Secondary OCPD

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brintonkeith

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Hello fellow electricians. My situation is this. We are installing a step down transformer for machines that require a 240 rating. Which would be a 480delta/240delta transformer. It's rated at 150kVA. Some fairly sized machines.

Anyway, my curiosity is this. The transformer that we are installing is in fact a 480/240 delta-delta. But the secondary also has a phase that is center tapped for line to neutral loads. However we are not using it for that application. Every load will be a 3phase load only.

1. Should it be corner grounded or can I ground the center tap to protect from over voltages? (On the secondary side, also I know I don't want the system to be an ungrounded system)

2. Since it technically will be a completely balanced 3-wire primary and 3-wire secondary does that mean the panel fed by this transformer can be MLO as stated in article 240 section 240.4(F)?

Thank you all in advance.

Also my thoughts are to just bond the center tapped phase to make it a grounded system to circumvent the white colored wire of I corner grounded, and in my estimation the panel can be MLO. If I understand the situation correctly.
 
If your transformer has a center tap you need to use that as your ground. .Follow 250.30 amnd install a system bond jumper, a supply side bond jumper and a grounding electrode system.
Per 408.36 you panel will need main OCP.
 
augie47 said:
If your transformer has a center tap you need to use that as your ground. .Follow 250.30 amnd install a system bond jumper, a supply side bond jumper and a grounding electrode system.
Per 408.36 you panel will need main OCP.
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Awesome. So I'm on the right path. I definitely knew to bond the center tap to ground. I just need some confirmation. As for the section for panelboard OCP my mind was so focused on the secondary OCPD protection I didn't even think to the section you referred me too. Thank you so much.

But just for fun, if it wasn't a panelboard I was feeding and it was going to a load directly. Would the secondary side of my transformer need OCP? As per the 240.4(F)?
 
Good question and I don't know the answer. Technically you have a delta-delta transformer so the primary should protected the secondary BUT you have a delta-tapped delta. Hopefully someone with greater knowledge on the subject will respond.
 
augie47 said:
Good question and I don't know the answer. Technically you have a delta-delta transformer so the primary should protected the secondary BUT you have a delta-tapped delta. Hopefully someone with greater knowledge on the subject will respond.
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Having the center tap on the output create a 4Wire system, so secondary side rules, like 240.21(C), would apply.
 
jim dungar said:
Having the center tap on the output create a 4Wire system, so secondary side rules, like 240.21(C), would apply.
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If you connect anything to it you have a 4 wire system, if you leave it floating you have a 3 wire system.

I don't think there is a rule that you must use it.

If you used three single phase transformers with center tap secondary's to build a three phase delta bank you would have three of these center taps. IMO you have a choice there to use one of them to derive a high leg delta secondary, or to use none of them and derive ungrounded or corner ground system.

If you do ground that center tap but don't connect any load to it, you still technically have a 4 wire system and can not use the primary OCPD to protect the secondary conductors AFAIK.
 
Ok so I bonded the the X4(center tap) to the ground bus to make it a grounded system. But we aren't using a neutral to feed the panelboard. So if my understanding is correct. In this scenario, secondary OCPD would be required at any point on the supply side to the panel board. (Which would make the panelboard a main breaker panelboard). Seems right to me.
 
Thank you all for your responses. Installing proper electrical systems is exciting l. Especially when knowledge increases. Thanks everyone.
 
Jraef said:
Side note: if any of the machines have VFDs or Servo Drives on them, you should check to see what they require regarding connecting to a delta source.
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Though one should look into it anyway, I think is more common to be able to connect with no additional requirements on a 240 delta than on a 480 delta system.
 
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