2 houses being combined into 1 - electrical rewiring options

[stadamack]

Hi all,

2 adjacent terraced houses are being joined into one. The electricity company meter cabinet is towards the front of each house and the consumer units are over the rear door in both instances. The heating system is oil and there is no storage electric or heat pumps in either unit, therefore, it is likely both houses are 12kVA supplies with 16mm2 mains tails. The houses were built in 2002.

The final house would have only 1 no. cooker with oil heating and so a 12kVa supply should suffice for the final two units combined,

It is proposed that 1 of the electricity meters would be disconnected and the associated consumer unit would be connected with a new 16 mm2 SWA to be installed to the other consumer unit. This would serve to minimize the amount of rewiring required,

Question:
1. do electricity utilities have particular requirements for supplies to previously separated houses?
2. Any obvious oversights in the above proposal?

Thank you,

 

[davidbeach]

It probably doesn't matter what "electricity utilities" requirements are, only what the requirements for one particular utility. Ask them.

What is a "consumer unit"? Off hand 12kVA sounds rather small for a single dwelling, let alone one twice the size of one that had a 12kVA supply.

 

[ScottyUK]

David,

From the terminology it sounds like a UK installation. A 'consumer unit' is a domestic single-phase distribution board, typically rated at 100A and containing between six and perhaps twenty single-pole MCB's or RCBO's. British properties are typically provided with a service fuse at the utility cut-out of 60A, 80A or 100A depending on how large the property is. Most British properties have a mains gas supply so the power-hungry loads such as water heating are typically gas-fired rather than electric, although in this case it's oil. On a smaller property which is well-insulated and with energy-efficient appliances a 60A service isn't unusual. On a larger property 80A or 100A is more typical.

Stadamack,

Approach the utility and see what options exist to uprate the service to 100A.

 

[waross]

Under Canadian codes, we would have to recalculate the demand of the combined dwellings. Part of the demand calculation is a Watts per square foot allowance.
David: Here we have demand charges on rural properties.
The utility doesn't measure the demand, they limit the demand with the installation of a circuit breaker.
I have a nominal 5 KVA demand service. Although 5 KVA works out to 20.8 Amps the utility is quite generous and gives me a 35 Amp breaker. This ahead of my 200 Amp service.
The the code demands a 100 Amp service for a house of this size, but the builder went overboard.
Gas heat and hot water but electric cooking, clothes drier, deep well pump, septic pump and about 800 Watts for the stock water.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[cranky108]

I know I might be missing something here. But why would a utility, that makes money on sales of its product, limit how much a customer uses?

That said, we do limit what a customer uses in a specific rate class, but that has more to do with equipment/service/wire ratings.

And yes I have seen a melted 200 amp meter (well over 200 amp).

Yes we give away LED light bulbs, and push energy saving equipment, motors etc. But why limit your customers?

 

[davidbeach]

So, if the "consumer unit" is a panelboard, "over the rear door" sounds like a bad installation. Certainly wouldn't meet the maximum height requirement in the NEC and presumably other codes. As with cranky, I find it surprising that a utility would limit usage and therefore revenue.

 

[waross]

They have a KVA charge, but they don't meter the KVA. If I want more than a 5 KVA demand, I have to pay more every month and they will put in a bigger breaker.
Interestingly, I spent some time in Chile over 20 years ago. I believe that they had a similar arrangement with a utility sealed supply breaker. If your demand tripped the breaker you could reset it. If tripping became a nuisance you could pay a higher monthly charge for a higher rated breaker.
Is there anyone here from Chile who can give us an update on present practice?

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[ScottyUK]

David,

Yes, I agree that the location sounds bad, it doesn't meet any of our present-day requirements either and it didn't meet them in 2002.

The reality is that a 60A service is large enough for the vast majority of small properties - remember our single phase supply is 240V, not 120V. The utility doesn't limit the consumer but equally has little desire to install infrastructure which is never going to be fully utilised or anywhere close to it. The trend seems to be that the service fuses to new-build properties are reducing in size as homes become more energy-efficient. If a customer wants an uprated supply then it's not refused although it is chargeable.

 

[waross]

Our single phase supply is also 240 Volts. We can utilize either 120 Volts or 240 Volts but we have the same kW/KVA as you do in a service of a given amperage.
Amps times 240 Volts.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[cranky108]

So I am thinking your housing is much smaller than ours.

In the US, I believe the smallest service that I have seen of late is 70 amp/240V which was on a mobile home (really small).

