240 volt , 3 phase , need neutral

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kwired

Electron manager
Location
NE Nebraska
We keep calling it a "derived neutral" mostly because nobody really knows what to call it.

It is really just an ungrounded conductor that depending on conditions may be operating near ground voltage.

If you want the conductor in question grounded - it needs be supplied by the main source grounded conductor, or come from a separately derived system where it is permitted - maybe required - to be grounded, but if separately derived you are not in parallel with the source winding like you would be with an autotransformer if you grounded the conductor in question.
 

kwired

Electron manager
Location
NE Nebraska
Take a look at 210.9 and 215.11 Circuits Derived from Autotransformers (in branch circuits and feeders, respectively). NEC requires that a grounded conductor be brought to and used by the autotransformer(s), with exceptions for certain line-line loads.

You might be able to use the derived neutral for something...but you would have to derive the neutral from a circuit which includes the neutral.....

-Jon
Does that mean when I have used a autotransformer in the past to derive 277 volts from 240 volts it violated this section? Have also gone from 277 to 240 before but transforming that direction usually does have a grounded conductor involved.

If it is acceptable for the OP I still say it is not code compliant to make the conductor in question white or gray, as it technically is not a grounded conductor.
 

Smart $

Esteemed Member
Location
Ohio
We keep calling it a "derived neutral" mostly because nobody really knows what to call it.

It is really just an ungrounded conductor that depending on conditions may be operating near ground voltage.

If you want the conductor in question grounded - it needs be supplied by the main source grounded conductor, or come from a separately derived system where it is permitted - maybe required - to be grounded, but if separately derived you are not in parallel with the source winding like you would be with an autotransformer if you grounded the conductor in question.
I agree that it would work... but the Code sections Jon cited expressly prohibit such.
 

winnie

Senior Member
Location
Springfield, MA, USA
Occupation
Electric motor research
Does that mean when I have used a autotransformer in the past to derive 277 volts from 240 volts it violated this section? Have also gone from 277 to 240 before but transforming that direction usually does have a grounded conductor involved.

It would appear to be a violation to go from 240 to 277 using an autotransformer.

I don't know the reasoning, but I can guess:
As you note, most situations where you have 277, it is 277 L-N. Most situations where you are starting with 240 it is 240V L-L, so you are changing it to something that is 277V L-L (unbalanced L-L but not usually relevant). However if you are now connecting a device which is designed for L-N operation, you might have a problem.

You would have a similar potential problem going from 277 to 240 (which _is_ permitted); a load expecting 240V L-L is instead seeing 240V L-N.

As far as the load is concerned, L-N or L-L is not a relevant. The potential issues are switching (single or double pole) and things such as screw shells or exposed conductors.

The exceptions appear to be allowing one type of common L-L to be converted to another common L-L supply.
-Jon
 

kwired

Electron manager
Location
NE Nebraska
It would appear to be a violation to go from 240 to 277 using an autotransformer.

I don't know the reasoning, but I can guess:
As you note, most situations where you have 277, it is 277 L-N. Most situations where you are starting with 240 it is 240V L-L, so you are changing it to something that is 277V L-L (unbalanced L-L but not usually relevant). However if you are now connecting a device which is designed for L-N operation, you might have a problem.

You would have a similar potential problem going from 277 to 240 (which _is_ permitted); a load expecting 240V L-L is instead seeing 240V L-N.

As far as the load is concerned, L-N or L-L is not a relevant. The potential issues are switching (single or double pole) and things such as screw shells or exposed conductors.

The exceptions appear to be allowing one type of common L-L to be converted to another common L-L supply.
-Jon

I can kind of see the logic there yet doesn't quite make full sense. My guess is they didn't think of all possibilities when making this rule and never occurred that it may not be that big of a deal in some instances. In my application of 240 to 277 it was for a bunch of 277 volt only HID luminaires the owner had bought used. If they would have been the usual quad tap ballasts I would have likely connected them to 240 volts leaving the lampholder shell at other then ground potential anyhow. The only difference here was the ballast supplied was same electrical characteristics other then they did not bring out multiple voltage taps. I was not aware of 210.9, but at same time still don't really see any harm in what was done. I don't plan to go change what was done a few years ago on this one either.

