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Control Panel Main Breaker Sizing

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I have a couple 5HP VFD's in a panel with their own fuses for short circuit protection. What I want to do is over size the main circuit breaker for the entire panel instead of using 115% of the sum of the motor's FLA. Is this possible and if yes, how high can I go?

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Not sure where you came up with "115% of motor FLA". NEC has a much different derating system than this for STARTERS. But since you are using a drive, this throws a monkey wrench into it. With a drive, you don't have to shoot for dealing with the inrush of the motor plus some overload handling. You need to look at your drive's expected power draw and there should be adequate documentation on this. What you will have is a set of loads (VFD's, and some other things) which are considered nonlinear loads by NEC standards so for the purposes of wiring alone, everything has to be derated 80% (100/80=125%). Due to the nature of how a VFD works, it's NOT a starter and Section 430 of the NEC doesn't apply in the same way. Instead, you've got to look at the drive itself as a load when doing sizing for the purpose of your circuit breakers and adding up the loads of all the branches. More to the point of "how high you can go"... The circuit breaker is providing branch circuit protection for downstream devices. So you've got to size it to protect all the wiring downstream of it, which means between the circuit breaker and the feed side of your fuses as well as anything else attached. The size of that wiring matters a great deal in determining just how big it has to be. You must provide adequate protection here. Whenever you try to parallel multiple VFD's as you described, it frequently drives the wire size way up and you've got to account for this. When you find yourself getting creative about ways to make the lug sizes bigger, back up and look at breaking the system down into multiple buses. There are two other things limiting "how high you can go". Pay close attention to your coordination curves and the available fault current. If you make it so big that it is not doing anything in the circuit (never trips) then you have no coordination and the upstream device is doing all the protection, so we have a miscoordination situation here. If this is greek to you, look on Cooper Bussman's web site. They have an excellent "book" on protective circuits. Most of the time a good understanding of coordination curves alone solves the problem. The reason to go "as big as you can" is mostly to escape nuisance tripping, and properly sizing and coordinating everything eliminates this concern. The final limitation concerns arc flash. Although most circuit breakers (unless specifically designed for it) don't do any sort of current limiting, increasing the circuit breaker rating increases the time before the circuit breaker actually trips out. Arc flash energy is directly proportional to the trip time so most folks these days are driving to decrease their trip times, not increase them. Normally as a first pass, I try to shoot for setting the circuit breaker instantaneous trip (or short time trip if I've got a coordination issue with instantaneous trips) to be at around 66% of the available fault current without taking things like line impedances into consideration (only the transformer). This quick-and-dirty estimate is based on knowing that IEEE 1584 arc flash fault calculations first determine the available fault current in detail and then rerunning the same calculation at a value of 85% of the available fault current. So giving myself a little margin to account for additional circuit impedances usually does the trick as long as energy storage in the downstream circuit isn't really large and as long as fuse current limiting doesn't impose hard limits on the available fault current which impede circuit breaker instantaneous tripping. Keep in mind however that there are a LOT of assumptions built into this "quick-and-dirty calculation" so it works most of the time, but there are always cases where the heuristic doesn't work. Edited by paulengr

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Paul, thankyou for that explamation but my question is can I oversize the panel's main disconnect breaker by say 200-300%. I want to be able to add more loads in the future.

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As a general rule, I'd answer either emphatically yes or no, but it is very situational dependent. Based on the sizes you are throwing around, this will be nontrivial. Depending on the panel doing something like this is usually either incredibly easy or difficult. For example, take a recent MCC arrangement that I had to deal with. It started with 3/C 500 MCM connections. I needed to increase the current draw about 50% with a new MCC compared to the original. The internal MCC bus was rated to 800 A horizontal/300 A vertical so that wasn't really a problem, and the transformer was thankfully from the 1970's and also fed a large GE AK style switch gear so the upstream side was vastly overrated (no problems there). There was no available wiring space left in the cable tray (which had already been overloaded), but I took the opportunity to run new cable tray and 350 MCM's. It also exceeded the frame rating of the breakers so I had to replace them with bigger units but I also switched to an electronic protection relay which afforded me better control and faster switching, so the overall arc flash rating dropped. Still, at about $10K per circuit breaker if you buy retrofits directly from a GE rep (the used/rebuild market is about half that), never mind the extra cost of wiring, cable tray, etc., this was a huge additional expense. All of this was just so that I could rearrange my MCC's and eventually tear down a 7th MCC within the next year that was not set up properly (isolating individual plant process units from each other). The alternative would have been to add an additional switchgear lineup (not enough breaker space in the GE AK lineup) and a bunch of dinky MCC sections or put in lots of MCB's with large incoming lugs (manually wire it together like you are talking about doing). Doing this took days of checking old drawings, new drawings, no drawings (opening up buckets and doing inspections) and taking lots of measurements before I could make a decision on whether or not this was practical. At the same time, I frequently have issues with doing something like increasing circuit breaker or fuse size within a panel to say go from 2 A to 5 A. It doesn't change the wiring size appreciably. The major concern with doing something like this is usually simply finding space for additional terminal blocks for distribution. The decision making process in this case is a matter of minutes. Again, the issues here are: 1. Does the panel's wiring (what you are protecting) support this? ie, do you need to increase the wiring size (almost always)? If you do, does this in turn create issues with lug sizes (are they rated for the larger wiring)? Are you going to exceed the size of the associated conduit/wire ducts/wireways where you need to upsize things? Usually the wiring issues become a factor. 2. Is the SCCR of the downstream equipment (what you are protecting) high enough to withstand the increased rating? 3. Is the panel UL 508/508A rated? If so, do you require it to be Listed? If so, do you have sufficient space within the panel according to UL rules to remain UL rated? Are you changing the panel sufficiently that you are out of UL 508/508A rules but can bring in an inspector to requalify the panel under the more general UL rules (508/508A is very restricted but drastically reduces the need for UL interaction)? 4. Does the additional breaker size and/or equipment exceed the panel's thermal capacity or space capacity? 5. Are you creating a miscoordination issue with the upstream protection? 6. Are you raising the arc flash rating within the panel (not usually...depends on design)? If so, is it going to become objectionable? 7. Are you changing the frame size of the breaker to the point where it won't physically fit any more? 8. Even if you increase the breaker in the panel, can the upstream wiring, distribution (transformer), and protection support the additional load? I might have missed something here but you need to essentially start out at the end (deciding what all your loads are) and slowly working your way upstream to determine what (if any) changes need to be made, or else go the other way and start with what the maximums are (load study, coordination study, arc flash study) at the feed end and work your way downstream.

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