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#208066 - 12/22/12 09:37 PM Size feeder??  
sparkync  Offline
Joined: Oct 2002
Posts: 821
I have an industrial customer who is wanting a 200 amp feeder to feed a 120/208 3 phase panel. They sent me the specs for the machines that are going to be used in the panel. He list (3) douple pole 20 amp 240 volt circuits,(5) 30 amp 240 volt circuits, (4) 20 amp. 120 volts, (1) 15 amp 120 volt circuit, and (1) 5 amp 120 volt circuit. At this point I can only assume that this is the running current. The total amperage of these machines is 310 amps. If I read the code right for these machines there are no reductions for this kind of feeder that can be applied, and a 200 amp feeder will be too small. The feeder is to come off of a 120/208 buss duct and feed the panel. Any input on this? Am I reading the code right here? I guess I need to see if the information he sent me is the actual running current or just the overload protection device. Any input would be appreciated. Also I have plans of using a fusible plug in unit on the buss duct and feeding to a main lug panel. The plug in disconnect will be around 20 to 25 ft. in the air. Is this still permissable to use the main like this even if it is not readily accessible? Thanks again.. Steve
Also I guess I need to ask him if all these machines will be or will be capable of running at the same time...

Last edited by sparkync; 12/22/12 09:38 PM.

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#208070 - 12/22/12 11:49 PM Re: Size feeder?? [Re: sparkync]  
Tesla  Offline
Joined: Jun 2004
Posts: 1,273
Sacramento, CA
To simplify the math for the 3-phase calculations...

The first three 30A 2-pole breakers are identical in load to two 3-pole breakers if they are mounted down the left side:

[ A&B, C&A, B&C = A&B&C + A&B&C => ckts 1,3,5,7,9,11 used.]

The next two 30A 2-pole breakers PLUS the first two 20A 1-pole breakers are LESS demanding than two more 3-pole breakers at 30A:

[ A&B, C&A,B,C < A&B&C + A&B&C => ckts 13,15,17,19,21,23 ]

Lastly, the last two 20A 1-pole PLUS 1 15A 1-pole is LESS demanding than one more 3-pole breaker at 20A:

[ A,B,C < A&B&C => ckts 25,27,29 ]

The last, odd 5A 1-pole circuit can be dismissed as irrelevant; it's too small to affect the calculation as we can see:

We have nearly balanced our phases with four (2+2) 30A 3-pole equivalent circuits and one (1) 20A 3-pole circuits.

Each feeder phase will see only its draw which is 4 x 30A PLUS 1 x 20A --- 140A per leg of Y connected power.

Since you've got 3/0 if not 4/0 conductors you're way, way into the clear. 125% of 140A is 175A.

The 5A 1-pole circuit is just frosting on the calculation. It doesn't matter which phase it hits.


As regards Bus Duct taps...

They have their own tap exceptions in the NEC...

And in an industrial setting this arrangement is the norm -- nothing special at all.

Talk to these fellows. They work that end of the street all the time. They deal in scrapped out Detroit assembly lines.

If your patron can accept second-hand industrial rated gear, some bucks can be saved.

(I have no connection to Salinger Electric. Web search is your friend.)

Last edited by Tesla; 12/22/12 11:59 PM.


#208080 - 12/23/12 08:56 PM Re: Size feeder?? [Re: sparkync]  
Tesla  Offline
Joined: Jun 2004
Posts: 1,273
Sacramento, CA
One last point:

Breakers used to feed motors are sized to withstand the inrush current that they draw.

This means that, unlike resistive loads, the running current from a breaker to a motor load will typically be about half of the nominal breaker rating.

e.g. a 35A 2-pole breaker will be mated to a motor that draws 17 A in normal, full-load service.

So, the customer's feeder conductors are even more conservatively sized than the above, quick-snap calculation implies.


His actual running loads are not even tapping half the available feeder current -- almost certainly.

Last edited by Tesla; 12/23/12 08:57 PM.


#208084 - 12/24/12 11:23 AM Re: Size feeder?? [Re: Tesla]  
Trumpy  Offline

Joined: Jul 2002
Posts: 8,214
SI,New Zealand
Circuit protection is designed to protect the down-stream cables, not what is on the end of them, this is electrician 101.

