jjmiller_08
Member
- Location
- North Dakota
What is the benefit of a 3 phase 208 panel to a single phase 240?
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Because I dont get it...
- Thread starter laketime
- Start date
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kwired
Electron manager
- Location
- NE Nebraska
Carultch
Senior Member
- Location
- Massachusetts
Maybe because you have sensitive equipment on the load side, that isn't dual rated for use with 208V.
kingpb
Senior Member
- Location
- SE USA as far as you can go
- Occupation
- Engineer, Registered
You can operate 3-phase loads.
kwired
Electron manager
- Location
- NE Nebraska
That response changes my whole perspective on what was being asked. I thought some kind of conversion from 208 three phase to 120/240 single phase was being asked about. But if it was just a question of advantages of having 208/120 three phase compared to having 120/240 single phase, then yes the ability to operate three phase loads is at the top of the list.
Next thing below that is the ability to gett nearly double the capacity (173%) by adding just one more conductor and one pole to the switch gear instead of having to go to the next standard "frame size". By that I mean 200 amp 208/120 can supply somewhat similar load as 400 amp 120/240 single phase can supply but only uses 200 amp gear and conductors instead of 400. The upfront costs are not so different at this level, but the price gap gets bigger when talking 600, 800 and 1000 amp plus gear sizes when comparing what is needed for single phase vs three phase.
jjmiller_08
Member
- Location
- North Dakota
The job I have now the mdp is feeding a 480vlt 400 amp panel. .A 75 amp breaker feeds a 75kva to step it down to the 208 panel. I am not sure what the panel is rated for, there is no main on it. Which is my next question I do have a small lite load do I need a main cb?
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jjmiller_08
Member
- Location
- North Dakota
Thanks for all your help
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jjmiller_08
Member
- Location
- North Dakota
I have a better understanding on the single phase to 3phase
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ron
Senior Member
- Location
- New York, 40.7514,-73.9925
typo
kwired
Electron manager
- Location
- NE Nebraska
About the two situations where you are permitted by NEC to protect the secondary side of a transformer via the primary side protective device is when you have single phase two wire output or when you have delta to delta three wire transformer.
A three wire single phase or a wye system are easy to unbalance and overload at least a portion of the transformer even though the net rating has not been exceeded. For example your 75 kVA 208/120 secondary will have a full load rating of about 208 amps. If you connected 45 kVA of 120 volt loads from A to N but had little or no loads on B and C phases - you are well under the total 75 kVA the transformer can deliver, but that one coil for A phase is only designed to carry 25 kVA and you have put 45 on it. Primary protection will not see any overcurrent condition here, but proper secondary protection in all three output conductors will.
petersonra
Senior Member
- Location
- Northern illinois
- Occupation
- engineer
the single phase panels are much cheaper.
ptonsparky
Tom
- Occupation
- EC - retired
You may not be required to have secondary protection for your transformer, but @ 75 amp for the primary, you may not get it to stay on.
See 240.21(C)(1) The last sentence.
Look at 240.21(C)(2), while your so close.
See 240.21(C)(1) The last sentence.
Look at 240.21(C)(2), while your so close.
JoeStillman
Senior Member
- Location
- West Chester, PA
Even though you don't need secondary protection for the tranny, you have to protect the secondary conuctors. 240 or 208, you should have an MCB.
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Only if there is a neutral conductor run from the secondary. 3-wire single phase and 4-wire three phase systems are required to have secondary conductor protection, or where otherwise and primary protection times voltage ratio is greater than ampacity of secondary conductors. As always, details, details, details.
208 75kVA transformer requires 200+ amps. With the same power at a higher voltage, the amp output should be cut in half. Well that's what p=iv states.
kwired
Electron manager
- Location
- NE Nebraska
Actually the current would be half if we were converting 240 volts to 480 volts. @ 208 volts to 240 volts the ratio is about 2.31:1 instead of 2:1.
480?Actually the current would be half if we were converting 240 volts to 480 volts. @ 208 volts to 240 volts the ratio is about 2.31:1 instead of 2:1.
kwired
Electron manager
- Location
- NE Nebraska
Yes, that is what I intended to say there.
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