Conductor size for 225a feeder

[LarryFine]

Me again. I have to run a 3ph feeder from a fused switch to a M/L panel in a commercial space. As there is no tenant yet, we have no load calcs, but I'm 99% sure it will never approach 225a.

We're also running a 200a 1ph feeder and panel in the same space, so I'm doubly sure we'll not see anywhere near 225a on the 3ph. Both are from a high-leg transformer bank, by the way.

How do I determine whether to price for and install 250 or 300 MCM aluminum? The conduit is 4" EMT, both because of downsizing from the original 400a panel, and in case of future needs.

 

[don_resqcapt19]

If the OCPD is 225 amps, you have to use a conductor with an ampacity equal to or greater than 225 amps. That will required 300 kcmil AL.

 

[JFletcher]

If the OCPD is 225 amps, you have to use a conductor with an ampacity equal to or greater than 225 amps. That will required 300 kcmil AL.

250 MCM has an ampacity of 205 amps... Would one be permitted to use the next size up breaker, which is 225 amps, provided the load is less than 205 amps?

I priced 250 MCM at $1.14/ft, 300 MCM is $1.58, a $0.44/ foot difference, which is going to add up quickly if your pull is 200 foot or more.

You could also parallel sets of 1/0 or 2/0, those run 60 to $0.70 per foot. 2 sets of 2/0 derated to 80% for 6 current-carrying conductors give an ampacity of 240 amps, 216 amps for the 1/0.

How long is the run?

 

[JFletcher]

Also note that you could use the wiring at 90* if both ends are joined to Polaris connectors. Probably not cost-effective, however it is another option.

 

[electrofelon]

250 MCM has an ampacity of 205 amps... Would one be permitted to use the next size up breaker, which is 225 amps, provided the load is less than 205 amps?

I priced 250 MCM at $1.14/ft, 300 MCM is $1.58, a $0.44/ foot difference, which is going to add up quickly if your pull is 200 foot or more.

You could also parallel sets of 1/0 or 2/0, those run 60 to $0.70 per foot. 2 sets of 2/0 derated to 80% for 6 current-carrying conductors give an ampacity of 240 amps, 216 amps for the 1/0.

How long is the run?

I agree, one could use 250 AL and next size up to 225. Just depends on if you think you will ever need that extra 20A.

That will be a luxurious pull in 4" EMT. I pulled some 300' foot 350 feeders in 4" and it was a breeze. Used AL THHN and practically had to hold the wire back

 

[Dennis Alwon]

If the plans call for a 225 amp feeder then Don is correct. I would not install a smaller conductor without approval from the architect

 

[electrofelon]

If the plans call for a 225 amp feeder then Don is correct. I would not install a smaller conductor without approval from the architect

I agree. It depends on what exactly "225 amp feeder" means.....With 250, it would be a 205 amp feeder

 

[LarryFine]

The run is actually about 40', and we're sticking with the 4" to avoid reducing and to allow for future up-sizing. The present 4" is there because the original supply was 400a.

225a fuses will fit the same clips as 400a fuses, won't they?

 

[petersonra]

Me again. I have to run a 3ph feeder from a fused switch to a M/L panel in a commercial space. As there is no tenant yet, we have no load calcs, but I'm 99% sure it will never approach 225a.

We're also running a 200a 1ph feeder and panel in the same space, so I'm doubly sure we'll not see anywhere near 225a on the 3ph. Both are from a high-leg transformer bank, by the way.

How do I determine whether to price for and install 250 or 300 MCM aluminum? The conduit is 4" EMT, both because of downsizing from the original 400a panel, and in case of future needs.

The provision to allow for the next size up rating on an OCPD requires that the load calculation be be less than the ampacity of the conductors so if you don't have a load calculation I don't see how you can take advantage of that provision.

 

[david luchini]

The provision to allow for the next size up rating on an OCPD requires that the load calculation be be less than the ampacity of the conductors so if you don't have a load calculation I don't see how you can take advantage of that provision.

I don't believe that is correct. There is nothing in 240.4(B) about load calculations.

 

[LarryFine]

Well, you both can't be right.

 

[Dennis Alwon]

I don't believe that is correct. There is nothing in 240.4(B) about load calculations.

