430 is not my friend, and 240 isn't helping either

[charlie b]

I am assisting our cost dept in pricing out a project, and it's been a long week .

• Existing: 40 HP motor (52 FLA), 125 amp breaker, 150 amp rated manual transfer switch, conductor unknown (perhaps #6).
• Upgrade: 75 HP motor (96 FLA), 250 amp breaker, likely #1 conductors.
• Question: Can I keep the 150A MTS?

I know the existing MTS can handle the FLA of the larger motor. But the upstream breaker is higher than the MTS rating. Is that an issue, given that the same breaker is also higher than the conductor ampacity. 430 allows that because a current that could damage the conductor would trip the motor overload. Can I take credit for this same overload as protecting the MTS as well?

I am inclined to tell the cost engineer to include replacement of the MTS. Opinions?

 

[jap]

Does the Tranfer Switch happen to indicate a Horsepower Rating by any chance?

JAP>

 

[infinity]

A disconnect switch is only required to be rated at 115% of the motor FLC {430.110(A)} I would if that could also apply to the MTS.

 

[charlie b]

Thanks for the replies. The photos I took of the MTS do not show a HP rating. I didn't ask to have the enclosure door opened, so I don't know if there is any HP rating shown on the inside of the door. If the required disconnect for a motor can be rated at 115% of the FLA, even if the upstream overcurrent protection device can be even higher still, then a case can be made for keeping the existing MTS. But the disconnect is explicitly mentioned in 430, and 430 is mentioned in table 240.4(G), and an MTS is not mentioned in either. So I would not have great hopes for making that case.

 

[steve66]

I would check the MTS's withstand ratings. I think Asco ATS's often list specific breaker or fuse SCC/Withstand ratings for breakers or fuses larger than the ATS rating. For example, a 600A ATS may have ratings for an 800A breaker.

Your MTS may have something similar.

If the 150A MTS happens to be rated to withstand more fault current than you have available for the specific breaker you have, then I think it should be OK.

Or maybe even the MTS's general withstand rating states what size breaker or fuse they are basing the rating on.

 

[electrofelon]

.....But the disconnect is explicitly mentioned in 430, and 430 is mentioned in table 240.4(G), and an MTS is not mentioned in either. So I would not have great hopes for making that case.

Just shooting from the hip here, but I don' recall anything saying that switch much be protected at it's rating by an OCPD. There is the 115% for disconnects, and I guess it's arguable whether the MTS is a/the disconnect, but seems like you are good either way.

 

[steve66]

I probably should have phrased my earlier post using the opposite scenario:

If there is no withstand/close-on rating for you 150A MTS when protected with a 250A circuit breaker, I don't see how it could be code compliant.

For example, here are the ratings or an ASCO 300 MTS:

ASCO Ratings

The 150A MTS only has a withstand rating for a maximum 200A fuse. And the only higher ampacity breaker that is rated is a 250A SQD HR breaker. But that only applies at less than 240V.

 

[electrofelon]

I probably should have phrased my earlier post using the opposite scenario:

If there is no withstand/close-on rating for you 150A MTS when protected with a 250A circuit breaker, I don't see how it could be code compliant.

For example, here are the ratings or an ASCO 300 MTS:

ASCO Ratings

The 150A MTS only has a withstand rating for a maximum 200A fuse. And the only higher ampacity breaker that is rated is a 250A SQD HR breaker. But that only applies at less than 240V.

Yes agree good to check manufacturer specs/instructions even if NEC says ok. I recall some instances in the past where switch instructions required certain OCPD requirements. Also SCCR is often ignored or overlooked.

 

[kwired]

Just shooting from the hip here, but I don' recall anything saying that switch much be protected at it's rating by an OCPD. There is the 115% for disconnects, and I guess it's arguable whether the MTS is a/the disconnect, but seems like you are good either way.

Have run into similar situations with Square D safety switches. 20 HP 480 volt three phase motor - if using their non fused disconnect you can use a 30, it is rated for maximum of 20 HP @ 480V. But if using a fused disconnect you need a 60 because you can't put the needed 40 amp fuses in the 30 amp disconnect.

I believe their non fused switches have same kA rating as the fused switches if there is fuse protection ahead of them. If not fuse protection then I think they go to a 10 kA rating.

 

[charlie b]

Let me clarify that I am not interested in the fault current rating of the MTS. I am concerned about protecting a 150 amp rated MTS with an upstream breaker rated at 250 amps.

 

[steve66]

Charlie: Yes, I understand.

But often the fault current rating is exactly what tells you the maximum breaker size that can be used. Or the maximum fuse size that can be used. That's shown in the link I posted for ASCO 300 series MTS's.

Say your MTS only has one rating, and its for 10,000 amps when protected with any 200 amp circuit breaker. If you try protecting this MTS with a 250A breaker, instead of a 200 amp breaker, then what's the withstand rating with the 250A breaker? There isn't one. So how can it be code compliant?

Can you post the brand and model?

 

[charlie b]

Here are photos of the MTS and its nameplate.

 

[iceworm]

Here is my understanding:

250A TM CB

|​

contactor

|​

overload

|​

ASCO MTS

|..........|​

DSQ1...DSQ2

If true, my best suggestion is to apply Part IX, Disconnecting means:
430.108, every disconnecting means
430.109 lists the types.

I looked on ASCO site but did not see any data relating to this switch. I can't tell what type of switching elements are in the MTS box. May have to pry off the lid and look.​

430.110 give the ampere ratings and interrupt ratings

If the switching elements meet one of the types in 430.109, it is likely okay.
430.110.A General​

requires 115% FLC

Minor issue (that could be a problem)
If the switching elements are molded circuit breakers, 150A TM CB could have nuisance trips with a 75hp motor (or not)

If the switching elements are molded case switches, it's golden.

