phase current calculation assumed for the induction electromagnets - used to calculate the current; equivalent to Ib (emphasis again I think and reiterate I am asking for help).

So the SF comes from a number of things I have seen online for overload conditions - I know this doesn't protect the cable I assumed this protected the electronics in the hob. 6mm and 10mm will do the job as per the table I mentioned in ref method A for standard calcs, Part P onsite guide duplicated the current carrying capacity tables you mention 4 which I have cross checked against the singles table 4d1a and tabulated current It (as per 7671A2).

The hob is pretty smart, it has power limitation settings (though completely unclear how they work) - I would not be surprised if it did detect single or three phase supplies. But I don't know, again this post is asking for help and experience.

Why don't you show me how you would do a NEFF 11kW 3phase hob installation with insulation, and what you would select and do?


Otherwise many thanks, I don't think this forum is for me.

 

[nicebutdim]

Otherwise many thanks, I don't think this forum is for me.

I wouldn't give up on the place as there's no end of useful advice, however forum posts can be difficult to discern intent from and some text can appear quite blunt when no offence is meant.

From time to time people post here expecting someone to do their job for them, having undertaken work that is far beyond their expertise, and literally post basic details with the expectation that someone will perform calculations on their behalf. Clearly that's not the situation here, but a lack of meaningful instruction from the manufacturer left your opening post quite light on detail.

I honestly don't see how anyone can provide empirical answers to your problem without further input from the manufacturer and I'm certainly invested in seeing how helpful (or not) NEFF will be in providing that. Press them for details and follow up on this thread (please).

 

[nicebutdim]

In this pdf NEFF state that protection should be provided by 16A breaker for 3 phase installation, suggesting 16A is indeed the highest power setting for 3 phase supply (common sense when you think about it) and they've neglected to provide anything meaningful for single phase supply.

No books to hand, but I'm guessing OSG tables might suggest 4mm for singles in conduit running through insulation. Cable calcs might even allow lesser CSA, when all factors are taken into account.

 

[Rockingit]

You'll probably find that this ships with single phase links anyway so this is almost certainly being massively over-thought. 32A MCB to 6mm is almost certainly more than sufficient unless there are some serious cable de-rates needed.

 

[nicebutdim]

You'll probably find that this ships with single phase links anyway so this is almost certainly being massively over-thought. 32A MCB to 6mm is almost certainly more than sufficient unless there are some serious cable de-rates needed.

Property has 3 phase supply.

 

[smokeyc]

Hi All
Thanks for the input, and I cannot believe it was there staring at me in the product information. I guess sometimes you cant see the wood through the trees. I feel like I have a good strong answer and now know with certainty and confidence. I do agree I think you can over think it; and I probably have. Though I am going to say putting those details in the manual, or assembly instructions would be much smarter. Thank you all

Hope this posts will be of some use to others working with 3 phase supplies.

 

[DefyG]

Though I am going to say putting those details in the manual, or assembly instructions would be much smarter.

Agreed but when some are clearly 'Badly' translated into English confusion results! which may have been the case albeit in part here.

 

[davesparks]

Hi Dave

As stated this is my first 3phase induction install.
Your comments seem a bit unkind and unfair, I am after all asking for help and advice.

I can't see that I've said anything unkind or unfair, for the most part I have just asked questions.

I made the assumption that you are an electrician and understand AC theory, 3 phase, cable calculations etc.

You mentioned breaker sizes for single phase and asked about 0.8; 0.8 is a worst case Power Factor used in a 3phase current calculation assumed for the induction electromagnets - used to calculate the current; equivalent to Ib (emphasis again I think and reiterate I am asking for help).

0.8 is not the worst case power factor, power factor could be far worse than that.
Power factor applies to both single or three phase.

So the SF comes from a number of things I have seen online for overload conditions

Are you sure you aren't looking at advice being given for installations in the USA? They do have some odd rules about percentages for safety factors and oversizing breakers.

In the UK we don't that sort of nonsense, certainly not for a simple appliance supply anyway.

Why don't you show me how you would do a NEFF 11kW 3phase hob installation with insulation, and what you would select and do?

Without knowing the full details of the job I can only hazard a guess, but I would expect a 20A supply in 4mm to be adequate. As far as the actual installation goes I would be doing my level best to install the conduit in such a way that it doesn't end up buried in insulation.

I know assumptions can often lead to problems but fact that this is being installed in conduit suggests that this is a commercial installation so I assume there will be voids bigger than the depth of the insualtion you could put the conduit in.

 

[Rockingit]

Property has 3 phase supply.

Doesn't mean it has to be installed as a 3ph device! How many 3ph ring mains and lighting radials do we normally install?.......

 

[brianmoooore]

Doesn't mean it has to be installed as a 3ph device! How many 3ph ring mains and lighting radials do we normally install?.......

It would be a strange decision to install it as single phase if a 3 phase supply is present. TP 20A MCB, 4mm2 supply, and 2.5mm2 HO7RN-F for the final connection.
Everything well within rating, running nice and cool, and the hob can be used flat out if desired.