277/480 panelboard used on a 480 3 wire no neutral

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Jeff35

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I’m trying to order a panel for a change out I’m going to do. The building service is 277/480 but the old feed to this panel is 3 phase 480 no neutral. My gear guy is telling me I can’t install a 277/480 panel because I have to have a neutral. He’s saying it has to be a 480 delta panel with straight rated breakers (more expensive and way longer lead time this day in age)
I just want to make sure I wouldn’t be in any violation of a code I’m unable to find. End of the day I want it correct. Any code reference would be appreciated.

Thanks Jeff
 
Need clarification. Are you saying that you have a 480/277 service with a neutral brought at least to the service disconnect and you then have a feeder from that service to a panel that does not include a neutral?
 
Since the actual utility service is a grounded wye, I believe that current code requires that a POCO neutral be carried as far as the service disconnect, whether it is actually used in the building or not. You are not required to carry the neutral beyond the service disconnect, but it has to be there and bonded to the building GES at that point.
Your gear guy would be correct if the actual service were an ungrounded delta. Using a 277/480 panel would both be misleading and would allow you to install breakers which were not rated for the actual supply voltage.
But the service really is a grounded wye, and a 277/480 panel with slash rated breakers is perfectly appropriate.
But it may be interesting getting a POCO neutral to the panel.
If the service disconnect is four wire, and you have a 3 wire feed to a subpanel, you can still use a 277/480 panel, but make sure that you indicate that you cannot attach line to neutral loads at that panel.
 
Jeff35 said:
My gear guy is telling me I can’t install a 277/480 panel because I have to have a neutral. He’s saying it has to be a 480 delta panel with straight rated breakers.

Thanks Jeff
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The straight rating vs slash rating, refers to the fact that the nominal voltage to ground cannot exceed the lower voltage stated in the rating. Given that your main supply is derived from a 277/480V wye service, you have 277V to ground. Both at the main service panel, and at any subpanel you derive directly from it. This is the case, regardless of whether you bring the neutral to the panel or not.

Derive from 277/480V wye, and you can use slash-rated 277/480V breakers. Had it been derived from a 480V corner-grounded delta or floating delta, you would need 480V straight-rated breaker.
 
Yes, sorry. This is a old sub fed panel from the main service MDP. The service has a neutral but there is not one at my sub panel. I believe he’s wrong but he was very sure of him self
 
Jeff35 said:
Yes, sorry. This is a old sub fed panel from the main service MDP. The service has a neutral but there is not one at my sub panel. I believe he’s wrong but he was very sure of him self
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He is misinformed. This is not an application where you need straight rated breakers based on your description of the system.
 
Jeff35 said:
Yes, sorry. This is a old sub fed panel from the main service MDP. The service has a neutral but there is not one at my sub panel. I believe he’s wrong but he was very sure of him self
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Yes he's incorrect. The fact that it's a sub-panel with no 277 volts loads means that the neutral is optional and slash rated CB's are fine.
 
Slightly off topic, but still on topic.
Say you have a 480/277 Volt generator. Would there be any problem leaving the neutral off of the generator and just running 3 conductor 350 MCM to the transfer switch? The 3 conductor 350 MCM cables from the transfer switch feeding the MCC have no neutral and don't need or have any 277V loads, nor is there any plans for 277V loads. All grounding would still be done to code.
Basically, will the 480/277 generator run fine and dandy if you leave its 277V neutral wire off?
 
At work. Sorry I spoke to quickly and wasn’t thinking that you can just do your system bonding jumper at the geny. These 2am starts are starting to get to me :sneaky:
 
powerpete69 said:
Slightly off topic, but still on topic.
Say you have a 480/277 Volt generator. Would there be any problem leaving the neutral off of the generator and just running 3 conductor 350 MCM to the transfer switch? The 3 conductor 350 MCM cables from the transfer switch feeding the MCC have no neutral and don't need or have any 277V loads, nor is there any plans for 277V loads. All grounding would still be done to code.
Basically, will the 480/277 generator run fine and dandy if you leave its 277V neutral wire off?
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Just be sure there is a N-G bond at the gen and a GEC run from the gen since it is a SDS, and good to go.
 
Jeff35 said:
Yes, sorry. This is a old sub fed panel from the main service MDP. The service has a neutral but there is not one at my sub panel. I believe he’s wrong but he was very sure of him self
Click to expand...
Just to back up this issue, probably 90%+ of MCCs are exactly like this, because they generally only feed motors or other non-neutral loads. It confuses a lot of people, but it's perfectly legit.
 