I think the smaller main panels I can find in home-depo is about 125 amp/240V.
There are smaller panels,but are mainly for an out building, or air-conditioning unit.

But granted if one uses gas for heat, hot water, and dryer, their is not much left unless you add air-conditioning, and some part of the US don't need that (or use a swamp cooler).

 

[ScottyUK]

Hi cranky,

Just some square footage and cost figures grabbed from the website of a company building mid-market homes in a decent town near me in the north-east of England:

£425k 1900 sq.ft
£410k 1800 sq.ft
£350k 1600 sq.ft
£315k 1400 sq.ft

Prices in the north-east are half or less of a similar property in the south-east, but it gives you some idea.

 

[Mbrooke]

For the US folks, a typical British consumer unit and how it works:

https://www.youtube.com/watch?v=2R3L_nvNP6Y

BTW,I know 100amps is minimum for a home in the US but truth is you could get away with a 50amp service in many cases. The NEC is grossly over engineered.

 

[davidbeach]

The NEC is grossly over engineered.

It's not engineered at all. It's a safety document that for the most part tries to avoid the need for detailed engineering to meet most of the requirements. There are a lot places where one might be able to more efficiently engineer an installation than what the code would come up with, but that might turn out to be a much less safe installation unless the reengineering applied completely across the board.

Back when I dealt with the NEC on a daily basis it was my estimation that the code ampacity of many conductor configurations was probably too high for the actual conditions. But the rules for calculating loading also overstated the load the conductors would be subjected to. If those were in fact accurate impressions of the code, they counteracted each other. "Fix" the load diversity problem and you'll wind up with conductors too small because the ampacity is overstated. You've got to just accept the whole thing.

In the case of my house, I'm one of nine houses all served from a single 75kVA transformer. That says I have an effective capacity of about 35A to serve my 200A service. I just added air conditioning, I may well exceed 35A at times this summer. Hopefully that doesn't push the transformer too much, but if I start having problems I know who to call ;-). I'm also a heretic who cooks, heats, makes hot water, and drys my clothes with gas rather than electric power so maybe I don't go too far over the 35A value even with the AC.

 

[waross]

My 200 Amp service is fed from a utility owned 35 Amp breaker. The monthly demand charge is a significant part of my bill.
This is the second house that I have owned that is on a 35 Amp breaker.
One trip in ten years is acceptable.
Electric kitchen and electric clothes drier.
Your 35 Amp share is not breaker limited like mine so you should be fine.
The minimum size service panel in Canada is 60 Amps if the floor area exclusive of the basement is less than 80 Sq meters.
But the minumum number of breaker spaces is:
60 Amps, 16 spaces
100 Amps, 24 spaces
200 Amps and greater, 40 spaces.
I have 38 out of 40 spaces in use in my panel.
Although a 100 Amp service would have been adequate for my house, it is doubtful if the builder could find a 100 Amp panel with 40 spaces.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[Mbrooke]

David, all other codes like BS7671 also lack detailed engineering however the final numbers are smaller. Nothing is failing or tripping. Even MCBs used all over the world are rejected by UL489 so something as simple as a 240/415Y data center requires greatly over priced 480 volt panelboards.

Simple example: In Canada I can wire a 4500 watt water heater with #14 NM and put it on a 25 amp breaker. In the US I must us #10. A car charger in Canada can take #3 NM, but under the NEC I must use #1 cu in conduit because NM isn't made over 2 gauge and restricted to the 60*C column unlike Canada at 90*C. 2.08mm2 is restricted to 15amps, but else where in the world thinner 1.5mm2 is typically driven at 16 amps, 20amps if clipped direct. Similarly 2.5mm2 which is smaller then 3.31mm2 carries 20amps, 27 if not in contact with insulation. No 80% rule. No dedicated circuit requirements. Services come out at 1/2 to 1/3 in size adjusted for voltage.

There will come a point where limited resources will force the NEC to change adapting to the rest of the world.


I know someone will make the argument about higher gauges being necessary due to the NEC lacking a mandate on maximum loop impedance, but the larger sizes will not stop someone from running 2000 feet of #12 UF to a shed. A simple table of max lengths for a given voltage with ungrounded and grounding conductor combinations would suffice either way.


Now regarding your pole pig I would not worry. Those can take a +200% overload for an hour if given enough time to cool in between cycles. Each home will not draw 35amps evenly either, if one is pulling 65 another will be pulling 10. And your lucky lol, where I live those nine homes would get up on a 50kva even with electric stoves and clothes dryers.

