Feeder cable sizing

[wwhitney]

Tables 310.15(B)(2)(a) says to multiply the allowable ampacity by the correction factor shown in table not divide

If you have the table ampacity, and want to know the corrected ampacity, you multiply the table ampacity by the correction factor.

If you have a target corrected ampacity, and want to know what table ampacity you need, you divide the target corrected ampacity by the correction factor to determine what table ampacity you need.

Cheers, Wayne

 

[xptpcrewx]

If you have a target corrected ampacity, and want to know what table ampacity you need, you divide the target corrected ampacity by the correction factor to determine what table ampacity you need.

The only issue I’ve had with the divide method is it that it’s not exactly clear what the wire ampacity (or remaining capacity) is when rounding to the nearest size. Doing the multiplication after a size and table value is selected would still need to be done to determine the ampacity of the circuit.

 

[wwhitney]

The only issue I’ve had with the divide method is it that it’s not exactly clear what the wire ampacity (or remaining capacity) is when rounding to the nearest size.

I don't follow? If you account for all the factors that need to be applied, then you've calculated a minimum table value. You pick the next higher table entry, you don't round to the nearest size.

Cheers, Wayne

 

[xptpcrewx]

I don't follow? If you account for all the factors that need to be applied, then you've calculated a minimum table value. You pick the next higher table entry, you don't round to the nearest size.

Cheers, Wayne

Exactly this is what I mean. You select the next higher entry. (What I’m calling round up)

 

[wwhitney]

Sounds like we are in agreement. I agree that after you calculate the required table value and pick the wire size whose table entry is at least that value, you need to apply the adjustment and correction factors to find out the actual ampacity. Which is a good check for your computation. And if you aren't using the "division" method, then you're doing some sort of iterative method, so you'll be do that application of adjustment and correction factors multiple times.

Cheers, Wayne

 

[xptpcrewx]

Sounds like we are in agreement. I agree that after you calculate the required table value and pick the wire size whose table entry is at least that value, you need to apply the adjustment and correction factors to find out the actual ampacity. Which is a good check for your computation. And if you aren't using the "division" method, then you're doing some sort of iterative method, so you'll be do that application of adjustment and correction factors multiple times.

Cheers, Wayne

Yup. After selecting the next higher table entry, apply the correction/adjustment factors to that value. This would be the actual ampacity of the wire and gives you information about how much more load can be added under the conditions of use before exceeding that ampacity.

 

[kwired]

What about 125% of the largest motor? (220.50)

Yes, 125% when you plug the value into your 220 calculations.

Next section (220.51) for electric space heating says to use 100%, though 424 says electric space heating is to be considered a continuous load.

Any place you are allowed to use a demand factor is not a continuous load, demand factors exist because of the likelihood that not all the load will run at same time, and most loads you can use a demand factor with are cycling types of loads.

Your 220 calculation already has the 125% included where needed and final result of the calculation is applied at 100% for the service or feeder you are calculating.

 

[hhsting]

Let's try this:

If a feeder is only serving the lighting load, in a dwelling unit that's 3VA per sqft before any load calculation, so if you are dealing with 2500sqft you have gross 7500VA. 220.42 now applies. 1st 3000VA at 100% 3000/240 is 12.5A, leaving 4500VA at a factor of 35% leaving 1575VA/240V is 6.56A so the load is 19.06A. If none of that load is continuous then for 215.2(A)(1)(a) you use only 100% or 19.06A. Lighting in a commercial space could fit the definition of a continuous load and then would be calculated at 125%, or 23.825A for 215.2(A)(1)(a).

Now for 215.2(A)(1)(b) you apply adjustment and correction factors to the table ampacities to determine the conductor size, but only at 100% of the load, even if the load is continuous. If an adjustment needs to be made per 310 for temp 310.15(B)(2)(a) 75° column, ambient temp 87-95° of 0.94, you need a conductor whose table ampacity is 19.06/0.94 = 20.28A to end up with an ampacity of 19.06A.

So if this 19.06A load is a continuous load, (a) controls and you will need a conductor with a table ampacity of 23.83A. While if this 19.06A load is a non-continuous load, then (b) controls and you will need a conductor with a table ampacity of 20.28A.

Cheers, Wayne

Lets say now in your example all light load would be all feeding from MLO panelboard. What would the breaker size feeding the panelboard be? Compare that to your feeder size

 

[wwhitney]

though 424 says electric space heating is to be considered a continuous load.

It says that under "branch circuit sizing," so I take that to mean for feeder and service sizing, you don't have to consider electric space heating a continuous load.

Any place you are allowed to use a demand factor is not a continuous load, demand factors exist because of the likelihood that not all the load will run at same time, and most loads you can use a demand factor with are cycling types of loads.

That might be true in practice, I'm not familiar with all the demand factors in 220. But it's not true a priori, there could be a type of load that is continuous, but you still get to apply a demand factor to it. E.g. it is judged that it tends to get used once a day or once a week and so in large multi-dwelling unit buildings they are never all going to be in use at once.

Your 220 calculation already has the 125% included where needed and final result of the calculation is applied at 100% for the service or feeder you are calculating.

Disagree; I think the lighting discussion is the obvious counterexample. 220 tells you the lighting load, but doesn't tell you if its continuous or not.

Cheers, Wayne

 

[gh0st]

I only do commercial design and I've never had issues approaching load calculations as follows:

• Receptacle: 1st 10kVA rule... these are receptacles that are purely convenience (180VA)
• Lighting: 125% of calculated load, or lighting power density as dictated by the energy code.
• Appliance loads: 100% of rated load. This includes all known loads even if connected to a receptacle.
• Elevators, Kitchen: As per the demand factors allowable per code.
• Motor loads: Largest motor @ 125% + 100% of remaining motors
  • Consider redundant loads...I typically apply an additional demand factor depending on motor types.
• Loads that gets the 125% factor
  • EV chargers, (<120g) water heaters, electric signage, project specific loads known to be connected >3hrs, existing
• Consider noncoincident loads.

Size the feeder to carry total load. Note...Apply 125% for continuous loads OR apply adjustment factors (NEC 210.19) - the intent for both is to avoid overheating of conductors. Use the larger of the conductor sizes between either calculation.

Size the breaker to protect the conductors.

 

[Fred B]

Lets say now in your example all light load would be all feeding from MLO panelboard. What would the breaker size feeding the panelboard be? Compare that to your feeder size

Article 215.3 covers that with a statement consistent with the calculation of feeder sizing in 215.2(A)(1)(a), the combination of 125% of continuous plus 100% of non-continuous loads. Additional considerations are listed referring to Article 240 part I.
215.3 exception 1 allows the beaker to be sized to only the sum of continuous plus non-continuous loads if breaker and all of the assembly is rated to operate at 100% of its rating.
It is about protecting the feeder conductors in event of overload. I think all the calculation involved would particularly useful for such a situation you highlight for a MLO panel.

 

[david luchini]

Your 220 calculation already has the 125% included where needed and final result of the calculation is applied at 100% for the service or feeder you are calculating.

There's nothing in Article 220 about including 125% for continuous loads.

 

[hhsting]

Article 215.3 covers that with a statement consistent with the calculation of feeder sizing in 215.2(A)(1)(a), the combination of 125% of continuous plus 100% of non-continuous loads. Additional considerations are listed referring to Article 240 part I.
215.3 exception 1 allows the beaker to be sized to only the sum of continuous plus non-continuous loads if breaker and all of the assembly is rated to operate at 100% of its rating.
It is about protecting the feeder conductors in event of overload. I think all the calculation involved would particularly useful for such a situation you highlight for a MLO panel.

Not really for panelboard breaker protection 408.30 and 408.36 says section see 220. If you do the continous plus non continuous then it maybe come out greater than 220 method but then 408.30 and 408.36 say you cannot do this thus reason for asking the question.

