[noddybrian]

Hi - trying to help out doing some electrical work on a farm & could use a 2nd opinion - we have a qualified electrician on site but he is only familiar with single phase - due to change of use of some out buildings they need to run a new supply in from the incoming power supply - the power company cannot provide more than a 100 amp supply without incurring huge costs but are happy to provide 100amps per phase 3phase - most of the power will run single phase showers / lighting / cookers - the question came up what size cable is needed - the electrician looked this up & made suitable allowance for distance & I agree if the load was across the 3 lines but as each line will be loaded against neutral I made the point that logically the neutral is carrying back the total load of all 3 lines so should be of 3 times the cross section of the lines - as all SWA cables I'ver seen have equal conductor size this means the cable would need to be 3 times the theoretical size looked up - I offered the opinion that options could be use a transformer taking 3phase in & giving a single phase output of the combined line capacity which would be expensive & waste some energy - or use a 3core cable for the lines & run a separate bigger neutral - or my personal choice is not to use 1 cable but do 3 runs of concentric cable so each line has it's own cable & matching size neutral this is easier to work with & more flexible given that it will need dragging in through some wall cavities - any thoughts from the electricians out there ? I know we have several knowledgeable guys on here ( in the UK so it's 3 lots of 240 volt ) many thanks in advance for any informed opinion - I have no liabilities involved as the electrician will be signing off any work done - he's just a bit out of his experience zone on this.

[TraditionalToolworks]

noddybrian,

Believe it or not I think the wire size is the same for single and three phase. The only real difference is there are 3 wires rather than 2 wires which are hot. It depends also on whether you run copper or aluminum.

Here's a chart that may help you. It looks like you'll need 1 AWG for 100 amps. This may or may not vary depending on the country and/or the standard being used. I believe this is what we use in America (AWG). I can't remember where you're located at and you don't have it on your post.

http://wiresizecalculator.net/wiresizechart.htm

[noddybrian]

Many thanks for the fast reply -
we can easily look up conductor size for current / distance to the spec in this country - my view was that while the size will be the same for single or 3phase if the load will be 3 separate buildings using 1 phase each driving single phase loads - all 3 phases load will be drawn against the common neutral as over here all single phase is 240volt between line & neutral & all 3phase is 440 across any2 lines - we don't have 110 at all or any of the set up used in the USA with 110volt - I realize the majority of members who post regularly are in the USA so maybe can't help as our power distribution is a bit different - hoping maybe a member outside of the US can help - it's possible I'm just over thinking it ! we should have a guy coming up from the power company soon to inspect the enclosure for the service head so I'm hoping to ask him - thanks again for your opinion.

[TraditionalToolworks]

That's an interesting thought on the neutral, I hadn't thought of that, but it seems it would need to be able to handle the amount of amps for the entire service, which changes depending on location. I am not sure if neutral feeding back to the power company is based on the service or not, so will defer to someone that may know.

I just got a 320 amp service installed at my property last night, a technician called about 8:20pm to tell me he had just finished...I was surprised he was working so late, but it was still light out, surprisingly. He said he had gone out earlier but the rep had miscalculated the distance from their pole, telling him it was 60', so he took 100' of cable and was about 5' short.

I don't think they charge me, I am only being billed $75 for the hookup as I went with 320 amps. They wanted $15k for the transformer to connect 3 phase...instead I decided to get a Phase Perfect for the 3 phase I'll be using.

This will be the most amps I've ever had for my shop/home in my life.

[Coldman]

Hi Nodds,
Even though the polarity alternates with AC power, the current always flows from the active phases. The current is consumed by the load so there is little current in the neutral which is why the neutral conductor is always the same size as the phase conductors. If you think about it, if the neutral had significant current in it, that would be a short circuit resulting in breakers tripping or smoke happening. Neutrals always end up to ground. Not the same place as your property earth necessarily but in a measured way from power poles all the way back the the power generators depending on what type of system you're connected too. In remote areas, like rural agricultural, you might only get actives fed to you to save cost of long conductor installation and you have to ground your own neutral and earth. By the time the neutral reaches the power station the neutral has been fully grounded and dissipated.
Enjoy your Tim Tams.

[v5cvbb]

I don't have an answer, but I see why you are concerned with all the neutral current. It would seem like each circuit should be ran from the source transformer as single phase where hot and neutral would be the same size.

I can't be of any help but I'm interested in the solution you find. Good luck.

