Sizing feeders for continuous and noncontinuous loads

[gporter]

For sizing feeder or service conductors, Article 220 gives you the rules by which to compute the total load. These rules contain no provisions for increasing loads by 25% for continuous loads (i.e. 220.15 says that electric heaters are to be computed at 100% of the connected load, not 125% even though they are likely to be continuous loads). However 215.2(A)(1) and 230.42(A) requires feeders and service conductors to be sized for the sum of non-continuous loads plus 125% of continuous loads. Does the load computed by Art. 220 take precedence over the sizing requirements of Art. 215 and 230?

 

[wirenut1980]

Re: Sizing feeders for continuous and noncontinuous loads

Article 220 is for computing the load for a facility. 215.2(A)(1) and 230.42(A) deal with choosing ampacity of conductors to feed that computed load. So they are not dealing exactly with the same principle. Best to think about it as 2 separate steps in a process of designing say, a service.

215.2(A)(1) General. Feeder conductors shall have an ampacity not less than required to supply the load as computed in Parts II, III, and IV of Article 220...

And it says the same type of thing in the beginning of 230.42(A)

 

[john m. caloggero]

Re: Sizing feeders for continuous and noncontinuous loads

The requirements for "continuous loads" last appeared in 1996 NEC, Section 220-3. It has been deleted since the additional 25% was not a true load value. The requirements are found in the other articles for sizing the conductors and the overcurrent devices. Refer to 215.3 Overcurrent Protection, Section 230.42(A)(1), and 210.19(A)(1)where the continuous loads and non-continuous loads are addressed. The size of the overcurrent devices and wire size are determined by the preceeding rules.

 

[radiopet]

Re: Sizing feeders for continuous and noncontinuous loads

You need to make sure you keep the two different calculations just that...different.

When sizing for the service you calculate it using guidelines of 220.

In the lastest codes the Continuous impact is now felt in the indivisual calculations rather than in the old method. Simply calculate the service and using the guidelines of 220 will compensate for the requirements because the continuous loads are now configured in the actual branch feeders calcs and so on.

 

[gporter]

Re: Sizing feeders for continuous and noncontinuous loads

Thanks for the replys--however

I?m still not certain what load to use. I?m trying to determine if an existing building has a large enough feeder to be able to add a new load. It seems to me that article 220 recognizes that there will be some diversity of loads, and lets you use a smaller feeder because not everything will be on at once. However, if I calculate the load on the feeder using the installed branch circuit loads (non-continuous plus 125% continuous) I get a much higher load ? because of this 125% factor. It seems like this would happen for any service or feeder designed using article 220 loads ? when you analyze the finished installation using 125% continuous plus non-continuous, you will be overloaded!

gp

 

[wirenut1980]

Re: Sizing feeders for continuous and noncontinuous loads

gporter, I think everyone thought you were talking about designing a new service or feeder, but given this new information, I agree that the Article 220 calculations and actual demand will be very different from one another. As you know, adding loads to an existing building requires that you have a very good idea of what the existing peak loading is. I think I am getting in over my head.

 

[petersonra]

Re: Sizing feeders for continuous and noncontinuous loads

There are also times when you need to use common sense in thinking these things through. A lot of times, you are either close to the max load on a panel board, or no where need its capacity.

If you are close to using most of the current capacity of an existing panelboard that should tell you to run a new circuit regardless of what the calcs might tell you.

If you have a 200A panelboard and it typically uses only 50A, that should tell you something as well, again, regardless of what the calcs tell you.

Once your brain decides what you should do or can get away with, then try and decide if you can justify what you want to do using the code.

You will usuually find that the code makes a lot of sense most of the time. If you use it wisely, you don't have to think everything out to the nth degree everytime.