Let's face it, these days if you're not young, you're old - Red Green grin

#208088 - 12/24/12 07:28 PM Re: Size feeder?? [Re: sparkync]  
Tesla  Offline
Joined: Jun 2004
Posts: 1,273
Sacramento, CA
I'm not going to dive into the upside down peculiarities found in 430.

I'll just reference the fact that -- in the extreme -- for motor circuits only -- the NEC actually permits a 50A breaker to service a #12 conductor pair.

Uniquely, the high inrush currents of inductive motors don't mate up well with the common NEMA circuit breakers that we all know and love.

So, the Code permits a separate set of calculations -- and ( belt & suspenders ) insists upon what amounts to a 'mated' OCPD -- permitted to be even integral to the design of fractional horsepower motors. (With NEMA dictating their design, too.) When the motor is larger, the NEC specifies that this second current protection be provided via one scheme or another. The most popular, by far, is by time-delay fuses set in a disconnecting means close by the utilization motor.

The conductors will be actually protected by the time-delay fuse -- and the breaker reduced to being a switch -- with the breaker providing a casualty-trip back-up 'trip' in case of collision or earthquake.

For motor circuits, the conductors are sized for normal running at full load PLUS an 'insurance' margin of 25%. NEMA fuses, for motor circuits, are designed to tolerate the in rush and yet blow before the conductors are stressed -- a value that will always be lower than the start-up surge that the conventional NEMA circuit breaker must let pass.


It was decided ages and ages ago, that it would be most unwise to have two circuit breaker time-current performance curves circulating all over America. For, it was a certainty that someone would substitute the wrong C/B -- leading directly to either fire -- or a penny patch-around.

So, NEMA decided that -- for ordinary 1 and 2-pole circuit breaker only one time-current characteristic ( actually a band of acceptable performance ) would be: that of lighting and appliance loads.

Further, that all small motors would be protected by other NEMA standards so that mass produced consumer retail devices were impossible to screw up even if installed by a hack or homeowner.

And finally, that 3- phase devices, stuff always installed by professionals (almost), would be protected by provisions in the NEC, and not integrally within the machines.


If you're not wiring up multi-horsepower motors in a NEMA environment, then the above does not apply.

Overseas, especially France, fuses mated to the load are the norm. In such a scheme, none of the above logic/ design trade-offs apply.

In a fuses only scheme, you install the correct fuse for the load type -- and are done with it.

Exotic 3-pole C/Bs do exist that have motor-startup time-current performance mated to loads. NEMA does make them. These days, they can even be adjusted in the field. But they're not priced like Chicklets -- and are spec'd by EEs. ( With a big assist from the factory tech rep.)

Having said that: it's as common as dust for up-sized conductors to run to all moderate motor and HVAC loads. It's often called out in the contract specs... or dogged conservatism rules the day.

No-one ever gets red-tagged because they wasted copper on over sized conductors. Such a thing only hurts the person paying for it.


I once ran a cost-plus job, a retro-fit. My employer ran grossly oversized HVAC feeders at every turn. He just &^%$ the owner-developer -- who was squeeling every step of the way. He hired on an independent expert only too late.

The HVAC system used (25 x 2) 50 hp at 200V 3-ph.

He ran twin 2.5" EMT stuffed with 400kCMIL 3-ph, each and mounted a 400A NEMA3R diconnect. (Installing it was brutal, it was on the roof of an unfinished retrofit. No elevators, missing stairs, etc.)

Though wildly overbuilt, it sure passed inspection; and without a murmur.

(Came in after it had been 40% installed. I was shocked as to its oversizing.

The prior foreman had been red tagged for installing EMT below grade, in concrete. That use was specifically prohibited by local (city) code.

I did get all of my work inspected by the city's top inspector. No-one else was permitted to inspect downtown office high-rises, by order of himself.)

Last edited by Tesla; 12/24/12 07:36 PM.


#208105 - 12/27/12 05:36 AM Re: Size feeder?? [Re: sparkync]  
Scott35  Offline

Broom Pusher and
Joined: Oct 2000
Posts: 2,707
Anaheim, CA. USA
Nice Motor Circuitry descriptions post, Tesla!