So if the calculated load is 220 amps could you use a 205 amp conductor.... I don't think so. It may not say it but it is there in other parts of the code. You cannot have a conductor with less capacity than the required load.

 

[electrofelon]

A load calc is pretty much always technically required. I am not aware of any code language that makes a load calc for one situation "more required" than another

 

[JFletcher]

The provision to allow for the next size up rating on an OCPD requires that the load calculation be be less than the ampacity of the conductors so if you don't have a load calculation I don't see how you can take advantage of that provision.

I agree. And with a 40-foot run, I would just go ahead and order 300 MCM aluminum and be done with it, the extra $0.40 of foot cost over 250 will be about $50 in the overall job.

Edited to add... Regarding the fuses, I am not sure where the cut-offs are between fuse sizes. I would think that even if a 225 amp did not fit in the 400 amp frame that some sort of adapters would be made to fuse downward.

 

[david luchini]

So if the calculated load is 220 amps could you use a 205 amp conductor.... I don't think so. It may not say it but it is there in other parts of the code. You cannot have a conductor with less capacity than the required load.

If 125% of the continuous load plus the non-continuous load is 205A or less, then you could use a 205A conductor. Other parts of the Code say you can protect the 205A conductor with a 225A OCPD.

But that is completely off point from my previous post.

 

[kwired]

I agree. And with a 40-foot run, I would just go ahead and order 300 MCM aluminum and be done with it, the extra $0.40 of foot cost over 250 will be about $50 in the overall job.

Edited to add... Regarding the fuses, I am not sure where the cut-offs are between fuse sizes. I would think that even if a 225 amp did not fit in the 400 amp frame that some sort of adapters would be made to fuse downward.

Anyhting over 200 but under 401, should all have same dimension within each class/volt rating.

If 125% of the continuous load plus the non-continuous load is 205A or less, then you could use a 205A conductor. Other parts of the Code say you can protect the 205A conductor with a 225A OCPD.

But that is completely off point from my previous post.

Well you either need to measure load or calculate it. If you are allowed to apply demand factors and want to use them - the end result is a calculated load.

 

[LarryFine]

In this case, with no tenants yet, the owner has no figures to give me yet. HVAC and lighting/basic receptacles are all we have.

 

[kwired]

250 MCM has an ampacity of 205 amps... Would one be permitted to use the next size up breaker, which is 225 amps, provided the load is less than 205 amps?

I priced 250 MCM at $1.14/ft, 300 MCM is $1.58, a $0.44/ foot difference, which is going to add up quickly if your pull is 200 foot or more.

You could also parallel sets of 1/0 or 2/0, those run 60 to $0.70 per foot. 2 sets of 2/0 derated to 80% for 6 current-carrying conductors give an ampacity of 240 amps, 216 amps for the 1/0.

How long is the run?

Since you posted he told us only about 40 feet. Thing with paralleling is a 225 amp main breaker or main lugs in a panel likely only rated for single conductor termination. If he is feeding from a 400 amp fused disconnect, which I think he is, it is more likely to have terminals that will accept two conductors per termination. Transitioning from two conductors to a single because the gear only accepts one, on only a 40 foot run, probably cost more than running 300 kcmil.

 

[JFletcher]

Since you posted he told us only about 40 feet. Thing with paralleling is a 225 amp main breaker or main lugs in a panel likely only rated for single conductor termination. If he is feeding from a 400 amp fused disconnect, which I think he is, it is more likely to have terminals that will accept two conductors per termination. Transitioning from two conductors to a single because the gear only accepts one, on only a 40 foot run, probably cost more than running 300 kcmil.

Agree, tho see post number 14.

 

[Adamjamma]

In code I have not seen too much except for the call for calculations, but since there is no tenant there are no loads to calculate, if I understand correctly.

So, if it was me, which it is not, I would size for the maximum the box can handle, which was stated as 225... then leave it for the tenant to finish up from there...
Because if I run it for 180 or for 195 and the next tenant needs 225... how do they get it?


Of course, if it is close enough you could just put in two large enough conduits to allow a 400 amp service to be run to it, and then leave the rest until a customer wants to get power...