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# Sub Main Design Calculations

Discuss Sub Main Design Calculations in the Electrical Wiring, Theories and Regulations area at ElectriciansForums.net

#### pga.elec

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Hi all, new to here.

Just designing a sub main, very long time since I have done this so just wanted some thoughts. Its to a summerhouse sort of set up that will be at the bottom of a very long garden at a later date (sockets, lighting and hot tub planned from the board). Patio is getting done soon so they are wanting the cable buried under that ready for when the rest of the garden is done, and it will be buried under the path that gets done.

Was thinking of installing 16mm 3 core XPLE SWA, fed from an 80amp DP switch fuse. The current meter tails are into a henley block before DB in house, so separate set of tails from the henley block to incoming side of switch fuse, and then sub main out.

The calc I have done is for part of the run, approx 58m which will be from an weatherproof box on the side of the house, down into the ground at a depth of 0.5m (internal of the house to the DB was going to calc down to a smaller cable given the run, clipped direct, and come in the back of the weatherproof box, so as not bringing a 16mm or bigger cable out the side and having to chop some of the render away as to get a decent bend). Cable will be grouped with another SWA for outside lighting on the patio (have accounted for cables touching).

Ib = I have allowed 15kw (not sure on ratings of hot tubs etc but thought I would allow more on this initial design) 15000/230 = 65.21amps
In = 80amp switch fuse (board in summerhouse will have a 63amp RCD incomer on 3/4 way board)
Factors of 1.03 (buried direct 0.5m) & 0.75 (cable grouping) assuming ground temp of 20 deg and thermal resistivity of soil of 2.5 both giving factor of 1
It = Ib/Cd*Cg = 65.21/1.03*0.75 = 84.41amps
Table 4E4A ref method D direct in ground is 16mm capable of 91amps
Volt drop = 5% so 11.5v therefore 65.21*58*2.9/1000 = 10.97v therefore acceptable

Is there anything I may have missed? Keep on second guessing myself.

PGA

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#### Spoon

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Was thinking of installing 16mm 3 core XPLE SWA
XLPE cable is only suitable if the devices it is connected to are rated for 90 deg C

#### APE37

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Allowable volt drop for lighting at 3%.

#### Spoon

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(internal of the house to the DB was going to calc down to a smaller cable given the run, clipped direct, and come in the back of the weatherproof box, so as not bringing a 16mm or bigger cable out the side and having to chop some of the render away as to get a decent bend).
Not sure what you mean by this.
What size are you thinking?

#### Charlie_

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Arms
The vd for section of cable from cu to outdoor box

#### Midwest

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Will the main property have a big enough supply, do they have a 3 phase supply?

#### Charlie_

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Arms
I think you are over estimating the size of the hot tub? What size do you envisage it being? Single phase hot tubs are usually half that rating.. larger then onto 3 phase = smaller cables

#### Ian1981

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As your using a cable underground you need to apply the correction factor cc which is 0.9 as the ambient temperature given for reference method D is at 20 degrees rather than the standard 30 degrees.
Using 0.9 corrects from 20 to 30 degrees if overload protection is required for the underground cable see regulation 433.1.203.
Also use the 70 degree cable ratings for swa rather than the 90 degree thermosetting ratings unless it is has been verified by the manufacturer of the accessories and switchgear that they are suitable for a 90 degree operating temperature.
Volt drop is from the origin of the installation to the point of utilisation ie a light fitting for example and should not exceed the recommendation of 3% , ultimately the voltage needs to be acceptable for the lighting point according to manufacturers instructions such as say 220-250 volts for example

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#### Strima

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TBH I probably would consider grouping factors either as both circuits would probably never get to full operating temperature to have an effect on each other.

Also if you have an 80 fuse then your RCD must now be = or > than the rating of the fuse as apparently we're not allowed and diversity on switch gear ratings.

If this is going to be a standard summer garden set up you would more than likely have plenty of wiggle room with 50 amp OCPD at source unless they go out and buy a massive hot tub, diversity gives a fair bit of room on these installations.

With LED lighting, some entertainment system and a fridge how much current will general use circuits draw? Once the hot tub is up to temperature then the current draw will stay fairly low as the tub cycles through heating and pumping.

#### Charlie_

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Arms
Strongly recommend installing a duct.
Let them crack on and do the patio, give you time to work out your existing capacities and size your hot tub to suit..
Also they duct will come in handy for when you wish you had installed a cat6 cable

Arms
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I like ducts.

#### pga.elec

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Appreciate the response guys.

Charlie - Tbh I never even thought of putting ducts in the ground and letting them crack on with the patio. Good point in having another one for cat5 etc as well. I am assuming the hot tub at most will be a 32amp one, which when looking online is usually the 6/7 seaters, if they go that big.

Midwest - The main supply is a single phase supply, TNC-S set up. Unsure at cut out fuse rating.

Strima - It will more than likely be the set up you have just described. I can't myself see there been much else than what you have listed. 50amp OCPD at source and then 50amp RCD main switch inside summer house is what you are suggesting?

Ian1981 - Will have another proper look at it tomorrow, try and dust off the cobwebs with the regs book. As mentioned, long time since having to do something like this, appreciate the help.

PGA

#### Strima

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I would probably go with a main switch rated to the supply OCPD and have RCBOs for individual circuits.

Will you be using the TNC-S or using TT at the load end?

#### pga.elec

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I would probably go with a main switch rated to the supply OCPD and have RCBOs for individual circuits.

Will you be using the TNC-S or using TT at the load end?
I was going to use the TNC-S, running a 3 core SWA down to the summer house.

#### Midwest

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Appreciate the response guys.

Midwest - The main supply is a single phase supply, TNC-S set up. Unsure at cut out fuse rating.
Thats a design consideration, before you proceed with anything else.

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