Electric Water Heater SCCR

L

lynchmatt23

Member
Location
Philadelphia, PA
Occupation
General Contractor
Reaching out regarding a common issue I run into when completing short circuit studies. For reference we are an electrical equipment supplier for commercial projects and our scope typically includes the power system study consisting of coordination / short circuit and arc flash analysis. Study is completed by 3rd party PE. On projects where there are electric water heaters (domestic water not HVAC) it seems that they always fail the short circuit analysis. I have yet to see a manufacturer indicate a short circuit current rating on their documents for these units and therefore the study engineer just assumes 5 kA. The manufacturers always say that the UL listing (either UL 174 or UL 1453) does not require units to be labeled with a short circuit current rating. I understand that there are many instances in the NEC where they call out requirements for SCCRs on particular types of equipment but I have yet to see a section requiring this for domestic water heaters (electric or otherwise). However, these heaters have control panels with terminal blocks / relays / fuses / etc. so there must be an overall rating for the controller. The discussions I have had with engineers of record on these projects are typically met with blank stares and I haven't come up with a real answer on the situation. At this point I think this is just a grey area in the code and at some point there will be an update to capture the SCCR requirements.

Has anyone else ran into this or have feedback on this issue? See links below for examples of units.

Durawatt - PVI

Electric water heater, 18 - 360 kW, 150 - 750 gallon tanks
www.pvi.com www.pvi.com

product - Rheem Manufacturing Company

www.rheem.com www.rheem.com

FlowCo | Eemax

www.eemax.com www.eemax.com
 
I agree that some of the larger electric water heaters probably should be tested and rated for a SCCR but it is not a requirement.

I would assume 5kA and modify the install accordingly.
 
SCCR does not apply to end use devices, only the power components leading up to them. It only applies to panelboards, switchboards, motor control centers and industrial control panels.

In some cases, PACKAGED equipment that has controls and power circuit switches incorporated INTO the equipment will need it too, for example a packaged air handler that has an internal panel with motor starters, disconnects or VFDs. So if the water heater had something like that as part of it and the entire unit with the controls has an NRTL listing as an assembly, then the listing will need an SCCR.

The concept is that if the END USE DEVICE has a short circuit inside, the components UPSTREAM of it can handle the stresses of the available fault current taking place. There is no expectation for that end use device to survive it, it is ALREADY damaged by virtue of being involved in the short circuit event.
 
Jraef said:
SCCR does not apply to end use devices, only the power components leading up to them. It only applies to panelboards, switchboards, motor control centers and industrial control panels.

In some cases, PACKAGED equipment that has controls and power circuit switches incorporated INTO the equipment will need it too, for example a packaged air handler that has an internal panel with motor starters, disconnects or VFDs. So if the water heater had something like that as part of it and the entire unit with the controls has an NRTL listing as an assembly, then the listing will need an SCCR.

The concept is that if the END USE DEVICE has a short circuit inside, the components UPSTREAM of it can handle the stresses of the available fault current taking place. There is no expectation for that end use device to survive it, it is ALREADY damaged by virtue of being involved in the short circuit event.
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This is a great write-up. I think a lot of people, me included, are ignorant of this. Are you able to provide a code reference for this, or is it not something defined by code?
 
Since the manufacturer says SCCR is not required for the water heater UL listing, they wont provide any help resolving the issue. They wont tell you what the SCCR is or what upstream OCPD can be installed to series rate. Therefore the only solutions are 1) for someone to try an establish an SCCR via UL 508A and by chance its higher than the calculated AFC 2) extending the circuit excessively or 3) installing an isolation transformer to knock down the AFC. I have yet to see anyone capable of accomplishing #1 and if the manufacturer isn't taking SCCR into account in their build, they probably are using cheap low rated components. #2 only works when the equipment in question is really close to passing. Therefore you're stuck with #3. It seems ridiculous that any time you install a large electric water heater, you have to install an isolation transformer.

Link below has photo of controller for the PVI water heater.

https://imgur.com/a/SP8CRqo
 
tjdilly said:
This is a great write-up. I think a lot of people, me included, are ignorant of this. Are you able to provide a code reference for this, or is it not something defined by code?
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Probably more listing requirements than code requirements.

Code mostly only requires us to follow any SCCR/AIC designations on a particular piece of equipment.

Similar concept would apply to general purpose snap switches or even receptacle outlets, you never see SCCR/AIC designations on those either.
 
tjdilly said:
This is a great write-up. I think a lot of people, me included, are ignorant of this. Are you able to provide a code reference for this, or is it not something defined by code?
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The term "SCCR" or "Short Circuit Current Rating" only appears in the NEC under Article 110 talking about switchgear, panelboards etc., 409 for "Industrial Control Panels" , 440 for packaged HVAC equipment and several places in Chapter 6 for "Special Equipment" like industrial packaged machinery, data center equipment, elevators etc. In a nutshell though, equipment that HAS circuit protection devices or power controls on or in them.
 
SCCR applies to incoming power, not the load. If the water heater shorts, then it is pulling the maximum current possible of the circuit and the upstream equipment better have a SCCR capable of handling this current and open properly. Why worry about the load? :unsure: Just imagine a short and that is why there is SCCR calculations. If the water heater shorts, it needs replacing. Or at least an element does, and that is why the manufacturer installed those fuses on multiple element heaters, to keep it in the repairable realm instead of meltdown.

