Sizing EBC for motor Branch Ckt

[iceworm]

This is just a recount of a discussion - hoping to elicit comments. Hopefully non-trolling in nature. Questions are posed just as a starting place.

Talking with the crew yesterday on sizing an EBC. Here is the test case:
Motor: 50hp, normal stuff, not energy efficient - say Design B, Code G, 3ph, 460V, Nameplate current 62A, sf 1.15

Feeder is xhhw in RSC, specs say "green wire ground".

Conductors are 65A x 1.25 = 81.25A ==> #4CU (85A) (T430.250)

Overload set to 140% nameplate 62 x 1.40 = 86.8A

Branch Ckt OCP (430.52)

Instantaneous 1300% 65 x 13 = 845A Not to exceed ==> 800A
Thermal-Mag 250% 65X 2.5 = 162.5A Next size ==> 175A
Thermal-Mag 400% 65 x 4.0 = 260A Not to exceed ==> 250A​

Select EBC (250.122.D)

We normally use an Instantaneous, so, size per 250.122.D.2 - max time delay fuse per 430.52.C.1, ex. 1

65 x 1.75 = 113.75A next size up ==> 125A
per T250.122 ==> #6CU
Interesting Note: Under 2005, 250.122.D, the EBC was sized by the overload, and would have been a #8Cu​

Supposing we used a TM CB, 175A EBC ==> #6Cu
Supposing we used a TM CB, 250A EBC ==> #4Cu

So:
Electricians:

Field installed, no prints - Is this what you guys (and girls) are doing (seeing)?
Installed per prints - Are you seeing EBC sizes in this range? Smaller? Bigger?​

Inspectors:

How are you looking at this? Is this anything you ever check?​

Design Engineers:

How are you guys (and girls) doing this? If you use TM CBs do you account for possibly needing to change out CB for a 400%?​

Field Dogs (engineers): I already know what you will do. Still, comments welcome

ice

 

[augie47]

This is just a recount of a discussion - hoping to elicit comments. Hopefully non-trolling in nature. Questions are posed just as a starting place.

Talking with the crew yesterday on sizing an EBC. Here is the test case:
Motor: 50hp, normal stuff, not energy efficient - say Design B, Code G, 3ph, 460V, Nameplate current 62A, sf 1.15

Feeder is xhhw in RSC, specs say "green wire ground".

Conductors are 65A x 1.25 = 81.25A ==> #4CU (85A) (T430.250)

Overload set to 140% nameplate 62 x 1.40 = 86.8A

Branch Ckt OCP (430.52)

Instantaneous 1300% 65 x 13 = 845A Not to exceed ==> 800A
Thermal-Mag 250% 65X 2.5 = 162.5A Next size ==> 175A
Thermal-Mag 400% 65 x 4.0 = 260A Not to exceed ==> 250A​

Select EBC (250.122.D)

We normally use an Instantaneous, so, size per 250.122.D.2 - max time delay fuse per 430.52.C.1, ex. 1

65 x 1.75 = 113.75A next size up ==> 125A
per T250.122 ==> #6CU
Interesting Note: Under 2005, 250.122.D, the EBC was sized by the overload, and would have been a #8Cu​

Supposing we used a TM CB, 175A EBC ==> #6Cu
Supposing we used a TM CB, 250A EBC ==> #4Cu

So:
Electricians:

Field installed, no prints - Is this what you guys (and girls) are doing (seeing)?
Installed per prints - Are you seeing EBC sizes in this range? Smaller? Bigger?​

Inspectors:

How are you looking at this? Is this anything you ever check?

Just as you show a #6 based on DE fuse
​

Design Engineers:

How are you guys (and girls) doing this? If you use TM CBs do you account for possibly needing to change out CB for a 400%?​

Field Dogs (engineers): I already know what you will do. Still, comments welcome

ice

side question: Is you IT breaker a part of a listed combination motor controller ?

 

[petersonra]

IMO, the code is fairly clear about this.

The conduit can serve as the EGC. Therefore, there is generally no need for a wire to serve as the EGC.

The code is also very clear about the size of the EGC if you use a wire as the EGC. You have to size it based on the rating of the upstream OCPD per 250.122. There is a special rule for sizing EGC where the OCPD is an IT breaker.

