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Posted By Topic: Recommendation for long run to induction Hob

Jan 04 2018 16:05

Trying to come up with the best solution for a new job. Would appreciate a recommendation.

New build, SIP panel walls and roof (so no usable ceiling space), concrete slab with hydronic heating. Has a run of aprox 20-22m from the board to the Kitchen (depending on routing). Hob is induction unit max 32amp. Wall oven max 16amp. 4 other double outlets in kitchen.

Was thinking to recommend conduit(s) in slab from board to kitchen before slab goes down. But having trouble getting my head around on de-rating calcs in this scenario if I mix 6.3mm for hob and bunch 2.5mm for outlets in one or more parallel conduits.

Any recommendation appreciated.


Jan 04 2018 17:10

Run 2 conduits in the slab?

Jan 04 2018 17:18

All the derating factors you need are in AS/NZS 3008.1.2.:2017 Table 27 (1)

Get the current rating of circuits in a conduit from table 11 and the derating factors from table 27 (1) remember to reads Notes 1 and 2 carefully and apply the correct temperature (max) of the heated slab.

Jan 04 2018 23:41

Thanks Pluto, I get Table 11 for the capacity and Table 27(1) for de-rating in heated concrete slab. However its bundling different size TPS into same or parallel conduit in the slab that I am not clear on. Am I OK to use Table 22, Bunched on surface or enclosure? And that wouldn’t be accurate for having two different size cables in same conduit?

Jan 05 2018 08:55

I have made an error it should be table 10 not Table 11 for the TPS cable current ratings in col. 25.

Table 27 (1) for the heated floor slab is correct.

I would run one conduit for the induction hub and if the run long (more than 3 lenghts) I would use a generous size of conduit for the ease of the cable installation in the conduit in the slab. If the other conduits are some distance away (say 200 mm) the derating from other wiring enclosures (conduit) should not effect the cable heating due to current in the different conduits.

For the conduit that have the 2.5mm2 TPS cables, the bunching derating factor tyable 22 will also need to be considered.

BTW check the current rating of the cables if they are run in thermal insulation, the current limitation due to thermal insulation may be more that for the cable run in conduit in the floor slab.


Jan 05 2018 09:48

Yes use Table 22 for bunching.
It doesn't matter what size the cables of the various circuits are; what matters is how fully they are loaded. The Table is set up for worst case: all circuits fully loaded; ie carrying what the applicable CCC Table tells you is the max permitted current.

To help get your head around this, the basic CCC Tables are for the maximum current the cable can carry in the particular installation conditions, without exceeding a specified conductor temp. All cables of same type of conductor + insulation have same max conductor temp, eg Table 10 is for TPS with max temp 75 deg C. The difference installation conditions makes is how easily this internally-generated heat can be dissipated.

When you look at T 27, you'll note the factor for floor slab temp of 30 deg C is 1.0. That's because the entire set of tables is based on official ambient air temp of 30 deg C. So if the slab is run at lower temp, you actually increase the CCC derived from the basic Table. But for a hotter slab, you'll have a factor less than one, so a decrease in CCC, followed by another potential decrease for bunching.

As Pluto indicated, separate conduits next to each other doesn't automatically get around the "bunching" effect; though you won't need to spread them out to 200 mm. Note 5 to T 22 points you to the relevant spacings required; also clause 3.5.2 & Fig 1.
CCC for 2.5 TPS in wiring enclosure 26 A; though some of the run will be in walls etc so must use "partially surrounded as an absolute limit. But those sections won't be bunched, so for the floor section: bunching factor (2 circuits) 0.8; 26 x 0.8 = 20.8 A.
This is less than the "partially surrounded" rating, so becomes the governing CCC for the circuit. 3 circuits factor 0.7 so only 18.2 A.

With the route length being over 20 m, you may well want to increase the cable sizes for these circuits to cope with volt drop, so don't scrimp on conduit size. Kitchens these days tend to have lots of appliances, and circuits can be pretty fully loaded; so assume you will be carrying 20A on each circuit. Since you're at or close to fully loaded, use the "75 deg" Vdrop figures: for 2.5 18.02 mV / A / m x 20 A x 20m will lose you over 7 V, and every extra meter makes things worse.
Increasing the size means the circuits are no longer fully loaded, so the bunching effect is less.


Jan 07 2018 11:53

thanks guys, that gives me all the info I think I need.