When to Bond XO to ground in transformers and when not to?

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chasejordan22

Member
Location
Ohio
Occupation
Journeyman Electrician
I was wiring up a step up transformer from 208 to 480 the other day and my inspector happened to walk in and tell me not to bond the neutral to the ground on XO. He said it would go boom. Now Im curious how I would know when to leave out the bonding between neutral and ground in transformers.
 
Then the inspector is correct, Connecting the neutral in that situation causes improper current flow.
You treat is like a delta primary,.
If the secondary (480) is WYE you would bond that neutral.
 
Short, rather incomplete answer:
You bond and ground XO when the neutral is not bonded and grounded elsewhere, such as at the service. You do not bond and ground XO when the neutral IS bonded and grounded elsewhere.

(I wonder if your transformer is being used correctly as a step up. Sounds more like it's intended for step down. But I could be wrong.)
 
chasejordan22 said:
Now Im curious how I would know when to leave out the bonding between neutral and ground in transformers
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You almost never bond the XO if you are using the wye side of the transformer as the primary. Some rare cases when you have a wye wye transformer would be one.

Most transformers we, meaning electricians, use are Delta/Wye so you don't have a neutral to consider on the primary side.
 
infinity said:
Sounds like it's reverse wired which means that it needs to be listed for such a connection.
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It has to be listed for what? Can you show me an example of this listing? As far as I understood a typical transformer has a high side and a low side. I haven't seen where the line side and the load side is also designated. If this is so, I feel it is something I need to know.
 
Strathead said:
It has to be listed for what? Can you show me an example of this listing? As far as I understood a typical transformer has a high side and a low side. I haven't seen where the line side and the load side is also designated. If this is so, I feel it is something I need to know.
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As part of the manufacturer's instructions it would need to be permitted to be reverse wired.

From Mike Holt:

New with the 2014 NEC is the text regarding secondary voltage-supplied transformers. A transformer can be supplied at the secondary voltage the manufacturer's instructions permit that arrangement [450.11(B)].

Many people don't realize that transformers are suitable to be supplied at the secondary voltage only if the transformer is marked for this application. For example, a transformer that's marked as 480V primary, 208V secondary can't be wired with a 208V primary and 480V secondary.

According to UL 1561, only transformers marked in this manner can be installed this way.
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Also:

450.11(B) Transformers. Source Marking.

Here is a PV system inverter output circuit connection to an existing step up transformer. Based on the line diagram, is this a grounded or ungrounded PV system? How can you tell? What color wires should be used or not used for the negative source cuircuit conductors?
www.electricallicenserenewal.com
 
infinity said:
As part of the manufacturer's instructions it would need to be permitted to be reverse wired.

From Mike Holt:


Also:

450.11(B) Transformers. Source Marking.

Here is a PV system inverter output circuit connection to an existing step up transformer. Based on the line diagram, is this a grounded or ungrounded PV system? How can you tell? What color wires should be used or not used for the negative source cuircuit conductors?
www.electricallicenserenewal.com
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Thank you.
 
ron said:
With the Delta - Wye being used as a step up (wye on low side), you will need to either corner ground the delta (high side) or provide ground detectors as an ungrounded system.
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I ran into this recently on a PV system I partially designed that connected 480V inverters to a 208/120V service. The installer bought the wrong (large, expensive, and non returnable) transformer and installed it; instead of the 208V delta primary, 480/277V wye transformer I specified they got one with a 208/120V wye primary, 480V delta seondary. Luckily for them, the inverters they chose could run on a 480V ungrounded service. I got them to wire the primary correctly and install ground detectors on the secondary.
 
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