Keep in mind the differences between different kinds of grounding.

First up is the 3 wire delta. It can also be an ungrounded wye. The loads don’t know the difference. Can’t do line to neutral loads but phase to phase single or three phase loads are fine. On BOLTED faults the first fault to ground turns it into a corner grounded delta system but doesn’t trip anything. You are required to have ground fault indication/alarms and need to take steps to find and repair it. Since there is no ground fault current the way to find it is open breakers until the ground fault indicator clears which can be time consuming. If you get a second fault you get a line-ground-line fault which often doesn’t trip anything and can cause a burn down. Arcing faults interact with the system capacitance and cause line to ground voltages of around 6 to 8 times the system voltage which destroys motors. Motor failures are much higher on ungrounded systems leading to much higher maintenance costs.

Going to the other extreme is solidly grounded. Line to neutral loads are possible. Neutral and ground conductors are large so installations are more costly. A ground fault has almost as much current as a phase to phase short. In smaller breakers the phase short circuit protection can do double duty as ground fault protection. Must trip ASAP for that reason. But arcing faults don’t generate high voltages. In fact line to neutral voltages are limited so theoretically you could use 300 V insulation in a 480 V system but there’s no cost advantage.

Next up is the peg ground. I really can’t say enough bad things about this one. Technically this is what must utilities use but there are good reasons it works for them. Peg grounding is what the original Edison DC Dynamos used. He ran one wire to a string of lights and used a ground rod as the return path. Tripping is nearly impossible and you can’t control the ground path well. It works fine at long distances (more than a couple miles) where Earth impedance is less than a neutral wire. Lots of elevated energized situations with these systems, even panels unrelated to the fault where the path through some equipment just happens to be less than the Earth path.

You can convert an ungrounded system by grounding one phase making it corner grounded delta. Line to neutral is 480 and phase faults on the grounded phase don’t show up but other than that it’s almost identical to grounded wyes in terms of operation.

Next up is resistance grounding. In this case we stick a resistor between the transformer neutral and the Earth. You get most of the advantages of the ungrounded and grounded systems. No line to neutral loads. Maximum system voltage in arcing faults is line to line voltage so no 300 V insulation for instance. There are two types. Low resistance means the resistor is chosen based on line to neutral voltage divided by typically either 400 or 100 A. So a ground fault can never exceed that current. The resistor wattage is typically chosen to survive a fault for up to 10 seconds so must trip within 10 seconds. The faults are much less destructive but other than that not much difference between this and solidly grounded. High resistance grounding means picking a resistor so the ground fault current is at least 3 times the system charging current but not much over that. A typical value for 480 V systems is 15 A, which means a #14 wire can withstand it indefinitely. It is possible to use fault alarms only on high resistance systems. Smaller grounds are legal.it us very easy to find faults since there is actually a fault current. Damage is minimal in a fault. Ground fault trip units work well with this system. It keeps both initial install and long term maintenance costs low. Arc flash can occur but rarely does since the current is not really enough to sustain an arc. Above 10 kV high resistance grounding resistors become huge and low resistance becomes attractive.

Ungrounded systems can easily be converted. Just install a zig zag transformer or a delta-wye isolation transformer and the neutral appears on the wye output. Transformer is rated for the ground fault current only so they are small. You can step down too if desirable.

There are other schemes (Peterson, reactor grounded, etc.) but they.are mostly academic and don’t improve on the above.

So if I was upgrading an existing ungrounded system I’d install the transformer and switch to high resistance grounding for the cost and simplicity of it,
 
Jeff35 said:
At work. Sorry I spoke to quickly and wasn’t thinking that you can just do your system bonding jumper at the geny. These 2am starts are starting to get to me :sneaky:
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You still would need an EGC from the generator, but on the OP topic involved here there would be no problem using slash 277/480 breakers. If you were a separately derived system with a grounded phase the slash breakers would not be allowed.
 
Jraef said:
Just to back up this issue, probably 90%+ of MCCs are exactly like this, because they generally only feed motors or other non-neutral loads. It confuses a lot of people, but it's perfectly legit.
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But the MCC's don't typically have 277/480 breakers in them either they are likely either straight 480 or straight 600 volt rated and therefore you don't have to even consider that slash rating.
 
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