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Do you use one calculation for an MCC or two? i.e. 1 for the main and 1 for the feeders?
1 calculation for entire MCC
2 calculations – 1 for feeders, 1 for main
It depends (please explain)
No MCCs/I don’t do this
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 Post subject: Motor Control Centers Arc Flash Calculations
PostPosted: Sun Jul 30, 2017 3:20 pm 
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This question was brought up by one of our Arc Flash Forum members regarding motor control centers (MCCs).

Do you consider the entire MCC as one arc flash calculation or do you treat the main and feeders separately as two calculations? i.e. one calculation for the main using the clearing time of an upstream device and one for the feeders/other sections using the clearing time of the main? The reference to "calculation" means all that are required - arcing current, incident energy and arc flash boundary.


Do you use one calculation for an MCC or two? i.e. 1 for the main and 1 for the feeders?
1 calculation for entire MCC
2 calculations – 1 for feeders, 1 for main
It depends (please explain)
No MCCs/I don’t do this


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 Post subject: Re: Motor Control Centers Arc Flash Calculations
PostPosted: Sun Jul 30, 2017 9:20 pm 
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MLO (main lug only) has only 1 bus.

With a main breaker there are two zones...either the whole incoming section or only up to the main breaker, depending on interpretation (whole section is a bit more conservative). Loads also have ratings but are often labelled at the MCC. Quite often due to low instantaneous settings and fast tripping characteristics the vast majority of loads are well under 1.2 cal/cm2. There is a slight issue that as line lengths get long arcing currents can get very small, incident energy can exceed the load side calculation though.


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 Post subject: Re: Motor Control Centers Arc Flash Calculations
PostPosted: Mon Jul 31, 2017 8:00 am 
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When we run an arc flash study for an MCC that has a main breaker, we will print a "Line Side of" the main breaker (protected by the upstream device feeding the MCC) and the MCC Bus (protected by the MCC Main breaker.) We do not create an arc flash label for each bucket since the line side of the bucket MCP or feeder breaker is equivalent to the MCC bus.

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 Post subject: Re: Motor Control Centers Arc Flash Calculations
PostPosted: Mon Jul 31, 2017 11:03 am 
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Through discussion with our other plants, we have come to a consensus stating "unless we have arc rated MCC, we will only use one calculation for entire MCC" We discussed many aspects of arc cascading and propagation. We even had a situation on 4160 that basically arced many locations within a starter and sent carbon throughout the adjoining cabinets. Until further research is completed, We will continue to label line side for the entire MCC.


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 Post subject: Re: Motor Control Centers Arc Flash Calculations
PostPosted: Mon Jul 31, 2017 4:48 pm 
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Generally, one calculation for the whole MCC and label every other section. More often than not, the MCC already has too many labels and labeling every bucket just adds to the mess.


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 Post subject: Re: Motor Control Centers Arc Flash Calculations
PostPosted: Tue Aug 01, 2017 6:18 am 
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I used to use just one, until I realized how wasteful it was to use the top of the main breaker for the entire MCC.


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 Post subject: Re: Motor Control Centers Arc Flash Calculations
PostPosted: Wed Aug 02, 2017 8:27 am 
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Great discussion. Here is a bit of interesting (?) info.

Around 7 or 8 years ago, this question came up at an IEEE 1584 meeting. It was a simple “what if” question – nothing official. The question was basically like the one here i.e. Do you perform one calculation, two, or “it depends”. “It depends” was the overwhelming choice of the more than 100 people at the meeting. The discussion, much like the discussion here, was a matter of equipment design and weighing liability vs. practical.

The industry went down this road some time ago with switchgear too. In the early years, many modeled switchgear as a single node/calculation. Over time, many began using two calculations – one for the main/incoming based on something upstream and the second for the feeders based on the clearing time of the main.

Some may even recall the hesitancy to use a 2 second cut off in the early years. Today, it is much more common.
Industry practices continue to evolve – I would like think in part due to some great discussions from everyone here!

Great comments!


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 Post subject: Re: Motor Control Centers Arc Flash Calculations
PostPosted: Mon Aug 07, 2017 12:05 pm 
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I chose "it depends" because it really depends on the design of the MCC. For some reason in NEC Article 430, other than Service Disconnect, it was not written when we require Main Breaker for MCC and when it is not required. The closest article I found was NEC Article 225.31 that states requirement of disconnects for ungrounded conductors that supply or passes through building or structure. From my experience, I found that the Switchboard and ATS is located outside the building in NEMA 4X enclosure and the MCC located inside the building, but there is no Main Disconnect for the MCC. From the ATS, the conduit goes through the wall and lands on the MCC bus as MLO (Main Lug Only). If something happens inside, and had to trip the Service Disconnect, somebody has to run around 200 ft, exit through the door located at the other side, and run back to the Switchboard to turn off Service Disconnect that was actually separated by a thin wall.
On the other hand, I have seen both Switchboard and MCC located indoor/outdoor side by side, and the MCC has Main Disconnect in one section that feeds all horizontal bus structures.
So when we affix arc flash warning labels, we affix one arc flash warning label for MCC Main Disconnect and one label on each MCC sections in the downstream of MCC Main Disconnect for the MCC that has Main Disconnect. In this scenario, if the MCC Main Disconnect has incident energy of 2 cal/cm2, the MCC sections will have 0.5 cal/cm2 (this is an example only). However, if there is no MCC Main Disconnect and the conductors land on the MCC as MLO, then the entire MCC section will have only one label.
We design our MCC with Main MCC breaker when the switchboard is located outside the building or when the switchboard is located more than 50ft (line of sight) from the MCC. It is little expensive, but it provides vital protection in the event of emergency.


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