[Arno]

Ah.. The fun of 3-phase

Keep in mind that the neutral in this setup is not always even used. You'll likely have a Three Phase Four Wire Wye setup with 3 phases and a neutral in a connection using a 'star' setup.

Nice diagram:

https://ctlsys.com/support/electrical_s ... _voltages/

You can use it in different ways:

- Actual 3-phase equipment (much cheaper high power welding gear! ), usually does not use the neutral but uses the phase-difference between the 'legs' to operate.
- 208V single phase equipment connected between 'legs' of the phases, so no neutral connection here either.
- 120V single phase between a 'leg' and the neutral

The latter would be the only ones actually loading the neutral, but if you distribute your various connections to 120V outlets across the 3 'live' phases and try to load them fairly evenly then they actually cancel eachother mostly out again on the neutral, so the current flow on the neutral remains small.

This is why you normally don't need a 'fat' neutral conductor. The more you spread the single-phase loads across all the 3 phases + neutral the lower the sum of the current on the neutral gets.

Bye, Arno.

[noddybrian]

Thanks to all that responded -
when I've spoken to the power company guys I will post back on their view or if we manage to find the code that covers it here - but just to clarify I'm in the UK so our power is all 240 single phase or 440 volt 3 phase - neutral does carry the full return load on single phase & does go directly back to the power company transformer - earth is only provided locally by ground rods -I agree if the load was all 3 phase then it's between the lines & almost nothing on neutral so no problem on cable size but this load is 3 independent single phase supplies as the power company will not upgrade the incoming supply above 100 amp - it is rural & would involve re stringing a long distance of overhead cables to the transformer assuming this has any capacity left as it already serves several properties.

[Arno]

Thanks to all that responded -
but this load is 3 independent single phase supplies as the power company will not upgrade the incoming supply above 100 amp - it is rural & would involve re stringing a long distance of overhead cables to the transformer assuming this has any capacity left as it already serves several properties.

Should still not be a big issue as long as you distribute the single phase loads on/across the individual 3 phases as much as possible.

The neutral will in that case still work more are a common reference point and the phase-offset return power of the 3 phases will want to cancel eachother (partly) out when single-phase devices hooked to different phases are used together on the neutral.

Normally speaking the biggest sustained return load on the neutral would be when 1 single phase device is used alone and nothing else. Once more devices are powered up that are attached to the other (shifted)phases then the net result of return amps on the neutral conductor will decrease.

They key here is that you will need to be aware to spread as much of the load/devices you tend to use at the same time across the different (single) phases and it should all work fine dimension-wise for the conductors.It may mean that the 3-phase needs to be run to all buildings and there split in to separate phase feeds/panels to be able to get the desired load-distribution or be prepared for the future when devices are perhaps moved to different buildings.

Power company usually also likes it best when all 3 phases are loaded as equally as possible.

Still.. Might be good to see if you can find more of an industrial electric engineer that's used to 3-phase and how to dimension things to get it all checked over.

Bye, Arno.

[noddybrian]

@ Arno
Thanks for your reply - I agree with you and have previously explained to them about sharing the load as far as practical across the phases - the problem is that they do not want to replace existing cables to 2 out of the 3 buildings as they are already of adequate size & the location would require a lot of digging across roadways / recently landscaped gardens etc - these cables do not have enough cores to allow 3phase so the 3 buildings will only get 1 phase each - one is an outbuilding that is converted to a dwelling - one will have a commercial size kitchen & the other is a shower / washing block - all will see significant loads close to the maximum incoming supply capacity but it's unlikely they will be in use together so the load will be very un-balanced - the new supply will use the shower building as a distribution point to the other existing buildings so I hope to get them to use all 3 phases in that one - power guys hopefully call today so I'll see what they say.

[Jakedaawg]

I thought the neutral only completes the circuit, it doesnt carry much current?

[Oscar]

I thought the neutral only completes the circuit, it doesnt carry much current?

The way I understand it is: It depends on the phase difference φ between the hot leads. In 240 1-Φ system, if you have two identical 120V loads each on separate phases, the neutral going back to the power source ends up carrying no amperage because of the φ=180°phase difference of the two hot leads that they are getting their voltage from. The amperage [from two identical loads] essentially cancels out in the neutral due to the phase difference φ. So basically the Neutral amperage = | Load1 - Load2| when 180° out of phase. If they were on the same phase (hot), then the loads are additive, and you would get Load1+Load2 on the neutral.

[smithdoor]

The 100 amp 240 volt will work for everything you every do.
I like using #2 for 100 amps copper.
If want to how power is three phase in kva it is a multiple of 1.72.
Simply put it is 100 amp X 240 X 1.73 =KVA or 41.5kva or 41,000 .

Dave

FYI
If look up the transformer in till the KVA.
Most power company will put less than 15kva transformer.

If use 41kva that cost you at $0.1 per kva.
Or cost per hour $4.1 or by day $24.80 or 10 days $248.00.
You find you never use that much power.

Hi - trying to help out doing some electrical work on a farm & could use a 2nd opinion - we have a qualified electrician on site but he is only familiar with single phase - due to change of use of some out buildings they need to run a new supply in from the incoming power supply - the power company cannot provide more than a 100 amp supply without incurring huge costs but are happy to provide 100amps per phase 3phase - most of the power will run single phase showers / lighting / cookers - the question came up what size cable is needed - the electrician looked this up & made suitable allowance for distance & I agree if the load was across the 3 lines but as each line will be loaded against neutral I made the point that logically the neutral is carrying back the total load of all 3 lines so should be of 3 times the cross section of the lines - as all SWA cables I'ver seen have equal conductor size this means the cable would need to be 3 times the theoretical size looked up - I offered the opinion that options could be use a transformer taking 3phase in & giving a single phase output of the combined line capacity which would be expensive & waste some energy - or use a 3core cable for the lines & run a separate bigger neutral - or my personal choice is not to use 1 cable but do 3 runs of concentric cable so each line has it's own cable & matching size neutral this is easier to work with & more flexible given that it will need dragging in through some wall cavities - any thoughts from the electricians out there ? I know we have several knowledgeable guys on here ( in the UK so it's 3 lots of 240 volt ) many thanks in advance for any informed opinion - I have no liabilities involved as the electrician will be signing off any work done - he's just a bit out of his experience zone on this.

[TraditionalToolworks]

I like using #2 for 100 amps copper.

Code specs #1 for 100 amps.

[smithdoor]

That is very possible where you live
The NEC let's you use #2 al up to 100. Amps in steel conduit.
My self I do no like using al wire
Do worry the power company will till you what to use.

Dave

I like using #2 for 100 amps copper.

Code specs #1 for 100 amps.

[TraditionalToolworks]

I like using #2 for 100 amps copper.

That is very possible where you live
The NEC let's you use #2 al up to 100. Amps in steel conduit.
My self I do no like using al wire
Do worry the power company will till you what to use.

I somehow see hypocrisy in your comments.

Nothing new there.

Honestly I'm not exactly sure what it is you were trying to say. Nothing new there either... When my smithdoor decoder ring arrives in the mail I might do better at understanding exactly what it is you say.

[noddybrian]

Thanks for the various replies - not really seeing anything that helps with the cable size issue but it's created more comments than I expected - the power company guys showed up Friday in 2 vans - one got out to watch the other guy hit the pole once with a hammer & announce the pole was good ! - I asked when they expected to make the connections & we could use advice on the cable as they need to check the install prior to making it live - the reply I got was " no idea mate - not our job - that's another department " - I thought I'd time traveled back to the 1970's on a union job ! will post back when I get any more info - in the mean time I'm curious where that power calculation came from - it's a long time since I went to college but we were taught 3phase in watts is line voltage ( 440 ) * line current ( 100 ) * power factor of load ( use 1 if purely resistive load ) * square root of 3 - so in my case it would be 76.2 Kw - average price per Kw/hr is £0,15 so I would not want to pay the bill if the supply was used at capacity for long.

[Oscar]

in the mean time I'm curious where that power calculation came from - it's a long time since I went to college but we were taught 3phase in watts is line voltage ( 440 ) * line current ( 100 ) * power factor of load ( use 1 if purely resistive load ) * square root of 3 - so in my case it would be 76.2 Kw - average price per Kw/hr is £0,15 so I would not want to pay the bill if the supply was used at capacity for long.

That was just some random calculation he was throwing out there, not necessarily anything to do with your scenario.

The only correction to your calculation is to divide by the power factor, not multiply by it.

[noddybrian]

Thanks Oscar - must have remembered it wrong - been a while since I was in college - most wiring I do all the loads are on a data plate on the machines so I just need to get correct size cable & breakers.

[smithdoor]

My calculations come from NEC handbook.
Any time installing electric panel you follow local codes.
Where I at the bottom of meter ring is local code. Top of meter ring is PG &E (power company)

Dave

[noddybrian]

If using USA standards then yes on short distance depending on allowable line voltage drop you can get away with 2gauge cable - on longer runs no - without knowing the distance or drop % you cannot say 2awg is adequate - once much over 100 feet from the service head then size needs to increase - on the job I'm helping out on they have not yet given us the route they want the cable to go for the least disruption & digging - I paced it out on what looks like the easiest route & it's around 400 feet but I cannot assume this till the owner clears it - this would likely require 1/0 gauge I believe in your terms - over here however all cable is metric & the closest will be 50mm - I think they already priced & hoped to get away with 35mm - will post back when I hear any more.

[smithdoor]

In the USA we have 120 volt to neutral (white) the 2 legs make 240 volt in most homes
I a lot office still 120 volt to neutral the 3 legs make 208 volts at this point is also 3 phase 208 volt
Now in most shops it 120 volt to neutral the 3 legs make 240 volt but the 3rd leg is call hot mark red and to neutral about 190 volts.
Now large shop 480 volt all three legs and all three legs is 277 volt to neutral. The new standard the neutral is gray but old is white for 480 volt

Now if working with old electronics look out the green wire hot and black ground some in 1960's they change green wire to red or some other color

some countries will use for ground use green and yellow
There other odd color combos and have list some where but just say look out or

Some new electrician's do not know all that info until
Most on this site do not know that info

Dave

Thanks to all that responded -
when I've spoken to the power company guys I will post back on their view or if we manage to find the code that covers it here - but just to clarify I'm in the UK so our power is all 240 single phase or 440 volt 3 phase - neutral does carry the full return load on single phase & does go directly back to the power company transformer - earth is only provided locally by ground rods -I agree if the load was all 3 phase then it's between the lines & almost nothing on neutral so no problem on cable size but this load is 3 independent single phase supplies as the power company will not upgrade the incoming supply above 100 amp - it is rural & would involve re stringing a long distance of overhead cables to the transformer assuming this has any capacity left as it already serves several properties.

[TraditionalToolworks]

In the USA we have 120 volt to neutral (white) the 2 legs make 240 volt in most homes
I a lot office still 120 volt to neutral the 3 legs make 208 volts at this point is also 3 phase 208 volt
Now in most shops it 120 volt to neutral the 3 legs make 240 volt but the 3rd leg is call hot mark red and to neutral about 190 volts.
Now large shop 480 volt all three legs and all three legs is 277 volt to neutral. The new standard the neutral is gray but old is white for 480 volt

But wire size is based on amps, so how does any of these volts even come into play?

[smithdoor]

In the USA we have 120 volt to neutral (white) the 2 legs make 240 volt in most homes
I a lot office still 120 volt to neutral the 3 legs make 208 volts at this point is also 3 phase 208 volt
Now in most shops it 120 volt to neutral the 3 legs make 240 volt but the 3rd leg is call hot mark red and to neutral about 190 volts.
Now large shop 480 volt all three legs and all three legs is 277 volt to neutral. The new standard the neutral is gray but old is white for 480 volt

But wire size is based on amps, so how does any of these volts even come into play?

Need to know what you are doing.
Here one if 240 volt motor 10 hp you about #6 wire gauge if 480 volt #12 gauge wire.

But cost cost of 480 equipment is about 3X times higher.
Single phase is lowest in cost just need bigger wire which is shop is best price.

Even conduit can change the cost 8 gauge and above .

Dave

[BugHunter]

In the USA we have 120 volt to neutral (white) the 2 legs make 240 volt in most homes
I a lot office still 120 volt to neutral the 3 legs make 208 volts at this point is also 3 phase 208 volt
Now in most shops it 120 volt to neutral the 3 legs make 240 volt but the 3rd leg is call hot mark red and to neutral about 190 volts.
Now large shop 480 volt all three legs and all three legs is 277 volt to neutral. The new standard the neutral is gray but old is white for 480 volt

But wire size is based on amps, so how does any of these volts even come into play?

Wire size is based on Watts, not amps. A wire will carry a certain amount of power. That is it. 100 volts at 2 amps is the same power as 50 volts at 4 amps.

Arno covered one of the misconceptions about neutrals when you're dealing with three phase. Number one the neutral is rarely ever used. Secondly, the idea that the same amount of power comes back as what goes out is incorrect. This is why you wire a switch not to switch the hot but to switch the neutral. That is because the light absorbs a portion of the power so the switch does not have to handle that power. It only has to create a ground reference for the load, while all the power is on the hot line.