max load standard 100 A consumer unit (c/u)

[NrVolts]

hi there this is my first post so its likely i come across a bit unknowledgeable .ive got a customer who uses single phase furnaces for heating glass, ive been told that each furnace requires a 40A breaker.he intends to use 2 furnaces at first and then wants to add a third and possibly a forth.i know i should get the rated wattage for a clearer amperage of each unit, so i intend to find out tomorrow.i suspect if the 4 way c/u with 3 x40A plus 20A for a radial ring would overload the c/u on a standard 100A as the main switch is rated at 100A .im guessing each furnace would be about 35 amperes giving for 3 furnaces 105 amperes, and this doesnt even account for the 20A radial.so im guessing i will be having to tell him he can only run 2 furnaces and the 20A radial at once.the 20A is only for a few p.c.'s and display lighting.im worried if he uses 3 at once the c/u will probably ignite lol.done my 2330 but not much in it was about max load.

Thanks for reading.

 

[platty1005]

reply for paul.m : its small commercial outbuilding.there is already one existing dis board in this outbuilding fed from the main building board on a 63 amp breaker.a new cable of 50 mm2 has been run to the outbuilding with the intention off adding 100 breaker in the main board .the new 50mm2 arrives at the main dis board but not terminated yet.i think i best say no to letting him run anymore than 2 at a time.i need to get the specs of the intended main supply board 15m away and the specific load of a furnace.its a real concern for me about possible high loads for let me get more info.thanks for replying will add more info tomorrow.

Hi NR think about you replies
Existing supply fed from 63Amp breaker
New 50mm cable installed but not terminated
This sounds like a job started and not finished by somebody else.
Commercial outbuildings from a main building I guess it must be 3 phase even if the existing is only 1 phase.
I think the original plan may have been a 3 phase DB at 100A per phase in addition to existing and as you have admitted you are limited on 3 phase work look around for a local approved contractor and ask for help you won't get all the cash but you might learn a lot

 

[Knobhead]

As I see it your only way forward is to get 3 phase in place. You have room for expansion then. 4 kilns will just shift more load on to the neutral. Once the kilns are up to temperature and the thermostats start controlling the load the average will fall.

 

[NrVolts]

thanks glen, paul, hifly. best i ask to put this on hold.till i get to site..im getting no response form client as yet.the penny dropped last as i was going to sleep last night re reply "7" ..ureeeka !! neutral to phase is 230.split each load across the phases.i'l try do some kinda jpg as explaining the layout by word is not easy lol.

 

[GLENNSPARK]

As I see it your only way forward is to get 3 phase in place. You have room for expansion then. 4 kilns will just shift more load on to the neutral. Once the kilns are up to temperature and the thermostats start controlling the load the average will fall.

Thats what i was thinking of tony in post 14. I know that imbalanced 3 phase introduces loads on the neutral....hence the reason a properly ballanced 3 phase will not require a neutral. I may be wrong on this though as we havn`t covered polyphase systems at college (yet)...so if i am wrong on this then please explain a bit about it.
Thank you.
Glenn.

 

[rallyandy]

(posted at the same time)




Reason I said get a 3 phase supply is so he can 3 x 100A single phase supply so each furnace can be on its own phase, not for him to change them all to a 3 phase furnace.

Totally agree 3ph is the way forward

 

[Knobhead]

You won’t be able to do away with the neutral. If things do expand for your client then 4 kilns will be no problem to accommodate. The highest neutral currents you can expect assuming 40A per kiln is 80A. That is of course 2 kilns one phase and the other and 1 each on the others. There’s capacity for another kiln after that, I wouldn’t go beyond so that there’s capacity for ancillary equipment.

PM me your email address and I’ll send you a 3Ph Excel spreadsheet. Part of it calculates the neutral current in an un-balanced system. There’s a lot more to it than that.

Or you can use this
N = √((L1²+L2²+L3²)–((L1*L2)+(L1*L3)+(L2*L3)))

 

[NrVolts]

still no access to site.View attachment 7982 sorry this is sideways i cant see how to rotate it.
wont proceed any further till i get info on main board in main building like does it even have 3phase? i suspect it has but this hasnt even been investigated yet.
yes mr platty re reply 16, the 50 mm swa (supplied by the client)was put in by a colleague of mine who is quoting the works..
Really apprciate all the comments so far guys.especially the ammount of thought and time you guys have put in..thanks tony great info.bit beyond my capabilities but im sure others reading will find very useful.i guess this thread can be used for food for thought
hopefully the customer will go to an experienced contractor.i'l put in a close this thread comment if in 3-4 days from now the job has passed onto someone else.many thanks again.