I can plug a toaster into a receptacle and never give SCCR a thought. Even with our low impedance transformers and short cable runs in subdivisions or industrial plants. But, The breakers before it need to have a proper SCCR!
 
Frank DuVal said:
SCCR applies to incoming power, not the load. If the water heater shorts, then it is pulling the maximum current possible of the circuit and the upstream equipment better have a SCCR capable of handling this current and open properly. Why worry about the load? :unsure: Just imagine a short and that is why there is SCCR calculations. If the water heater shorts, it needs replacing. Or at least an element does, and that is why the manufacturer installed those fuses on multiple element heaters, to keep it in the repairable realm instead of meltdown.

I can plug a toaster into a receptacle and never give SCCR a thought. Even with our low impedance transformers and short cable runs in subdivisions or industrial plants. But, The breakers before it need to have a proper SCCR!
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This makes sense to an extent. For instance, perfectly explains why we don't care about SCCR for motors. However, how do you draw the line as to what exactly the load is? In this case I could see the heater elements as the load and everything upstream in the water heater controller would need to be properly rated. If we don't need to take SCCR into consideration for water heaters than why is it required for boilers, chillers, AHUs, etc.? What really gets me is its required for an electric boiler which is a widely similar product to the electric water heater but the difference is the hot water is for heating the building and not washing hands.
 
In an electric boiler, power is usually controlled by a contactor or SCR controller that turns on and off. So you have a power device that must hold together during a fault.

Now, in theory the thermostat in an electric water heater is also a device that is controlling power, so by rights should have a rating. But in a residential heater where the thermostat only opens one side of the circuit, maybe they are considering that “controls” since it only sees 120V? Interesting twist if that’s the case. I don’t know this to be fact, I’m just guessing at why they would not need it.
 
A Bradford-White 54KW 480 volt water heater I'm familiar with has JKS-50 fuses inside. They are rated for 200kA, so that should be OK for SCCR in most areas!

What are the fuses in your heater rated for? If no fuses, then like a normal thermostat controlled device, not an issue. SCCR is for opening a circuit under duress (like a bolted short) without hurting people. Things like wire do not have an SCCR rating. Neither do actual loads.
 
Jraef said:
In an electric boiler, power is usually controlled by a contactor or SCR controller that turns on and off. So you have a power device that must hold together during a fault.

Now, in theory the thermostat in an electric water heater is also a device that is controlling power, so by rights should have a rating. But in a residential heater where the thermostat only opens one side of the circuit, maybe they are considering that “controls” since it only sees 120V? Interesting twist if that’s the case. I don’t know this to be fact, I’m just guessing at why they would not need it.
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Do residential devices even require a SCCR rating?
 
Frank DuVal said:
SCCR is for opening a circuit under duress (like a bolted short) without hurting people.
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Actually SCCR is about being able to handle/tolerate short circuit current until an upstream protective device operates. AIC is the rating associated with interrupting/opening fault current. A panel bussing has an SCCR while it's breakers have AIC.

Over the years I have seen specifications requiring switchboard to have 100kA bussing SCCR and the OCPDs to have 65kAIC all the while the infinite bus fault current was substationally less, like only 20kA.
 
Jraef said:
In an electric boiler, power is usually controlled by a contactor or SCR controller that turns on and off. So you have a power device that must hold together during a fault.

Now, in theory the thermostat in an electric water heater is also a device that is controlling power, so by rights should have a rating. But in a residential heater where the thermostat only opens one side of the circuit, maybe they are considering that “controls” since it only sees 120V? Interesting twist if that’s the case. I don’t know this to be fact, I’m just guessing at why they would not need it.
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Take a look at the photo I posted of the PVI unit. This is 480V 3 phase unit for a commercial project. I am trying to get wiring diagram of the internal components but you can see the unit has power distribution block, fuses / fuseholder, contactors, transformer, staged control of elements, etc. Therefore, someone could identify all the components and use UL 508A to establish an SCCR for the overall assembly. However, the manufacturer wont provide that rating or help in that exercise in any way since they say the water heater UL listing (UL 1453) does not require them to.
 
Jraef said:
In an electric boiler, power is usually controlled by a contactor or SCR controller that turns on and off. So you have a power device that must hold together during a fault.

Now, in theory the thermostat in an electric water heater is also a device that is controlling power, so by rights should have a rating. But in a residential heater where the thermostat only opens one side of the circuit, maybe they are considering that “controls” since it only sees 120V? Interesting twist if that’s the case. I don’t know this to be fact, I’m just guessing at why they would not need it.
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Typical electric water heater thermostat has 240 volts across the contacts. They aren't too likely to carry a fault involving 240 volts but rather a fault to ground. Not impossible in the right circumstances to have one connected to high leg or corner ground delta either though and then you may have a 208-240 volt fault.

If anything some these sort of items maybe take into consideration they will often have a 10kAIC rated breaker on the branch circuit, as well as at least a minimal amount of smaller gauge conductors supplying them that will also have some current limiting effects.
 
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