250.122 Size of Equipment Grounding Conductors.
(A) General. Copper, aluminum, or copper-clad aluminum
equipment grounding conductors of the wire type shall not
be smaller than shown in Table 250.122, but in no case shall
they be required to be larger than the circuit conductors supplying
the equipment. Where a cable tray, a raceway, or a
cable armor or sheath is used as the equipment grounding
conductor, as provided in 250.118 and 250.134(A), it shall
comply with 250.4(A)(5) or (B)(4).

(D) Motor Circuits. Equipment grounding conductors for
motor circuits shall be sized in accordance with (D)(1) or
(D)(2).
(1) General. The equipment grounding conductor size
shall not be smaller than determined by 250.122(A) based
on the rating of the branch-circuit short-circuit and ground fault
protective device.

(2) Instantaneous-Trip Circuit Breaker and Motor
Short-Circuit Protector. Where the overcurrent device is
an instantaneous-trip circuit breaker or a motor short-circuit
protector, the equipment grounding conductor shall be sized
not smaller than that given by 250.122(A) using the maximum
permitted rating of a dual element time-delay fuse selected for
branch-circuit short-circuit and ground-fault protection in accordance
with 430.52(C)(1), Exception No. 1.

I see no reason to make the EGC any larger than is required just because "someone" might do "something" down the road.

 

[iceworm]

... Inspectors:

How are you looking at this? Is this anything you ever check?
Just as you show a #6 based on DE fuse​

So, this is something that wou regularly look for? And you will check the CB for a TM type, and calculate that as well? Or maybe this stuff is normally part of job types that get a plans review - and that is where the check is made.

Reason I'm asking is I get inspected plenty - but no one looking at this level of detail. It's up to me to get it right. And if I screw it up, the last check is the electricans: "Uhh Worm - What was your plan here?"

side question: Is you IT breaker a part of a listed combination motor controller ?

Whoa - I don't know. Is that important? :?:?

ice
(Just jerking your chain - hopefully in a non-agressive manner. Yes, we have read 430.52.C.3)

 

[iceworm]

IMO, the code is fairly clear about this. ....

Absolutely that's true. Mostly I am asking about what you are seeing.

... The conduit can serve as the EGC. Therefore, there is generally no need for a wire to serve as the EGC. ....

Again absolutely true. However:

... Feeder is xhhw in RSC, specs say "green wire ground". ...

... I see no reason to make the EGC any larger than is required just because "someone" might do "something" down the road.

I don't know where you are going on this one - I'm lost. Are you discussing:

... Design Engineers:

How are you guys (and girls) doing this? If you use TM CBs do you account for possibly needing to change out CB for a 400%?​

If you are, that's not '"something" down the road' - that's part of the job. Motor gets installed, feeder gets pulled and connected to MCC bucket. I don't generally work with TM combination controllers, however, the few I have seen are ordered with a 250% (round-up) CB. Supposing the motor load is high inertia and slow to come up. And the 250% TM CB trips. The CB is going to get changed to 400%. And it will likely be the installing electrians that do the change.

So couple of questions: How is this handled? Does anybody go back and look at the EBC? Would the inspector go back and look?

This has only happened once in my carreer. Years back - I was the junior engineer, only one left on the job, all the brains and authority had left - leaving behind the accountability. I know exactly what happened on this one. But I curious - What should have happened?

ice

 

[petersonra]

The "down the road" was in regards to someone coming in after the thing was pulled and changing something like a 240% CB to a 400% CB.

IMO, this is no different than if someone changed the motor from a 50 HP motor to a 75 HP motor. They made a change to the install. It is up to them to get it right.

Maybe someone decides that this is enough of a problem that they write into the spec that the EGC should be based on a 400% CB.

Personally, if I came across such a situation, and really cared enough to do it right, I would be inclined to disconnect the green wire and/or yank it out and use the conduit as the EGC. Personally, I am not convinced that this is even necessary. You have a legal EGC in the form of the conduit. I am not convinced that there is a code requirement that you not have another smaller EGC in parallel with a full sized one. In fact, it is fairly common to have multiple EGCs in parallel.

 

[augie47]

So, this is something that wou regularly look for? And you will check the CB for a TM type, and calculate that as well? Or maybe this stuff is normally part of job types that get a plans review - and that is where the check is made.

Reason I'm asking is I get inspected plenty - but no one looking at this level of detail. It's up to me to get it right. And if I screw it up, the last check is the electricans: "Uhh Worm - What was your plan here?"

