MCC Feeders, Neutral

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donny baker

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Where is the neutral usually derived from for single phase loads in an MCC. Is it grounded to some building steel or a ground rod underneath the MCC? I have seen a lot onf single phase load in and MCC design. Why not just pull another conductor along with the ground and the three phase conductors? Is it really because of cable cost (I have heard)?
 
If you have single phase line to neutral loads, the system must be fed from a wye transformer and you must run a grounded conductor to the loads from XO on the transformer. What kind of single phase loads are you talking about.
Don
 
Connecting the neutral bus to planet Earth does not ?derive? a neutral that can be used to serve single phase loads. That derivation must take place at the source. If you need to serve a single phase load from an MCC, then I can only offer you three choices. One is to bring a neutral wire to the MCC from the upstream panel. Another is to buy an MCC that has a step-down (or isolation) transformer within the MCC enclosure, along with a set of single phase breakers. However, I don?t know if anyone makes MCCs that have that option. The third option is not to supply the single phase load from the MCC, but rather from a nearby 120/208 panel.

Welcome to the Forum.
 
charlie b said:
buy an MCC that has a step-down (or isolation) transformer within the MCC enclosure, along with a set of single phase breakers. However, I don?t know if anyone makes MCCs that have that option.
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Yes that is available, and it is a nice clean looking installation. :smile:
 
If someone wants to put a tiny 20 amps of single phase load on a 600 A MCC, what can we derate it to? I read several length threads about this, so lets assume it is a purely resistive load; does it have to be at least a 3/0 (200A) neutral?
 
donny baker said:
If someone wants to put a tiny 20 amps of single phase load on a 600 A MCC, what can we derate it to?
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The term ?derate? does not apply here. That term is about changing the allowable ampacity of a conductor, not about determining the amount of current that the load will draw (i.e., ?required ampacity?).

Are you asking whether you need the neutral conductor from the upstream panel to the MCC to be the same size as the ungrounded conductors? That answer is ?no.? Generally, you size the neutral based on the calculated load. I have never seen a 600 amp panel with a calculated neutral load of 20 amps. So I am not sure what to advise.
 
I was thiniking about it a little more, shouldn't the neutral should have to be the same size as the phase conductors up untill a 3/0 or 200A to satisfy 220.61.

In theory: say to a small panel, could I technically have 60 amps of 3phase load and a few of single phase and have a tiny #12 neutral (for argument). Or could I have 180 amps of single phase plus a small imbalance and a #12 neutral. Electrically the neutral only has to carry the unbalanced current right, but if you loose a phase, your OCPD won't operate, the neutral will just burn up?

So I was asking if you have ANY single phase load on a 3 phase panel, the neutral has to be the same size as the phase conductors, OR up until 3/0. Con you confirm?
 
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Donny,
The neutral has to be the larger of what is required by 220.61 or 215.2(A)(1). The requirement in 215.2(A)(1) is new to the 2005 code. It says that in cases like your example with a very small calculated neutral load, it has to be sized based on the feeder OCPD and Table 250.122.
Don
 
georgestolz said:
Got a link? :)
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Yeah, most any manufacturer. :)

Pay for it and it will be built. :cool:

At this point you can pretty much get anything factory assembled. We do a fair number of jobs that the entire electric rooms come in pre-assembled on large trucks. Fire alarm systems, PA and phone systems along with switch gear, transformers and panels just waiting for the 'field connections'

Not saying I like it, just the way of the future. The cost of labor in the field will continue to push us this direction.
 
georgestolz said:
Got a link? :)
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Attached is a picture of a GE MCC with a transformer. The panel in this case is outside the MCC. I have another MCC with 480/277 and 208/120 panels inside. Next time I am up there I will snap a pic.
 
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