I have dealt with many complaining Persons, over the Years, when applying Article 430 allowances to Motor Circuits - be it in the Field, or per my Plan Sets.

Most Commonly heard statements:
"You Cannot use #6 THHN CU. with a 90 Amp Breaker"
"You Cannot use #4 THHN CU. with a 125 Amp Breaker"

I guess the responses:
"Right; I could have used a 125 Amp Breaker with the #6s"
"Right; I could have used a 175 Amp Breaker with the #4s"
was more smart-aXX than resourceful! wink

--Scott (EE)

Scott " 35 " Thompson
Just Say NO To Green Eggs And Ham!

#208111 - 12/27/12 02:50 PM Re: Size feeder?? [Re: sparkync]  
sparkync  Offline
Joined: Oct 2002
Posts: 821
On this 120/208 3 phase feeder, is there any deductions I can allow on the neutral? The Code calls for 3/0 for 200amps. for copper conductors. If I am reading right, according to 220.61 (C) in the 2005 Code( don't have my 2011 code handy) says there is no reduction allowed on a 4 wire 3 phase wye connected system. Just wanted to verify. Thanks...

#208113 - 12/27/12 08:58 PM Re: Size feeder?? [Re: sparkync]  
sparkync  Offline
Joined: Oct 2002
Posts: 821
"I'm planning on running 2" EMT with (4) 3/0 copper thhn conductors with a # 6 thhn ground. The length will be approximately 25ft. in the air, coming out of the buss duct switch. I'm planning on securing the emt with beam clamps (CADDY BC200-CD5B). The specs on the caddy clamps say the static load limit is
100 lbs. I assume that this means that the weight at the clamp cannot be over 100 lbs., therefore if I install sufficent amount of clamps, I should not have a problem with it falling or moving, right? thanks for the input..

#208114 - 12/27/12 10:23 PM Re: Size feeder?? [Re: sparkync]  
Tesla  Offline
Joined: Jun 2004
Posts: 1,273
Sacramento, CA
Strictly speaking, you've got zero three-phase loads.

You can't count on load balance -- one must design for the absolute worst load imbalances possible. At some point in the future they'll happen.

So, there's no way you can justify reducing the neutral in this situation.


In these situations it's extremely common to run MC in cable trays -- or to use industrial rated cable drops -- from Bus Duct Plugs.

I'd get on the phone to Salinger without delay.


In a typical factory, Bus Duct is run along either a wall or high, straight across the factory floor. ( Open space, flexible assembly )

From there power is tapped by Bus Duct Plugs (They're massive - no corelation to NEMA cord caps at all.) with cable drops down to manufacturing cells/ distribution panels at floor level. These drops always have some strain relief device and protection from mechanical casualty. ( Bollards and cages, e.g. )

Not uncommonly, there is a provision for a top-side disconnecting switch at the Bus Duct Tap which is operated by a pull cord/ rope. See the NEC Handbook for illustration.

The drops are massive cable assemblies that can stay in service for ever and ever.

The other 'new-wave' approach has been cable tray and industrially sized MC. (Even 750 kCMIL is available.)

There has been a long term shift towards aluminum and away from copper. It's the $$$$$.


Unless you're a blood relative to the client, I'd avoid getting so far afield from your prior experience.

One last thing: don't make any attempt to tap the Bus Duct while it's energized.

Bus Duct is a whole sub-specialty -- with all manner of gotcha details and materials -- that is sure to embarrass a noob.

You need to sub-contract with / hire on a j-man who's done the deed before.

As for 100# static loads -- I can't fathom your geometry/ installation set-up.

If the panel is to be set 25 feet below the Bus Duct -- why would you use anything but cable?

Last edited by Tesla; 12/27/12 10:27 PM.


#208115 - 12/27/12 10:35 PM Re: Size feeder?? [Re: sparkync]  
gfretwell  Offline

Joined: Jul 2004
Posts: 9,084
Yup that is the weight on each clamp. I think you are well within that using a 10' or less between supports.

Greg Fretwell

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