How many loads out there actually require a particular supply conductor to be grounded? Yes there are many that are intended to have a specific conductor grounded or even some that have the option of using a grounded conductor but it must be in a particular input lead if there is a grounded conductor, but same load will work just fine as long as rated voltage is applied whether there is a grounded conductor or not most of the time.
 

ggunn

PE (Electrical), NABCEP certified
Location
Austin, TX, USA
Occupation
Electrical Engineer - Photovoltaic Systems
So, are we back to a choice between installing a delta to wye isolating transformer for the entire load with a grounded neutral (separately derived) or pulling a neutral from the service?

The existing conduit is probably not big enough to add a neutral even if there is a way to pull it without endangering the existing CCC's. What a mess.
 
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Smart $

Esteemed Member
Location
Ohio
So, are we back to a choice between installing a delta to wye isolating transformer for the entire load with a grounded neutral (separately derived) or pulling a neutral from the service?...
It'd have to be a delta secondary if the equipment is looking for 240/120V 3? 4W, but otherwise, yes.
 

big john

Senior Member
Location
Portland, ME
So let's make a list of the important things we've learned from this thread:
1) Never ask for the manufacturer's opinion about anything.


...I think that about covers it.
 

kwired

Electron manager
Location
NE Nebraska
So, are we back to a choice between installing a delta to wye isolating transformer for the entire load with a grounded neutral (separately derived) or pulling a neutral from the service?

The existing conduit is probably not big enough to add a neutral even if there is a way to pull it without endangering the existing CCC's. What a mess.
If this is only to supply a control circuit no reason it can't be 14 AWG if it has no more load then a 14 can carry, as long as you are pulling it with branch circuit conductors and not with feeder conductors.

215.2(2) says it must not be any smaller then the required EGC, but that is for feeders, there is no such requirement for a branch circuit.
 

kwired

Electron manager
Location
NE Nebraska
So let's make a list of the important things we've learned from this thread:
1) Never ask for the manufacturer's opinion about anything.


...I think that about covers it.
Correct.

If the machine is new and you want to avoid warranty issues - why wasn't it ordered for the proper voltage setup? Good chance that machine can be ordered for multiple voltage configurations. If it is not new - there likely is no warranty and I will make necessary changes to work with what I have. UL listing - at very least that expires once the machine is put into service as things begin to degrade even if very slightly and at that point and you no longer have what was listed when it left the factory, you just have used equipment.
 

topgone

Senior Member
Correct.

If the machine is new and you want to avoid warranty issues - why wasn't it ordered for the proper voltage setup? Good chance that machine can be ordered for multiple voltage configurations. If it is not new - there likely is no warranty and I will make necessary changes to work with what I have. UL listing - at very least that expires once the machine is put into service as things begin to degrade even if very slightly and at that point and you no longer have what was listed when it left the factory, you just have used equipment.

Lesson learned: be specific what you want to buy. That's basic, IMO.
 

Smart $

Esteemed Member
Location
Ohio
If this is only to supply a control circuit no reason it can't be 14 AWG if it has no more load then a 14 can carry, as long as you are pulling it with branch circuit conductors and not with feeder conductors.

215.2(2) says it must not be any smaller then the required EGC, but that is for feeders, there is no such requirement for a branch circuit.
But I thought we were discussing a piece of equipment looking for 240/120V 3? 4W on 4 terminals. As such the neutral has to be rated for the entire load, not just the control components.

IIRC ther is only one section, albeit an exception thereto, which permits one to use a reduced neutral: 210.19(A)(3) Exception No. 2. It is regarding a 3-wire circuit supplying household range, oven, or cook top. And even for that case, reduction is not permitted under 70% and not less than 10AWG.
 

kwired

Electron manager
Location
NE Nebraska
But I thought we were discussing a piece of equipment looking for 240/120V 3? 4W on 4 terminals. As such the neutral has to be rated for the entire load, not just the control components.

IIRC ther is only one section, albeit an exception thereto, which permits one to use a reduced neutral: 210.19(A)(3) Exception No. 2. It is regarding a 3-wire circuit supplying household range, oven, or cook top. And even for that case, reduction is not permitted under 70% and not less than 10AWG.
What entire load? If the only 120 volt load is a contactor coil, how is the neutral ever loaded to more then a few volt-amps?