Interesting note: 430.109.E Isolating Switch rated at 115% does not apply. The motor is not over 100hp.

I'd say if the elements are not TM CBs, it is okay. It is just a matter of does it meet code.

Let us know what is in he box.

 

[Nuber]

I am assisting our cost dept in pricing out a project, and it's been a long week .

• Existing: 40 HP motor (52 FLA), 125 amp breaker, 150 amp rated manual transfer switch, conductor unknown (perhaps #6).
• Upgrade: 75 HP motor (96 FLA), 250 amp breaker, likely #1 conductors.
• Question: Can I keep the 150A MTS?

I know the existing MTS can handle the FLA of the larger motor. But the upstream breaker is higher than the MTS rating. Is that an issue, given that the same breaker is also higher than the conductor ampacity. 430 allows that because a current that could damage the conductor would trip the motor overload. Can I take credit for this same overload as protecting the MTS as well?

I am inclined to tell the cost engineer to include replacement of the MTS. Opinions?

The branch circuit and all of its components should be sized according to 430 requirements. The breaker is the exception to standard 125% rule because it feeds a motor.

I would treat the MTS as a part of the branch circuit (it is neither an overcurrent device, an overload device, nor a disconnecting means) - so 125% of 96 amps is 120 amps. So the 150 amp MTS is adequately sized for the branch circuit, same as the #1 conductors (hopefully THHN) are.

The breaker breaks the traditional 240 rules because of the motor, it is oversized to allow the motor to start, and all of the branch circuit components in between do not match the rating of the breaker by necessity and code compliance.

In other words, size all of your branch circuit items off of the motor, not the breaker. Let 430 size the breaker according to 430.

The MTS is fine. Keep it.

P.S. - I would not ignore fault current or fault withstand ratings in this analysis nor would I ignore the MTS manufacturer info. If the MTS nameplate says 200 amp max overcurrent device or words to that effect, then you are stuck.

P.S.S. - Why is the 250A breaker needed? If you are using the 250% rule from 430.52, then this is a maximum number. As long as the motor starts without the OCPD tripping, you are code compliant. You have a range of 125 amps to 250 amps to play with here. You theoretically could put in a 150 amp breaker and see if the motor starts.

 

[Jraef]

The reason you can use a larger breaker in a motor circuit is BECAUSE you will also be required to have running OL protection for the motor sized at a lower value than the breaker, which then presumes that same protection serves all components in the branch circuit including the conductors which, under any other circumstances would have needed to be sized per the 250A breaker. It is the motor's running OL protection that actually makes the difference, not just that it is a motor. So since in your case the MTS is in the branch circuit, it is protected in the same manner as the conductors.

 

[steve66]

So its a Asco 386 MTS. It looks like that has the same ratings as an Asco 300.

IMO, from the attached chart (which comes from the link I posted earlier) 200 amps is the maximum circuit breaker size you can use at 480 volts. And that only works with a SQD JG, JJ, or JL breaker.

If it were a 208V application, you could use a 250A SQD HR breaker (from the 1st line in the chart.)

Otherwise the MTS has no withstand or short circuit rating.

 

[Another C10]

I am assisting our cost dept in pricing out a project, and it's been a long week .

• Existing: 40 HP motor (52 FLA), 125 amp breaker, 150 amp rated manual transfer switch, conductor unknown (perhaps #6).
• Upgrade: 75 HP motor (96 FLA), 250 amp breaker, likely #1 conductors.
• Question: Can I keep the 150A MTS?

I know the existing MTS can handle the FLA of the larger motor. But the upstream breaker is higher than the MTS rating. Is that an issue, given that the same breaker is also higher than the conductor ampacity. 430 allows that because a current that could damage the conductor would trip the motor overload. Can I take credit for this same overload as protecting the MTS as well?

I am inclined to tell the cost engineer to include replacement of the MTS. Opinions?

Without discrediting any other great points made by others, I'm reading your note which to me says you have a 125 A Breaker protecting a 52 A. FLA motor source which in turn is being fed from a Main Transfer Switch or Main Thermal Switch which has a 150 A output capability.

then your upgrading a motor to a 75 HP at 96 FLA while upgrading the wiring as necessary, from what I can tell you can almost use the same 125 A. Breaker, 125% of 96 is 120 A, at least off my calculator, even if you up sized it to a 150 A I'd say you'd be good. As far as the mentioned MTS? 150 A should be fine, worse case you just up-size the main if in fact a problem of thermal shut down occurs. Just make sure the wiring can handle the calculated current flow keeping in mind distances and other overheating factors.. Just a thought.

 

[kwired]

Without discrediting any other great points made by others, I'm reading your note which to me says you have a 125 A Breaker protecting a 52 A. FLA motor source which in turn is being fed from a Main Transfer Switch or Main Thermal Switch which has a 150 A output capability.

then your upgrading a motor to a 75 HP at 96 FLA while upgrading the wiring as necessary, from what I can tell you can almost use the same 125 A. Breaker, 125% of 96 is 120 A, at least off my calculator, even if you up sized it to a 150 A I'd say you'd be good. As far as the mentioned MTS? 150 A should be fine, worse case you just up-size the main if in fact a problem of thermal shut down occurs. Just make sure the wiring can handle the calculated current flow keeping in mind distances and other overheating factors.. Just a thought.

Conductor needs to be 125% of motor rated current, but chances are that ~125% breaker is not always going to hold during starting of that motor. Code allows you to go to 250% and even more if that won't hold during starting.