Forgive the NEC rant. Its not directed at you but rather the NFPA CMPs. When you sit down and put physics to the code or compare how its done elsewhere you soon wind up with a red pill that is rather jolting. I can wake you up, but the question is do you want to live with high blood pressure lol.

 

[itsmoked]

Mbrooke; Thanks for the UK panel vid. Seems pretty clever. I like that it could all be on one DIN rail. I'd have lots of fun with that!

The one thing that was horrid was the busbar game. Yack! It looks like you have to remove the bus bar from everything to change one breaker. The buss bars are also under there to light-you-up if you forget. Seems it would be dangerous even to work on just 'one half' since the other busbar is still there waiting for its opportunity.

NEC domestic panels are always worked on hot with single circuit work not blacking-out any other circuits.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[itsmoked]

Geez Scotty around here:

$1,000k 1900 sq.ft
$900k 1800 sq.ft
$850k 1600 sq.ft
$800k 1400 sq.ft



Keith Cress
kcress -

http://www.flaminsystems.com

 

[ScottyUK]

It's possible to swap a breaker without removing the busbar. The observations about live working are on point though - I don't work on domestic very often, but I switch it off on the occasions when I do. Schneider's 'Acti 9 ' range of boards has a sliding isolation link for each outgoing pole - it's a good system and makes for a relatively safe board to work in. In the image below the brown tabs slide down when the MCB is fitted and connect it to the bus. When the tabs are in the position shown the outgoing stabs are dead.

 

[waross]

What part of Canada, Mbrooke?
When I wire a 4500 Watt heater in my part of Canada I must use #10 AWG and a 30 Amp breaker.

but the larger sizes will not stop someone from running 2000 feet of #12 UF to a shed. A simple table of max lengths for a given voltage with ungrounded and grounding conductor combinations would suffice either way.

That would be rule 8-102 Voltage drop (see Appendices B and D)
(1) The voltage drop in an installation shall be based on the connected load of the feeder or branch circuit if
known; otherwise it shall be based on 80% of the rating of the overload or overcurrent device protecting
the branch circuit or feeder, and not exceed
(a) 3% in a feeder or branch circuit; and
(b) 5% from the supply side of the consumer’s service (or equivalent) to the point of utilization.
The simple tables are table D3 for 120 Volts AC and above.
Table D3
Distance to centre of distribution for a 1% drop in voltage on
nominal 120 V, 2-conductor copper circuits
This is a simple table. You simply extrapolate for other percentages and other voltages.
and table D4 for 6 volts DC and above for emergency lighting from batteries.
Table D4
Copper conductor sizes for 5% drop in voltage
on 6 V — Two conductors


Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[Mbrooke]

Those following the CEC (Canadian Electrical Code). Take a look at 62-114 (6). Im going by the 2014 code, but the rule should still be present in the 2018 code.

(6) Where a service, feeder, or branch circuit is used solely for the supply of energy to heating equipment, the
load, as determined using Rule 62-116, shall not exceed

(a) 100% of the rating or setting of the overcurrent devices protecting the service conductors, feeder conductors, or branch circuit conductors when the fused switch or circuit breaker is marked for
continuous operation at 100% of the ampere rating of its overcurrent devices; or

(b) 80% of the rating or setting of the overcurrent devices protecting the service conductors, feeder
conductors, or branch circuit conductors when the fused switch or circuit breaker is marked for
continuous operation at 80% of the ampere rating of its overcurrent devices.

(7) Service, feeder, or branch conductors supplying only fixed resistance heating loads shall be permitted to
have an ampacity less than the rating or setting of the circuit overcurrent protection, provided that their
ampacity is

(a) not less than the load; and
(b) at least 80% of the rating or setting of the circuit overcurrent protection.

(8) Notwithstanding Subrule (7)(b), where 125% of the allowable ampacity of a conductor does not
correspond to a standard rating of the overcurrent device, the next higher standard rating shall be
permitted.

The concept is similar to that of motor circuit sizing under the NEC. The circuit breaker provides short circuit (L-L) and ground fault protection (L-G) to the circuit, while the fixed nature of the heaters having no receptacles where load can be added provides inherent protection against overload.


Regarding voltage drop, yes it is mandatory in the CEC. And with table 68 in the 2015 CEC you now have an explicit limit on length of dwelling circuits. However in the NEC voltage drop is only a FPN and there are no loop fault impedance requirements either.

From the 2017 NEC: