Wire sizing

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ggunn

PE (Electrical), NABCEP certified
Location
Austin, TX, USA
Occupation
Electrical Engineer - Photovoltaic Systems
I keep getting confused about this. Say I have some rooftop DC wiring that I need to size and the heat load is significant - hot roof in Texas, wiring very close to it - with 90 degree wire and 75 degree terminals.

Calculate necessary ampacity two ways, one at continuous use, one at conditions of use, choose the higher.

1.25 X 1.25 X Isc for continuous use

1.25 X temp derate (with adder) reciprocal X Isc for conditions of use (fewer than 4 CCC's per conduit - no derate for that).

OK so far, but what confuses me is when to use the 75 degree column and when to use the 90 degree column. I remember a Holt video that says to use 90 degrees for one calculation and 75 degrees for the other, but which and why? If I understood the why I wouldn't have so much trouble the remembering the which.

I could play it safe and use the 75 degree column throughout (pretending that I am using 75 degree wire), but the difference in the temp derate between 75 and 90 degree conductors is significant and there is a lot of wire involved.
 

Smart $

Esteemed Member
Location
Ohio
Noncontinuous plus 125% continuous is for terminations and uses 60 or 75 degree column.

All conditions of use (beyond termination enclosure) use wire rating column (e.g 90 degree).
 

ggunn

PE (Electrical), NABCEP certified
Location
Austin, TX, USA
Occupation
Electrical Engineer - Photovoltaic Systems
Noncontinuous plus 125% continuous is for terminations and uses 60 or 75 degree column.

All conditions of use (beyond termination enclosure) use wire rating column (e.g 90 degree).
Thanks. One more question: When I am running an EGC in conduit with 1000V PV wire, does the EGC need to be 1000V as well? I wouldn't think so, since I see bare Cu bundled with PV wire all the time.
 

Smart $

Esteemed Member
Location
Ohio
Thanks. One more question: When I am running an EGC in conduit with 1000V PV wire, does the EGC need to be 1000V as well? I wouldn't think so, since I see bare Cu bundled with PV wire all the time.
That's my belief. IIRC, there is a section of Code just for that purpose, but I couldn't find it the last time I looked and still can't.
 

ggunn

PE (Electrical), NABCEP certified
Location
Austin, TX, USA
Occupation
Electrical Engineer - Photovoltaic Systems
That's my belief. IIRC, there is a section of Code just for that purpose, but I couldn't find it the last time I looked and still can't.
It was a silly question, now that I think about it. The conduit is grounded, so there's no way the EGC would need 1000V insulation.
 

Carultch

Senior Member
Location
Massachusetts
When I am running an EGC in conduit with 1000V PV wire, does the EGC need to be 1000V as well? I wouldn't think so, since I see bare Cu bundled with PV wire all the time.

I don't think there is a code section that specifically gives you that answer. I would think that if the NEC allows your EGC to be bare in any application, and you insulate it for reasons not dictated by the NEC, you should be able to insulate it with any type of insulation. And one reason you would want to insulate your EGC, is to reduce abrasion during the wire pull. It is overkill in my opinion, to run 1kV PV wire for the ground, just because the power wiring is also 1kV PV wire. THWN-2 should accomplish this purpose. You do have to run wire rated at the proper voltage for both ungrounded "live" AND grounded "neutral" conductors. The maximum voltage across any pair of conductors in the same raceway or across any conductor and ground.

The only counterexample I can think of, is sunlight resistance. Where if you install THWN-2 insulated wire outdoors exposed to sunlight, it doesn't automatically become a listed/code compliant bare wire after the sun erodes the non-sunlight resistant insulation. One reason why it wouldn't meet its expected purpose, is the stranding on bare wires is often coarser than that on insulated. Coarse 7-strand #6 can withstand physical damage a lot better than the standard 19-stranding of THWN-2, because of the potential for damaging to an individual strand.
 

Carultch

Senior Member
Location
Massachusetts
I keep getting confused about this. Say I have some rooftop DC wiring that I need to size and the heat load is significant - hot roof in Texas, wiring very close to it - with 90 degree wire and 75 degree terminals.

Calculate necessary ampacity two ways, one at continuous use, one at conditions of use, choose the higher.

1.25 X 1.25 X Isc for continuous use

1.25 X temp derate (with adder) reciprocal X Isc for conditions of use (fewer than 4 CCC's per conduit - no derate for that).

OK so far, but what confuses me is when to use the 75 degree column and when to use the 90 degree column. I remember a Holt video that says to use 90 degrees for one calculation and 75 degrees for the other, but which and why? If I understood the why I wouldn't have so much trouble the remembering the which.

I could play it safe and use the 75 degree column throughout (pretending that I am using 75 degree wire), but the difference in the temp derate between 75 and 90 degree conductors is significant and there is a lot of wire involved.

It is very counterintuitive to me, why we have to use BOTH 1.25 factors (which combine to become 1.56) for terminations prior to derating, but then we get to only use one of the 1.25 factors when applying the derating factors. I would think that both 1.25 factors would apply in all sizing calculations.

However, that being the case, here are the three sizing conditions your wire must meet:
#1: size wire for terminations at 1.56*Isc (which is 1.25*maxContinuousCurrent), without using any derate factors.
#2: size wire at insulating rating with 1.25*Isc (or the maximum continuous current), now applying both the temperature derate and the bundling derate factors together.
#3: the overcurrent protection device, where required, must protect both the wire at its derate factors and the terminations at their ampacities. Note that the next size up rule, 240.4(B) may very well come in to play in this calculation. Where the OCPD is not required, but is part of the circuit anyway, this rule does not need to apply.

When you are working with current-limited devices, such as inverters or power optimizers, the enhancement factor (first 1.25 factor) does not apply. The current listed on the datasheet, takes the place of 1.25*Isc in the above calculations.

Part #3 is curiously absent from the 2014 NEC, after having been part of the 2011 NEC in the 690.8 section specific for PV systems. However, it is still implied in the general rules for circuit protection. Part #3 can very well increase the wire size, above what both calculations in parts #1 and #2 would require. An example of this, is when you have numerous DC breakers (more than 2) at a central inverter's input. Obviously the breakers are sized by 1.56*Isc, but if the derated wire per part #2 is only sized for 1.25*Isc, the overcurrent protection device may not be protecting it, at its conditions of use ampacity.

An example when you might have an OCPD that you do not need, and would therefore not need it to affect wire size, is when you have two fused inputs to a central inverter. Fuses could be omitted, and it would still be compliant. But suppose they are there anyway, because it is how the inverter got ordered. In this case, you could put in 200A fuses for both circuits, and only have 150A of wiring. As long as wiring for both feeders is sized for the larger of the two ampacities needed.
 
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ggunn

PE (Electrical), NABCEP certified
Location
Austin, TX, USA
Occupation
Electrical Engineer - Photovoltaic Systems
It is very counterintuitive to me, why we have to use BOTH 1.25 factors (which combine to become 1.56) for terminations prior to derating, but then we get to only use one of the 1.25 factors when applying the derating factors. I would think that both 1.25 factors would apply in all sizing calculations.
One of the 1.25 factors is for insolation on the PV modules in excess of 1000W/m^2, so it's applied across the board. The other is for continuous use, which is either-or with conditions of use.
 

Carultch

Senior Member
Location
Massachusetts
I could play it safe and use the 75 degree column throughout (pretending that I am using 75 degree wire), but the difference in the temp derate between 75 and 90 degree conductors is significant and there is a lot of wire involved.

In a practical sense, most terminations & equipment you'll find are rated 75C. Most wire you find today and use in applications for PV systems is rated for 90C. Gotta be careful with multiconductor cables like UF or Romex though.

There is a rule about 100A and less terminations being rated 60C by default, unless listed and marked otherwise. And in most cases of parts manufactured today, it will be listed and marked otherwise. So this rule is mostly academic, however you still need to check.

So when conditions of use apply, use the 90C column when you can, then apply both applicable derate factors. At terminations, use the 75C rating without any derate factors.


Think of a steel chain, with a stainless hook at each end.
When initially manufactured, the chain can hold 100 lbs, and the hooks can hold 90 lbs. The assembly therefore can hold 90 lbs.
Now the chain rusts, but the hooks do not. The chain can hold 85 lbs, and the hooks still hold 90 lbs. The assembly therefore can hold 85 lbs. You only apply the "rust derate factor" to the component that rusts, i.e the chain. The assembly's strength is still only as strong as the weakest link.
 
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Carultch

Senior Member
Location
Massachusetts
One of the 1.25 factors is for insolation on the PV modules in excess of 1000W/m^2, so it's applied across the board. The other is for continuous use, which is either-or with conditions of use.

My point is:
Why is it that you exclude the "continuous use" factor, when you apply "conditions of use"?
I would intuitively think that the continuous use factor would apply BOTH for terminations without conditions of use AND for wires with continuous use.

I'm well aware of the first 1.25 factor, which applies to everything sourced directly from a PV module, and does not apply when sourced from a piece of current-limited equipment (optimizer, inverter, etc).

It probably isn't possible to receive a real answer to this question, but do others understand why I think it is counterintuitive?
 

Smart $

Esteemed Member
Location
Ohio
My point is:
Why is it that you exclude the "continuous use" factor, when you apply "conditions of use"?
I would intuitively think that the continuous use factor would apply BOTH for terminations without conditions of use AND for wires with continuous use.

I'm well aware of the first 1.25 factor, which applies to everything sourced directly from a PV module, and does not apply when sourced from a piece of current-limited equipment (optimizer, inverter, etc).

It probably isn't possible to receive a real answer to this question, but do others understand why I think it is counterintuitive?
Here's how I think it got to be this way...

Conditions of use are beyond the terminal enclosure... and where derating applies. The conductors must have the ability to carry the max calculated current, so no factoring for continuous loads.

In terminal enclosures, the NEC so-called conditions of use, other than termination temperature limitation, do not apply. Max' calculated current is padded to 125% continuous load to prevent the terminations from ever operating at the limit for an extended period under normal operating conditions.

You might say what establishes the normal operating conditions? In general the ambient temperature must be not more than the maximum operating temperature minus the rated temperature rise of the equipment. Some equipment provide details on how to derate ratings where extreme cases violate this approach, such as mounting a panelboard on a rooftop exposed to sunlight....
 

wwhitney

Senior Member
Location
Berkeley, CA
Occupation
Retired
Conditions of use are beyond the terminal enclosure... and where derating applies. The conductors must have the ability to carry the max calculated current, so no factoring for continuous loads.
Here's what I don't understand: when you apply the 125% continuous load factor you end up among other things increasing the breaker size. So with the larger breaker, why isn't the 125% factor applied for the entire circuit?

For example, if you feed a 40 amp continuous load with a 50 amp breaker using 75C conductors with an unadjusted ampacity of 50 amps and no adjustment factors apply anywhere in the circuit, everything is hunky dory. But if part of the circuit away from the enclosures runs through an area of increased ambient temperature where a 0.8 adjustment factor applies, the conductors in that area now have a 40 amp ampacity. According to the above that is OK, as we can drop the 125% continuous load factor when applying derating factors.

But the conductors are still on a 50 amp breaker. So the load could suffer an overload condition and start drawing 50 amps intermittently without the OCPD tripping. Wouldn't that condition run the risk of damaging the conductor insulation in the area where the conductor ampacity is only 40 amps?

Cheers, Wayne
 

Smart $

Esteemed Member
Location
Ohio
Here's what I don't understand: when you apply the 125% continuous load factor you end up among other things increasing the breaker size. So with the larger breaker, why isn't the 125% factor applied for the entire circuit?

For example, if you feed a 40 amp continuous load with a 50 amp breaker using 75C conductors with an unadjusted ampacity of 50 amps and no adjustment factors apply anywhere in the circuit, everything is hunky dory. But if part of the circuit away from the enclosures runs through an area of increased ambient temperature where a 0.8 adjustment factor applies, the conductors in that area now have a 40 amp ampacity. According to the above that is OK, as we can drop the 125% continuous load factor when applying derating factors.

But the conductors are still on a 50 amp breaker. So the load could suffer an overload condition and start drawing 50 amps intermittently without the OCPD tripping. Wouldn't that condition run the risk of damaging the conductor insulation in the area where the conductor ampacity is only 40 amps?

Cheers, Wayne
You've nailed down one of the pitfalls of the current system.

However, the allowable ampacity and derating system in place is very, very conservative. If you actually did a controlled scientific evaluation of the circuit you described, I'd bet that the conductor temperature in the "hot" section was still under 75°C during minor overload conditions.

And if you did further scientific evaluation, you'd likely discover that 75°C-rated conductor insulation doesn't start to degrade until several degrees over 75°C. Wouldn't surprise me if it didn't start to degrade until the ambient temperature reached 90°C.
 

GoldDigger

Moderator
Staff member
Location
Placerville, CA, USA
Occupation
Retired PV System Designer
I think that would be better stated as the insulation temperature reaching 90C.
If the ambient reached 90C there would zero or negative allowance for current related heating.
 

Smart $

Esteemed Member
Location
Ohio
I think that would be better stated as the insulation temperature reaching 90C.
If the ambient reached 90C there would zero or negative allowance for current related heating.
But if the insulation temperature is 90°C, the immediately adjacent materials, including the internal conductor and surrounding air, are also at 90°C. We're talking operating temperature, not nominal, zero-current ambient temperature.
 

Carultch

Senior Member
Location
Massachusetts
I think that would be better stated as the insulation temperature reaching 90C.
If the ambient reached 90C there would zero or negative allowance for current related heating.

If the ambient temperature reached 90C, we'd need a suit with an air conditioned interior, or else we'd be dead.
 

GoldDigger

Moderator
Staff member
Location
Placerville, CA, USA
Occupation
Retired PV System Designer
Thermodymics tells us that if the outside of the insulation is at the same temperature as the ambient air no heat will leave the insulation and conductor by that route.
The thermal equilibrium (worst case insulation temperature) will occur with all of the heat being generated in the wire being transferred through the insulation to the air (or raceway).
The inside surface of the insulation will also be at a higher temperature than the outer surface.
 

Smart $

Esteemed Member
Location
Ohio
Thermodymics tells us that if the outside of the insulation is at the same temperature as the ambient air no heat will leave the insulation and conductor by that route.
The thermal equilibrium (worst case insulation temperature) will occur with all of the heat being generated in the wire being transferred through the insulation to the air (or raceway).
The inside surface of the insulation will also be at a higher temperature than the outer surface.
Yes, the source of the heat is the conductor and the heat will dissipate provided the wide-field ambient temperature is less... but extreme near-field temperature will be almost identical, assuming no air circulation.
 

Smart $

Esteemed Member
Location
Ohio
If the ambient temperature reached 90C, we'd need a suit with an air conditioned interior, or else we'd be dead.
I'm not talking about personnel environmental air temperature. Just the air immediately adjacent to the outer skin of the insulation.
 

GoldDigger

Moderator
Staff member
Location
Placerville, CA, USA
Occupation
Retired PV System Designer
I'm not talking about personnel environmental air temperature. Just the air immediately adjacent to the outer skin of the insulation.
And I am saying that even the air immediately adjacent to the insulation must be lower than the insulation temperature for any heat transfer to take place across the interface.
 
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