Wetroom Store - Network Wetroom Suppliers
This official sponsor may provide discounts for members

Discuss Volt drop Question SWA in the Electrical Forum area at ElectriciansForums.net

Welcome to ElectriciansForums.net - The American Electrical Advice Forum
Head straight to the main forums to chat by click here:  American Electrical Advice Forum

Hello All,
I’ve been asked to install a cable 170m in SWA, this is to be pulled through ducting. The cable is straight from the meter to an isolator in a beach hut. As the property is so small with little electrical appliances, I have accounted for 45A max current and deduced a 35mm2 SWA. Is this correct, I’ll just fit a 40A RCD on the CCU. I only ask because if I apply 3% volt drop I end up with a cable between 70 and 90mm2. This seems massively ott. Any comments would be great as although I’ve been an electrician for 10yrs+ it’s mostly been on the commercial side and I’ve not really had much to do with domestic supply cables. Thanks very much in advance.
 
Last edited by a moderator:
Volt drop it taken from the source of supply to the furthest point of your final circuits so for lighting this is 3%. On submains a rule of thumb figure is often quoted at 1% to give you a 2% drop on the final lighting circuits but being a beach hut it is probably negligible. 35mm cable is going to exceed the volt drop limit.
 
Change the protective device to 32A if the final loading is only small.
 

Richard Burns

-
Mentor
Arms
Esteemed
If you only have fixed lighting that is designed for a voltage range that allows lower than 230V then you could stay with the 5% volt drop and also take account of the lower operating temperature and extended length to give more leeway on the volt drop.
I hope if you have 45A demand that your RCD would be capable of carrying more than 40A. I would expect far more problems with volt drop and cable length in a commercial setting than in a domestic.
 

Wilko

-
Arms
Esteemed
Hi - not sure I've understood correctly, but perhaps consider fitting an RCD to match the upstream fuse rating (or more), rather than picking one with 40A rating. One less thing :) .
 
B

Bobster

First thing to clear up, as others have pointed out.

Do you mean 3% volt drop or diversity, as the latter doesn't make much sense.
 
  • Thread Starter Thread Starter
  • #8
Sorry my terminology was bad, your correct. I went for 40A to oversize it really. The measured load will not be above 30A for sure. So my question is really.. what size cable would you use, an I correct in thinking 35mm2 is adequate? Thanks again
 
I have amended it so it says volt drop. I would have thought dropping to 30/32A is the best option.
 

Richard Burns

-
Mentor
Arms
Esteemed
Using 35mm² SWA underground and with an operating current of 40A you would be just outside the Vd for lighting after correction for operating temperature, but assuming unrealistically 30°C for the ambient temperature. If your design current is less, say 38A, then you would just be within at 6.78V dropped.
However it is up to you to ensure your calculations do ensure compliance for this installation.
 

davesparks

-
Mentor
Arms
Esteemed
Does lowering the fuse rating work out for discrimination with the final circuits?
 

Strima

-
Arms
Esteemed
How big is this beach hut and what are they growing? I would expect it to draw more than a few amps unless they have as shower installed?
 

davesparks

-
Mentor
Arms
Esteemed
Keep up Dave, selectivity :D
Oh yeah, we're not allowed to discriminate any more are we.

I guess black jap screws aren't kosher anymore either?
 

TJ Anderson

-
Mentor
Arms
Esteemed
What exactly are the loads you are supplying? You need to really bottom out a sensible and realistic design current before sizing selecting protective devices and sizing cables.
 
Scolmore Electrical Products
This official sponsor may provide discounts for members

Reply to Volt drop Question SWA in the Electrical Forum area at ElectriciansForums.net

Top Bottom