HOFR Cable

[SparkB]

I am looking to use HOFR cable on a job coming up - supply to a 12 KW single phase water heater unit. The unit has a maximum cable capacity on the terminals of 10mm. I have looked and feel HO7BN4-F (638TQ) is the best for the job - but was hoping someone could confirm the correct current rating table in the 17th edition to check the rating. One website says Table 4F2A (which looks correct), but other websites have also got their own tables with slightly different values. Can anyone help?

 

[Wilko]

Hi - my thought is Table 4F2A gives CCC for the cable at 90deg, but are the things cable connects also rated for that temp? If not, then Reg 523.1b applies and I think Table 4D1A is appropriate. The difference is 76A (85deg) vs 65A (70deg) for 10mm when clipped direct. Hope that helps. Cheers.

 

[Richard Burns]

12kW single phase, what do manufacturers think!

The problem you have is that the cables you are looking at are rated to 90°C and it is likely that the supplying connections would only be rated at 70°C, therefore the cable would be limited to a maximum temperature of 70°C and this would reduce the current carrying capacity of the cable. However the likely current carrying capacity of a 10mm² cable should be acceptable for the load if the cable is not prevented from losing heat by the installation method, ambient temperature or grouping.
Even 60°C flexible cable can take 51A in free air so a 70°C cable should be OK at 52A.

But Wilko beat me to it!

 

[SparkB]

Thanks for the replies guys.

It is a pain that the incoming terminals are limited to 10mm cable - which is what is the main issue.
The length of this 3 core 10mm cable will be short - 1 metre at most from isolator to heater. It will be clipped direct and general ambient temperature etc will not be an issue. However, The incoming terminals are mounted on a PCB, right next to the outgoing hot water supply!
I have checked the install manual, is there any way i can verify the rating of the supplying connections?
I am having to use a 63A MCB (circuit also RCD protected) as the heater will be using 52A under normal operation which is why I decided on this particular cable, but I am now unsure if I have to lower the rating to 70deg.

 

[Wilko]

Hi - Check heater manuf. spec for temp range, but 70deg rating probably best to assume. Perhaps also look into higher current rated iso switch as well (due to cts load).

 

[SparkB]

Under normal operation then it seems like the 10mm cable will be fine even working on the 70deg basis. However, I am going to have to use a 63A MCB for circuit protection, which is in excess of the 70 deg current rating for the cable, which cannot be done in case of overload/fault.

 

[Richard Burns]

That is what I mean about manufacturers not thinking if they had made an 11.5kW heater it would probably perform the same function and be much easier to fit.
You could consider using reducing pin ferrules on a larger cable or review 433.3 on the omission of overload protection.

 

[Wilko]

I like the reducing pin ferrule idea a lot

 

[Nickj]

Does the manufacturer state 12kW rating @ 230 or 240v? If they've stated it at 240v (they normally do) then surely a 50A MCB would suffice (just!) and solve all your problems?

 

[SparkB]

Yes I know what you mean, by limiting the cable size at the terminals, yet making it 12KW so it runs at just over 52A it has been a nightmare trying to design.
I am going to ring Santon and get their advice/perspective.
Thanks for everyone's advice so far.

 

[SparkB]

I rang Santon who were no help at all, offered no help with the temperature rating of the incoming connection terminal, suitable 10mm cable etc - claimed never to have been asked such questions by anyone before.
The only advice he gave was to put fibreglass sleeving on the cables at the terminals....

NickJ We are now thinking Along the lines of your idea - the 12kw rating is at 240V = 50A, 230v = 52.17A.

50A MCB, the unit is electronically controlled soft start up so this might be the best way to go.

 

[Nickj]

I'm unsure which product you have but I looked at the instructions for:

https://www.google.co.uk/url?sa=t&s...tEbKsm4fv_IEyUpWQ&sig2=drpPyIra1YjpTsfv2vJY3g

Which states their 12kw model is calulated at 240v. They show 11kw for 230.

Electrical rating...6.4/7.0kW @ 230/240V
.............................8.2/9.0kW @ 230/240V
............................11.0/12.0kW @ 230/240V

Therefore I'd be working my calculations based on 11kw load.

Happy to be corrected if I've misinterpreted

 

[SparkB]

It is this NickJ-
http://cms.williamsharp.net/_assets/media/shop/product/537-instructions.pdf
The 12KW version

 

[Richard Burns]

Looks like the 12kW is an 11kW for design purposes.
Specification
Ratings
4kW @ 240V, (3.7kW @ 230V).
6kW @ 240V, (5.5kW @ 230V).
9kW @ 240V, (8.3kW @ 230V).
11kW @ 240V, (10.1kW @ 230V).
12kW @ 240V, (11kW @ 230V).

Makes things easier.

 

[Nickj]

Specification
Ratings
4kW @ 240V, (3.7kW @ 230V).
6kW @ 240V, (5.5kW @ 230V).
9kW @ 240V, (8.3kW @ 230V).
11kW @ 240V, (10.1kW @ 230V).
12kW @ 240V, (11kW @ 230V)

I've taken that from your link. 11kw at 230.

To mix it up though:

Model Product Rating Temperature Weight Boiler Power supply Recommended
code @240V range output to boiler MCB
(kW) (ºC) (kg) (Btu/h) (amps) (amps)
PH4kW 94:022:011 4 65-80 7.5 13600 17 20
PH6kW 94:022:012 6 65-80 7.5 20500 25 32
PH9kW 94:022:013 9 65-80 8.5 30700 38 50
PH11kW 94:022:014 11 65-80 8.5 38000 45 63
PH12kW 94:022:015 12 65-80 8.5 41000 50 63

They suggest a 63a mcb but again they've made those calculations at 240v not 230v which suggests 50A mcb.

 

[SparkB]

Thanks for all the help everyone.
We are going to go with a 50A MCB, based on the above. Will let you know how it goes.