Disconnect size and customer facility sizing

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MFS-PDX-313

MFS-PDX-313

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Location
Tualatin, Oregon, USA
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Electrical design engineer, Industrial equipment
I designed a system that has a 30A main circuit breaker, based on 125% of max current. The system uses an AC disconnect that is rated for 30A. When the customer asked for a Facility Chart for the system (the Facility Chart also lists max pressure and flow for the various compressed gasses and fluids the equipment uses, FYI) we always list the max current as the main breaker size. In this case, 30A. The customer is saying that because the 30A listed in the Facility chart, they have to use a 40A breaker for feeding the equipment, based on 125% of the Facility Chart. Now the customer is telling me that I need to replace the AC disconnect because it is not rated for the 40A breaker that feeds the equipment.

Am I in the wrong here? What am I missing?

Also, I have decided that from now on, I will be listing 80% of the main breaker size for max current on our Facility Charts.
 
You could list the actual motor FLA(s) as the Max Current, even if you use a 30A main breaker. But you can can list a “Maximum Over Current Protection” (MOCP) value of 30A.

I think they are over thinking this though. They only need to use a 40A breaker if your machine will run for 3 hours continuously. If it’s an air compressor, that’s rarely the case.
 
FYI, there is no motor wired to this panel. The 30A breaker feeds a handful of branch circuits, that have AC to DC power supplies, and the largest load is a three-phase pump controller. The pump controller has its own unique multi-phase PWM output (it is a Levitronix pump)
 
Well still, just add up all of the individual loads and put that down as your max load. If they are a mixture of single phase and 3 phase, just convert everything to kVA and convert the highest phase kVA to amps, then list that as the peak load.
 
I trust it's not a continuous load and/or you have otherwise sized your breaker correctly. If so, the customer has a (stupid) rule that isn't required by code and if their spec wasn't explicit in your contract then they should pay the extra cost to replace the disconnect. Or maybe just tell them your 30A breaker already includes the required 125% factor, correct the chart so that max current is 24A or less.
 
MFS-PDX-313 said:
I designed a system that has a 30A main circuit breaker, based on 125% of max current. The system uses an AC disconnect that is rated for 30A. When the customer asked for a Facility Chart for the system (the Facility Chart also lists max pressure and flow for the various compressed gasses and fluids the equipment uses, FYI) we always list the max current as the main breaker size. In this case, 30A. The customer is saying that because the 30A listed in the Facility chart, they have to use a 40A breaker for feeding the equipment, based on 125% of the Facility Chart. Now the customer is telling me that I need to replace the AC disconnect because it is not rated for the 40A breaker that feeds the equipment.

Am I in the wrong here? What am I missing?

Also, I have decided that from now on, I will be listing 80% of the main breaker size for max current on our Facility Charts.
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Air compressors may have a MOCP. If not since it’s not a continuous load yuh need to follow the intermittent load rules.
 
So, the issue is that the customer is demanding that we change the AC disconnect on our equipment to meet the 40A breaker that their facilities installed. This doesn't make sense to me. Isn't the disconnect sized for the equipment, not the breaker that feeds it?
 
Yeah, pretty sure there's no code issue here and unless there was something in your contract that you overlooked they have nothing to stand on. I still say try to school them on the correct size breaker or ask them to pay to replace the disco.

"Sorry we misunderstood how to fill out your Facilities chart. The corrected max amps is [something equal or less than 24A] and you may connect our system to a 30A circuit breaker. This will avoid the cost of upgrading the disconnect beyond what was included in our contract." Something like that.
 
OK, so the customer is saying that 110.10 of the NEC proves that if they have a 40A breaker feeding our equipment, then the AC disconnect on our equipment needs to meet or exceed that 40A.

I'm not seeing this. Isn't Article 110 of the NEC referring to the wiring to the customer unit? When the word equipment is used in 110.10, am I correct in assuming that it refers to the facility's equipment?

If I have an AC disconnect on a system that we build, shouldn't I be specifying it to the size of the system, not the current available? I would think that the customer could supply power to our system with a 100A breaker if they wanted, but we would still size our disconnect based on the OCP of the machine.
 
MFS-PDX-313 said:
OK, so the customer is saying that 110.10 of the NEC proves that if they have a 40A breaker feeding our equipment, then the AC disconnect on our equipment needs to meet or exceed that 40A.

I'm not seeing this. Isn't Article 110 of the NEC referring to the wiring to the customer unit? When the word equipment is used in 110.10, am I correct in assuming that it refers to the facility's equipment?
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I'm not seeing it either. I don't see anything about disconnect switch sizing in 110.10.

"Equipment" refers to any equipment in the electrical circuit. The disconnect switch would be equipment in this case.
 
You built the equipment. The user must install it per 110.10 It would be wrong if they used a 40A device to feed your 30A equipment.
 
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jim dungar said:
You built the equipment. The user must install it per 110.10 It would be wrong if they used a 40A device to feed your 30A equipment.
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Would it? Wouldn't that just mean that the wiring from the 40A breaker in the panel has to be sized for the 40A?

Hypothetical situation: If in the future the customer disconnects this piece of equipment and installs a machine that has a 15A main breaker and a 20A rated disconnect. Would that mean that the customer would have to change the panel breaker to a 20A?

Don't get me wrong folks, I mean no disrespect. And I'm just trying to understand this. It just seems to me as though the disconnect of our machine needs to be designed to meet the requirements of the machine itself. And that the breaker in the panel that feeds our machine needs to be at least the current of the machine, but can exceed it.

If I'm wrong, please help me understand.
 
CA
MFS-PDX-313 said:
Don't get me wrong folks, I mean no disrespect. And I'm just trying to understand this. It just seems to me as though the disconnect of our machine needs to be designed to meet the requirements of the machine itself. And that the breaker in the panel that feeds our machine needs to be at least the current of the machine, but can exceed it.
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If you as the manufacturer say what the size of the feeder protection needs to be, then that is what must be installed. You may specify a maximum or a minimum, or even both. It is not up to an installer to dictate to a manufacturer, other than through pre-order specifications.
 
jim dungar said:
If you as the manufacturer say what the size of the feeder protection needs to be, then that is what must be installed
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We did not tell them what size of breaker to use. We did list 30A as the max current of the machine in a Facility Chart. We probably should have listed the full load current (which is around 20A). But the customer took that 30A and multiplied it by 1.25 to get their panel breaker size of 40A.

Why should either we or the customer have to change anything?
 
david luchini said:
It's hard to know if supplying your equipment with a 40A circuit is acceptable or not, not knowing exactly what your equipment is.
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It's an industrial chemical blender. It has 3-phase 208VAC wye that feeds a pump controller (pump controller outputs a multi-phase PWM to a Levitronix pump) and a handful of AC to DC power supplies (some for DC distribution and some specifically for powering PLCs) and that is it.
 
I've looked into this some more. According to 110.10 it states "...This fault shall be assumed to be either between two or more of the circuit conductors or between any circuit conductor and the equipment grounding conductor(s)..."

Doesn't that mean that the panel breaker is merely sized for the wiring to the machine?
 
david luchini said:
That's got nothing to do with breaker sizing.
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True.
The panel breaker and wiring sizing are done per the NEC, based on the requirements from the end use equipment manufacturer. The equipment manufacturer does not need to modify their equipment unless specified during the order process.
 
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