 

[petersonra]

The calculations are not there to protect the feeder conductors from overcurrent. That's what the overcurrent protection device is for. The calculations are there to make sure that you adequately size them. That's all. Protection is solely the responsibility of the overcurrent protection device. In any case if you manage to somehow run too much current through your feeder, and your overcurrent protection device is sized properly, it will protect your conductors by tripping.

 

[kwired]

There's nothing in Article 220 about including 125% for continuous loads.

correct, the one that was pointed out that I replied to in post 27 was isn't really there either, just a reference to other code section that may require it.

I still think after you finish a feeder/service calculation utilizing any method allowed by 220, your result is your minimum conductor ampacity and overcurrent protection level. No need to go back through and factor in what is continuous and what is not and make further adjustment. If something does require an additional 125% it should have been included before plugging into your 220 calculation process.

 

[david luchini]

correct, the one that was pointed out that I replied to in post 27 was isn't really there either, just a reference to other code section that may require it.

I still think after you finish a feeder/service calculation utilizing any method allowed by 220, your result is your minimum conductor ampacity and overcurrent protection level. No need to go back through and factor in what is continuous and what is not and make further adjustment. If something does require an additional 125% it should have been included before plugging into your 220 calculation process.

You have that backwards. You have to calculate the loads per Article 220. There is no 125% included there.

Then you add the 125% for continuous loads when sizing your feeder, per 215.2(A).

215.2(A)(1)(a) and (A)(1)(b) wouldn't make sense if "continuous loads" were accounted for in Article 220.

 

[Fred B]

Not really for panelboard breaker protection 408.30 and 408.36 says section see 220. If you do the continous plus non continuous then it maybe come out greater than 220 method but then 408.30 and 408.36 say you cannot do this thus reason for asking the question.

Here we go, what looks like more disjointed codes. 215.2 says to size conductors plus maybe any increases for continuous loads using 220.
215.3 says size protection for feeder by criteria in 240 part I.
240.3 indicated certain equipment to be protected by respective relevant section list in table 240.3, 408 is one, and supersede aspects of 240.
408.30 mearly says that it is sized for minimum feeder capacity required for a load calculated in accordance with parts III,IV,V of Article 220.
The feeder capacity when sized properly to Article 215 will be sized in accordance with those 220 parts. And if panel is sized appropriately will meet the amperage related to the feeder capacity.
215.3 also includes exception for use of a 100% breaker and equipment eliminating the 125% increase for continuous loads. If panelboard is listed to be rated to 100% of load capacity along with protective breaker it meets the requirements of 215.3 Exception 1. would allow a simple sum of loads without the 125% increase for the breaker, and panel capacity.
408.36 says you can't have a panelboard rated less that the protective protective device, so you cant have a panelboard that is rated for ie. 200Amp protected by a 250Amp breaker. So if you back this information out would simply indicate 2 options for the panelboard. 1. Get a bigger panelboard. or 2. Reduce the loads on the panelboard.

 

[david luchini]

Here we go, what looks like more disjointed codes.

None of those Sections are disjointed.

 

[Fred B]

None of those Sections are disjointed.

Just in sense that 408, 240, 215, 220, are not universally connected or referenced to each other and reference other code sections as well that influence the outcomes that are also not connected directly by reference to the part originally looking at. So you have to chase around within the code to get the answers. But I also said "looks like", not that it was.
But I have to say that is what this forum is good for, a lot of "I didn't notice that part" comes out.

 

[hhsting]

Just in sense that 408, 240, 215, 220, are not universally connected or referenced to each other and reference other code sections as well that influence the outcomes that are also not connected directly by reference to the part originally looking at. So you have to chase around within the code to get the answers. But I also said "looks like", not that it was.
But I have to say that is what this forum is good for, a lot of "I didn't notice that part" comes out.

You never know we all might be doing this wrong and their can be little pieces here and there in code that we might have missed right now