Neutral Current In 1 Phase Configuration

Provide several 3 phase 277/480 volt panels with 277 volt breakers. Each breaker serves one heater. The only neutral return is on each heater circuit. Since the panel can not be always balanced, there would be some neutral current on the feeder circuit to each panel. That should not be a problem.

Sounds like a wye type transformer configuration. In that case the currents in the Neutral could infact become imbalanced. If it wasn't a wye it is my understanding that if you could ensure that the loads would balance properly then the neutral current could be very low.

Say what?

How do you get a "neutral" current on anything other then a wye type transformer winding?

I believe it is common practice in rural areas of Pennsylvania to have delta transformer banks for 240/120 VAC service with a "neutral" tapped off middle of one leg of the transformer, creating 2 @ 120VAC legs and a "high" leg used only for three phase feeds. Power companies don't like to feed three phase into areas without any expected industrial facilities, and create these Frankensteins to get around that problem.

I know this, since as a green engineer just out of college, I caused quite a bit of grief for my employer when I designed a building with 120VAC breakers evenly distributed across all three phases of the service. (lighting panels were all three phase with single phase breakers) Good old PP&L provided the service as noted above, since this building was out in the boonies. The electrical contractor wired to my design. So, we had a few dozen fluorescent fixtures go poof when the power was turned on to the building. How the inspector missed this? I don't know. So, yes, there can be deltas with a neutral.

Ya... pretty common in the old days. But that is still essentially a proper neutral, albeit from a single phase of the delta transformer.

It is pretty clear the poster was talking about neutral currents in a delta transfromer.

You are talking about a 200 kW load for the area. Way beyond the power of a "small" 460V motor.

Install 5 new 100-Amp 480 VAC, 3-phase panels with 24 2-pole breaker spaces and 20 breakers per panel in the room fed by a individual 100-amp 3-phase breakers in the room with the primary power. Wire all of the feeds to the heaters individually from each breaker.

You can group the feeds in larger conduits to localized junction boxes (make sure that you size conductors based on multi-line conduit rules). You will only have about 4.2 amps per feed, so 12 gage wire should be adequate even with the multi-conductor derating.

Don't screw with single phase power or connections- You may even have a problem getting the right heaters.

The circuit breakers in your plan worked the best for balance and consumption, since we installed just this type of service and each heater had its own twistlock connector. There was no access problem for replacements, and the rest of the machine ran under lower stress levels.

yes your have to add another local disconnect swith per code for next 100 machines and use a 277V feed to each heater.The phase current will cancel each other out.So factor in any imbalance created by random on-off of the indivudual heaters in the phase schift of cause.

Best Regards

Mr joseph

RF Eng

You do not, necessarily have to add a local disconnect. If the branch breaker (assuming) has a "lock-off" device.

There are several issues to address in your post.

1. You are right about the current. The total neutral current will never exceed the highest current of any one phase conductor.
2. If your source is wye-connected and the wiring to the machine is across the line (no VFD or similar electronics), connecting the heaters phase to neutral will work just fine.
3. As long as the neutral is grounded at the source (grounded wye), there is no need to add another pole to the existing disconnect switch. Nothing in the NEC requires disconnecting the grounded neutral conductor.
4. If you phase balance the heaters, the worst case imbalance (and therefore neutral current) would be 245A. This would only occur if all heaters on one leg were on, and all heaters on the other 2 phases were off, at the same time.
5. As someone else already mentioned, you may not need the local disconnect. The code requires a disconnect within sight OR that the remote disconnect be lockable in the open position. See NEC 422.31(B).

VFD's get really unhappy when phase currents are not balanced. If you use VFD's or other power electronics, the imbalance will likely cause them to trip.

Mark in NJ,

As you have noted in the answers so far, there are many possibilities to choose from. I can't tell which one will be best for you because I do not have the full design and layout for the machines, the incoming power, and the controls. If your incoming power is 480v delta (no neutral), the cost of a delta-wye transformer will be significant.

Let me suggest one approach:

• For each machine, use a single 3-pole disconnect, with the strip heater and motor both being connected to the load terminals of the disconnect. Use a fusible disconnect (explained below). Use the suggested 480v heaters; no neutral is needed (eliminates one wire).
  
• Depending on the U/L listing of the starter you use, you may need an MCP for the starter or you may be able to have the fuses in the disconnect provide short-circuit protection for the motor. Use an IEC style starter, if you have a choice, because they include protection against single-phasing.
• Feed each group of 6 machines from a single circuit rated 30A. The load on this circuit will not exceed 16A for the heaters plus the load for the motors. I assume here that the motors are 3/4 HP or smaller.
• Use two 36-pole or larger breaker boxes with a 225A or larger bus (assuming the motors are 1/2 HP each; 300A feed and larger bus would be required for larger motors). Divide the machines equally between the two breaker boxes.
• Depending on cost and local conditions, you could use a single breaker box with a 600A bus. The 42 pole limit does not apply because this is not a lighting and small appliance panelboard (and this limit was removed in the 2008 NEC).
  

The "devil is in the details". You will have to choose how to proceed. I am sure that cost will be a significant factor in this, so 4-pole disconnects may be a problem (that's why I didn't suggest their use).

--JMM

You can connect in star and balance them This would be the best solution. You neutral current in worst case condition cannot go beyond phase current. You are likely to get bst results in the star connection.