I've got an old 480v to 120/240v single-phase transformer that I want to use for some electric heat. Using my multimeter, I get 180v to ground on one leg of the secondary and 80v to ground on the other leg. I'm sure this is because the secondary is not bonded. Right now, the secondary is wired strictly for 240v. Do I need to wire it for 120/240v so that I can create a neutral point for bonding purposes?
Transformer Bonding
- Thread starter A-1Sparky
- Start date
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- Location
- Logan, Utah
I agree with Gus.
250.20(B)(1) would require that this transformer be grounded.
Chris
250.20(B)(1) would require that this transformer be grounded.
Chris
winnie
Senior Member
- Location
- Springfield, MA, USA
- Occupation
- Electric motor research
Depending upon the rest of the system being considered, you might be able to get away with bonding one of the 240V terminals, giving a 2 wire 240V system with no neutral.
Your secondary side OCPD would need to be rated for 240V rather than slash rated for 120/240V, however as a 2 wire to 2 wire transformer, your primary OCPD may be able to serve as your secondary OCPD as well (see 240.21(C)(1) )
-Jon
Your secondary side OCPD would need to be rated for 240V rather than slash rated for 120/240V, however as a 2 wire to 2 wire transformer, your primary OCPD may be able to serve as your secondary OCPD as well (see 240.21(C)(1) )
-Jon
LarryFine
Master Electrician Electric Contractor Richmond VA
- Location
- Henrico County, VA
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- Electrical Contractor
Wouldn't he then also be able to use 1P OCP?
dbuckley
Senior Member
- Location
- Canterbury, New Zealand
I disagree that definition - if you bond one of the secondary treminals then that is the neutral.
If there is no bond, then you have a 240V system with no neutral.
winnie
Senior Member
- Location
- Springfield, MA, USA
- Occupation
- Electric motor research
dbuckly,
If you use 'neutral' as a synonym for 'grounded conductor', then I agree with you.
I believe that the following is current usage:
'Neutral' indicates a source terminal/circuit conductor that is in some sense 'equidistant' from the other source terminals.
'Grounded' indicates a source terminal/circuit conductor that is intentionally bonded to ground potential.
Two different words for two different concepts, which in most systems happen to apply to the same conductor.
-Jon
If you use 'neutral' as a synonym for 'grounded conductor', then I agree with you.
I believe that the following is current usage:
'Neutral' indicates a source terminal/circuit conductor that is in some sense 'equidistant' from the other source terminals.
'Grounded' indicates a source terminal/circuit conductor that is intentionally bonded to ground potential.
Two different words for two different concepts, which in most systems happen to apply to the same conductor.
-Jon
erickench
Senior Member
- Location
- Brooklyn, NY
You most certainly do. You need that balanced voltage if you intend to connect 120V loads.
I don't have any 120v loads--strictly 240v for baseboard heat. My question was based on code requirements for bonding the transformer. I need to make sure the OCPD will activate in case of a fault. The way that it's currently wired, I don't believe it will.
erickench
Senior Member
- Location
- Brooklyn, NY
In that case it's an ungrounded separately derived system. The EGC is bonded to the grounding electrode conductor. But if you only have 240 Volt loads, do you really need a neutral? NEC 250.21(B) requires you to install Ground Detectors.
winnie
Senior Member
- Location
- Springfield, MA, USA
- Occupation
- Electric motor research
If you do not ground the secondary of a transformer, then a single ground fault on the secondary side will not trip an OCPD. While ungrounded systems are intentionally used to provide continuity of service in case of a ground fault, I don't believe that this is applicable in your situation. The secondary of your 480V to 240V transformer needs to be grounded.
The question remains open as to _which_ terminal to ground. Common practise would be to ground the neutral of the secondary, giving a common 120/240V system. You would use normal 120/240V breakers, and your heaters would be served by double pole breakers.
As I stated above, I believe that you could ground one of the 240V terminals, and simply have a straight 240V system. This would be a 240V grounded system where one of the circuit conductors was a grounded conductor, and the other was the 'hot' conductor.
Additionally, for 2 wire to 2 wire transformers, the secondary current is a direct function of the primary current, and you may be able to rely on primary OCPD only for all of the overcurrent protection. It isn't a given that this would work (for example, if the OCPD required for proper secondary side protection is too small to prevent tripping on inrush), but if it does work this could make the installation much simpler.
What size transformer are we talking, and how many watts of heat?
-Jon
The question remains open as to _which_ terminal to ground. Common practise would be to ground the neutral of the secondary, giving a common 120/240V system. You would use normal 120/240V breakers, and your heaters would be served by double pole breakers.
As I stated above, I believe that you could ground one of the 240V terminals, and simply have a straight 240V system. This would be a 240V grounded system where one of the circuit conductors was a grounded conductor, and the other was the 'hot' conductor.
Additionally, for 2 wire to 2 wire transformers, the secondary current is a direct function of the primary current, and you may be able to rely on primary OCPD only for all of the overcurrent protection. It isn't a given that this would work (for example, if the OCPD required for proper secondary side protection is too small to prevent tripping on inrush), but if it does work this could make the installation much simpler.
What size transformer are we talking, and how many watts of heat?
-Jon
erickench
Senior Member
- Location
- Brooklyn, NY
Yes I agree. This may be your best bet if you intend on grounding the system. It would eliminate the abnormal voltages and give 240 Volts to ground on the high side.
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