The household cooking appliance application is a little more specific load - though most of them have very limited load on the neutral, some can have significant neutral load, not same at all as a motor driven appliance that only has 120 volt controls. Now if the appliance has more significant neutral load then yes we need a conductor sized accordingly.

In a nutshell 210.19 says Branch-circuit conductors shall have an ampacity not less than the maximum load to be served - and that is the first sentence in (A)(1). If you have a 100 amp motor you need 100 amp supply conductor, if the control contactor for said motor only draws a few VA, we kind of have a rule that says 14AWG is the smallest conductor to run for most general applications, otherwise 210.19 is ok with whatever size is acceptable to carry those few VA because that is the load served by that conductor.


I will add if the load is only those few VA and we are sizing a neutral for a feeder - then the 250.122 sized conductor is the minimum size. So are you suggesting I can run maybe a 8AWG feeder neutral (say this circuit is protected by a 100 A breaker) but from final branch overcurrent device I would need to increase the neutral to same size as the ungrounded conductors?
 

Smart $

Esteemed Member
Location
Ohio
What entire load? If the only 120 volt load is a contactor coil, how is the neutral ever loaded to more then a few volt-amps?

The household cooking appliance application is a little more specific load - though most of them have very limited load on the neutral, some can have significant neutral load, not same at all as a motor driven appliance that only has 120 volt controls. Now if the appliance has more significant neutral load then yes we need a conductor sized accordingly.

In a nutshell 210.19 says Branch-circuit conductors shall have an ampacity not less than the maximum load to be served - and that is the first sentence in (A)(1). If you have a 100 amp motor you need 100 amp supply conductor, if the control contactor for said motor only draws a few VA, we kind of have a rule that says 14AWG is the smallest conductor to run for most general applications, otherwise 210.19 is ok with whatever size is acceptable to carry those few VA because that is the load served by that conductor.


I will add if the load is only those few VA and we are sizing a neutral for a feeder - then the 250.122 sized conductor is the minimum size. So are you suggesting I can run maybe a 8AWG feeder neutral (say this circuit is protected by a 100 A breaker) but from final branch overcurrent device I would need to increase the neutral to same size as the ungrounded conductors?
I have no problem with your rationale. I'm just saying Code does not give us any avenue to reduce conductors that supply only part of the load. There is no this or that part of the load at the branch circuit level. The load is the entire load on the circuit.

Code treats service and feeder conductors differently, as plainly evident by 220.61. Also note the difference between 210.19(A)(1) first sentence and that of 215.2(A)(1).
 

kwired

Electron manager
Location
NE Nebraska
I have no problem with your rationale. I'm just saying Code does not give us any avenue to reduce conductors that supply only part of the load. There is no this or that part of the load at the branch circuit level. The load is the entire load on the circuit.

Code treats service and feeder conductors differently, as plainly evident by 220.61. Also note the difference between 210.19(A)(1) first sentence and that of 215.2(A)(1).


If we had 208/120 supply to a multiwire circuit and a 100 amp fused disconnect is it not code compliant to install a 70, a 90 and a 100 amp fuse in this disconnect and size the conductors accordingly if that is all the load that is connected to each respective line? I agree this is somewhat unconventional, but I also don't see a problem with it. The difference is the grounded conductor is not limited by overcurrent protection, the load limits the current for a grounded conductor and if the load is only a few VA of control contactors, that is all the grounded conductor will ever see during normal operation.
 

Smart $

Esteemed Member
Location
Ohio
If we had 208/120 supply to a multiwire circuit and a 100 amp fused disconnect is it not code compliant to install a 70, a 90 and a 100 amp fuse in this disconnect and size the conductors accordingly if that is all the load that is connected to each respective line? I agree this is somewhat unconventional, but I also don't see a problem with it. The difference is the grounded conductor is not limited by overcurrent protection, the load limits the current for a grounded conductor and if the load is only a few VA of control contactors, that is all the grounded conductor will ever see during normal operation.
Like I said, I have no problem with your rationale. All I'm saying Code does not allow it for branch circuits (other than the already mentioned exception).
 
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