Article 547 - Site Isolating Devices

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Senior Member
Reviewing article 547. So far, the only farm premises I have worked on have been smaller applications. I worked on one premises with an outdoor equipment rack that housed the main service disconnect and had feeders going to barn, house, other outbuilding. I've seen one installation where there were two separate services. I don't have specific experience with seeing an outdoor, site isolation device that is not considered service equipment, but reading the article, I wonder if it would require some revision.

First, 547.1 says the entire article shall apply to buildings or that portion of buildings that are similar in nature to those described in 547.1 A&B. Then, it goes on to describe that a distribution point like described in 547.9 is allowed. Many installations may have only a few of the several buildings with the hazards described in 547.1 A&B, but this is describing something more general to agricultural buildings regardless of dust/corrosion risks.

Perhaps the scope of the article should change, the there should be a part II added to cover specific wiring methods and protections for those portions of a agricultural premises exposed to dust/corrosion hazards.

Second thing that trips me up a little is 547.9(B)(3)-1. It requires the equipment grounding conductor between the site isolation device and the service disconnecting mean(s) be not smaller than the largest supply conductor. I would think that the reason for this requirement is that this conductor is being relied upon to handle fault current that would run all the way back to the site isolating device before completing the circuit through the neutral, and there is concern about it being undersized and not being able to enable the over-current device to operate upon a fault condition. The issue is that it says if the EGC is of different material, then it shall be sized per the equivalent size of table 250.122. If the goal is to ensure the ampacity (or resistance) of the EGC is at least as good at the ungrounded conductors, then it would seem that it should either just require it to have the same ampacity, or that it should reference the equivalent size table of that of article 310.

Does anyone agree on this?

Here's one reference:


Electron manager
NE Nebraska
When it comes to services, feeders to different buildings and such, I do believe today's farms have evolved to something other than what was common at time 547 was developed.

Reading through the scope of 547, this was only intended to cover buildings with livestock for the most part, and possibly feed and other closely related operations. Crop raising and other related operations are not even mentioned in the scope, though you can still run into similar environments from the perspective of dust, moisture, corrosion.

Houses, shops, or other non livestock operations are not part of the scope, they still may be supplied by the distribution point/site isolation device.

Other than some "hobbyists" or other small producers, many livestock facilities are more like mini industrial plants these days. Especially swine and dairy production. Facilities are large enough there is service run directly to the building and not multiple buildings supplied by a distribution point.

The old site isolation device used to be 200 amp or less single phase most of the time. Now there are a few 400 amp single and three phase, but a modern dairy farm or swine farrowing operation will have 1000 amp three phase service or more.

I have one client I have worked with before with two large farrowing facilities on same site - but each has it's own service to it. Never really worked in the service room much, but I believe they each had 1200 amp 208/120 service disconnect.

Another client has dairy farm - the old dairy barn at the old site was only single phase 200 amp, they still have some operations at this site but are not part of the active milking herd. The new milking site (now maybe around 15 years old) has at least 800 maybe even up to 1200 amp 480/277 service in a mechanical room that is typical of an industrial plant. Old milking room would easily fit into this mechanical room at the new place.

There is few other structures involved, but they are supplied by feeders from that main mechanical room.
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