I seldom see IT breakers on my installs so a quick check of the Inverse Breaker vs the EGC is something I commonly do.

Whoa - I don't know. Is that important? :?:?

ice
(Just jerking your chain - hopefully in a non-agressive manner. Yes, we have read 430.52.C.3)

Honestly I didn't even note who was posting or I would not have asked ..............
that said, I am surprised of the times I see a field instakll on a Inst.Trip breaker...

 

[iceworm]

I'm losing the point on why you are adversarial. This is not about if you like it or not, or agree with it or not, but rather on how have you see it dealt with.

The "down the road" was in regards to someone coming in after the thing was pulled and changing something like a 240% CB to a 400% CB.

IMO, this is no different than if someone changed the motor from a 50 HP motor to a 75 HP motor. They made a change to the install. It is up to them to get it right. ....

Well, it is different. The conversation is about an original installation, It's okay if you read the post to see that.

... I would be inclined to disconnect the green wire and/or yank it out and use the conduit as the EGC. Personally, I am not convinced that this is even necessary. You have a legal EGC in the form of the conduit. I am not convinced that there is a code requirement that you not have another smaller EGC in parallel with a full sized one. ...

I don't know what to say

... Feeder is xhhw in RSC, specs say "green wire ground".

I would hope that "yank it out" would get you directive to put it back.

... Personally, if I came across such a situation, and really cared enough to do it right, ....

Okay, you lost me again. Generally speaking, I get paid to care enough to do it right.

... In fact, it is fairly common to have multiple EGCs in parallel.

Hummm In paralleled runs - Yes. How does that apply to this?

I'm still lost as to why this is an adversarial issue.

ice

 

[iceworm]

... Honestly I didn't even note who was posting or I would not have asked ..............

aug - Relax. I'm just jerking your chain a bit. No intention to draw blood.

... I am surprised of the times I see a field instakll on a Inst.Trip breaker...

Yes. I have heard on here several times that mag-only combination controllers are uncommon. Not for me. With very few exceptions, and excluding field made-up starters, they are all mag only combination controllers. And as far back as I can remember, they had to be listed.

I can only think of one reason. With a mag only, the system can exhibit a failure mode where the current is way above the locked rotor and still way below the inst setting. The ovld will quickly open the contactor coil and the contactor will drop out under high current - way above jogging duty. I suspect (that's code meaning I don't know for certain) the listing checks for the contactor being robust enough to handle the high fault current (just under the max inst setting) without doing a Chernobyl act.

ice

 

[petersonra]

I'm losing the point on why you are adversarial. This is not about if you like it or not, or agree with it or not, but rather on how have you see it dealt with.

I'm still lost as to why this is an adversarial issue.

ice

No one is being adversarial. You seem to think that there is good cause to install an oversize egc on the chance that the ocpd gets upsized. This is not a code requirement and I just would not be doing it short of spec requirement. In fact, in general, if it was my choice I would rely on the metal conduit as the etc.

If someone else sized the cb, I would be generally inclined to base whatever holes there are in the design on nec minimums or some well recognized design criteria such as vd concerns. If the design has a 250% cb, that is what I would base an egc on if it was not otherwise stated.

IMO, it is not up to the installer to accommodate whatever might happen down road. Just what is installed.

Now IF I had a problem with needing to upsize the egc on certain types of installs on a regular basis, I might well go ahead and upsize those particular situations.

IMO, if the drawings show a250% cb, it is a legit change order if the CB to be changed out and the egc has to be changed.

 

[iceworm]

No one is being adversarial. ....

Oh, okay

.... You seem to think that there is good cause to install an oversize egc on the chance that the ocpd gets upsized. ...

Nope. Not at all.

I don't deal with TM CB combination controllers. Like I said, in 50 years (well, 48 anyway), I've only seen one case where a 250% TM CB would not start a motor and was changed to a 400% - that one was 28 years ago. I was not the engineer of record, but I was the one standing closest. After a couple of phone calls I got to tell the crew to change out the CB. I don't recall even thinking about the EBC - I don't recall anyone else saying anything either. I'm damned sure no one snuck in a changed it.

As I recall, it was a 480V, 200Hp driving a huge boiler ID fan, and we changed out the 600A CB for an 800A - cause that was the biggest we could fit in the cabinet. So we only went up to a 333%

I don't recall me ever sizing an EBC for a TM CB combination controller.

Up until the 2005, all of the EBC that I personally speced (mag-only combination) were sized per the ovld, generally 140% nameplate FLA. So for this 50hp case they would have been #8 - not even the #6 as required now.

.... In fact, in general, if it was my choice I would rely on the metal conduit as the etc. ....

I got that - really, I did. In fact, I am pretty sure we all know you think this.

... IMO, it is not up to the installer to accommodate whatever might happen down road. Just what is installed. ....

And that is still not an issue. Cause this is not "down the road". What I am questioning is part of the commissioning.

....... IMO, if the drawings show a 250% cb, it is a legit change order if the CB to be changed out and the egc has to be changed.

Yeah - that's true. And .... ?

What I'm asking is: How is this handled? What I hearing is, "Don't know - never saw it happen." I'm thinking:

• With today's software, coordination issues are seen well enough in advance that the starters are ordered with the oversize CB if needed.
• I recall a post a bit back about someone wondering if it was okay for the engineer-of-record to upsize the CB from the gitgo and not wait for commissioning to find out that the CB need to be higher rating. Maybe today's engineers are just smarter as well as having better software.
• That would include the DIY engineers that "roll your own" on projects. I was going to say "small projects", but actually any size.
• Maybe, no one is ordering TM CB combination controllers for anything above small motors. Essentially, up to a 5hp motor TM CBs will all be 15A

Still, there is time for others to chime in.

It likely won't change what I do - but I am interested in how others have seen this handled.

ice

 

[petersonra]

I do not show field wiring sizes on drawings anymore. I can't control what the installer does anyway, and that leaves them the option to use copper or aluminum, or to run parallel conductors if they decide that is most advantageous.

In the end if the CB I gave them does not work, it is a design problem and not an installation problem. I don't recall it ever being a problem though. So, the situation where one has to remove a CB and replace it with a 400% CB maybe never happened to me.

These days I mostly use type F starters for smaller loads.

This is akin to a question about whether one should upsize a motor because it is going to run close to 100% HP, on the theory that it might need to be made larger due to the actual conditions in the field being a little worse than what was anticipated.

I am not a fan of upsizing stuff just because of what might happen. If it has happened in similar situations I might be inclined to consider upsizing those things, but otherwise, I am going with just what is needed and not adding a lot of cost.

One thing I have tended to upsize is feeder conductors inside of control panels. This is because I have run into people who want to power other things from a control panel that they did not bother to mention up front. But, we are talking about a relatively small cost to increase 10 or 15 feet of 2/0 conductor to maybe 3/0. However, the last few years I have mostly gotten away from that by getting rid of distribution blocks and wiring things directly to lugs on MCCB. Many of my panels no longer have any large feeder wires because there are no distribution blocks. If there happens to be an unused point on a MCCB lug, maybe they can wire in there to something else, but otherwise, there is not much in the way of provision for such things anymore.

 

[iceworm]

... These days I mostly use type F starters for smaller loads. ...

What's a type F starter?

 

[iceworm]

... This is akin to a question about whether one should upsize a motor because it is going to run close to 100% HP, on the theory that it might need to be made larger due to the actual conditions in the field being a little worse than what was anticipated. ....

Not really - At least I don't think so. I think it is a high inertia load/long starting time issue. Class 10 overloads trip. So, overloads get set up to class 30 and then the CB trips. And in the 50hp range, TM CBs don't tend to have adjustable trips. But the steady state motor loading is within the Hp rating. I'm saying it this way cause I have reviewed very few cases using TM CBs - So, I don't know for a fact.

In my experience, the 100% loadings show up somewhat different:
The mechanicals come in with a process pump application and say, "We want to run the motor up in the service factor rather than get the next size up motor."

For some unfathomable law of nature, this only happens when the change is from a 25hp to 30hp, or 50hp to 60hp.

My response, "You are going to absolutely guarantee me your hydraulic calculation are absolutely perfect. There is no way you could ever need an impellor change to balance the process. There is absolutely no way you could need another 5% over design. Keep in mind that the change from a 25hp to 30HP (or a 50hp to 60hp) requires a bigger starter and it won't fit in the same MCC space."

Amazingly, no one has ever taken me up on designing to run in the service factor.

ice

 

[petersonra]

What's a type F starter?

http://www.schneider-electric.us/do...port-resources/ul-508a-support/8538DB0401.pdf

It is a combination starter that has built in over current and short circuit protection.