Sizing mast for overhead feeder

[Pascal2]

NEC is not very helpful:
225.17 Masts as Supports (A) Strength. The mast shall have adequate strength or be supported by braces or guys to safely withstand the strain imposed by the overhead feeder or branch-circuit conductors.

Is there some convention to calculate the required size for a mast? I'd like to find a general calculator or table or something, but in the current case I need a mast for a 25' span of 6 AWG quadruplex to a 9' tall detached residential garage with a nearly flat roof. 225.18(2) requires 12' clearance above grade so I'll probably extend the mast 4' above the roof with a weather head on top and cable attachment at 3.5'. Based solely on conduit fill, it appears I only need 3/4" conduit. The cable itself only weighs 0.162 lbs per ft, so 4 lb total, plus hardware, but this is Colorado so ice and wind load are not insignificant.

 

[fdes1879]

2 1/2" galvanized pipe minimum for all masts services. Regardless of wire size

 

[Little Bill]

2 1/2" galvanized pipe minimum for all masts services. Regardless of wire size

That's strictly a POCO (Utility) call. I deal with several POCOs and most of them have different requirements concerning mast size. So the OP needs to contact his POCO to get their requirements.

 

[infinity]

I don't see any mention of a service in the OP.

 

[480sparky]

Size is based on the local POCO requirements. 'Round here, it's 2"minimum, and may be up to 4" depending on the distance to the next pole.

 

[Hv&Lv]

NEC is not very helpful:
225.17 Masts as Supports (A) Strength. The mast shall have adequate strength or be supported by braces or guys to safely withstand the strain imposed by the overhead feeder or branch-circuit conductors.

Is there some convention to calculate the required size for a mast? I'd like to find a general calculator or table or something, but in the current case I need a mast for a 25' span of 6 AWG quadruplex to a 9' tall detached residential garage with a nearly flat roof. 225.18(2) requires 12' clearance above grade so I'll probably extend the mast 4' above the roof with a weather head on top and cable attachment at 3.5'. Based solely on conduit fill, it appears I only need 3/4" conduit. The cable itself only weighs 0.162 lbs per ft, so 4 lb total, plus hardware, but this is Colorado so ice and wind load are not insignificant.

I believe your forgetting catenary loads.
Ice and wind are on top of that

 

[electrofelon]

I would probably go inch and a quarter.

 

[kwired]

I would probably go inch and a quarter.

Me too, though for only a 25 foot span and that light of conductors I'd bet even 3/4" likely holds up pretty well in most cases, just don't stretch it very tight.

 

[mikeames]

You could calculate it. You can change the load by optimizing the sag. (within your clearance constraints). https://www.engineeringtoolbox.com/cable-loads-d_1816.html Personally I prefer to overkill it and probably do 1.5 to minimize creep.

That link has a simple calculator, plug in the values.

 

[Pascal2]

Apologies for the confusion. This question is about masts for overhead feeders (NEC 225) not overhead service-drop conductors (NEC 230). Requirements for the latter are typically specified by the power company. The former has nothing to do with them.

I was hoping to find a simple calculator that I can plug in cable size, height above last support, span length, etc. and it would calculate catenary, ice, wind, etc. loads and give me a mast size. Kind of like the many great conduit fill calculators out there, but somehow I didn't think it would be that easy.

 

[RumRunner]

Apologies for the confusion. This question is about masts for overhead feeders (NEC 225) not overhead service-drop conductors (NEC 230). Requirements for the latter are typically specified by the power company. The former has nothing to do with them.

I was hoping to find a simple calculator that I can plug in cable size, height above last support, span length, etc. and it would calculate catenary, ice, wind, etc. loads and give me a mast size. Kind of like the many great conduit fill calculators out there, but somehow I didn't think it would be that easy.

Assuming you are doing this project where you live-- (Chicago) it would seem prudent to call AHJ what their requirements are.
I did my projects like this years ago but our POCO is also owned by the largest municipality-run utility company.

The utility is owned by the City of Los Angeles-- therefore, their Building and Safety Department is in close coordination with the POCO in terms of requirement that has the semblance of this nature.

POCO has a different inspectors and they come out to inspect--after the final inspection.
L. A. Electrical Code Book . . . . about double the number of pages compared to the regular NEC Code Book.
There is a separate chapter that spells out the POCO requirements (ie). underground or overhead secondary conductors to the service panel or MCP .
I’m not saying that L.A. requirements mirror your particular case. . . I’m just citing it as an example to give you an idea.
If your riser is installed for say-- designed to support the service drop. . . that crosses a public passage way...you would be required to maintain clearance of 18 ft.

Most residential one-story structures will not meet this clearance.. . . and you will be forced to have a longer conduit (riser) or higher weather head.
The max height for weather head is 3 ft. from the roof or parapet if there is one

The point of attachment of the service drop conductors should be no more than 18 inches from the bottom.

If the riser is more than 3 ft in height (weather head) you are required to have or fabricate whatever is necessary to keep it in place. Subject to acceptance by POCO inspector doing the inspection.

Swinging wires during earthquake--could spoil the whole ball of wax.

In-a-nutshell—each situation is not typical. There is no “cookie-cutter” guideline.
POCO has the final say.
Good Luck

 

[infinity]

This is not a service drop so the POCO and their rules are not involved. The question is how do you size the mast for feeder conductors which is a great question because the NEC seems to be silent on the subject.

 

[RumRunner]

This is not a service drop so the POCO and their rules are not involved. The question is how do you size the mast for feeder conductors which is a great question because the NEC seems to be silent on the subject.

OK. I missed that part that says : "not service drop". MY BAD .

 

[Hv&Lv]

This is not a service drop so the POCO and their rules are not involved. The question is how do you size the mast for feeder conductors which is a great question because the NEC seems to be silent on the subject.

This is the reason I’ve been following this post, to see what the expert answers are...

This seems to be one of those like the boat lift motor question earlier.
It’s like it’s up to you to come up with an answer and convince an AHJ.
The AHJ would seems to have the final say.

 

[tom baker]

In Washington we have very specific requirements that have been in the state electrical code since the 1960s, with just a few modifications since then. The drawing does not specify the size of the mast, couplings are allowed as long as there are two means of support above the coupling. Most masts are 2" RMC. The drawings says service but it also applies to feeders. I would say use this drawing for your installation and tell the AHJ those folks in the Evergreen State have been doing this way since the 1960s

 

[kwired]

In Washington we have very specific requirements that have been in the state electrical code since the 1960s, with just a few modifications since then. The drawing does not specify the size of the mast, couplings are allowed as long as there are two means of support above the coupling. Most masts are 2" RMC. The drawings says service but it also applies to feeders. I would say use this drawing for your installation and tell the AHJ those folks in the Evergreen State have been doing this way since the 1960s

OP had only 25 feet and 6 AWG (probably aluminum) multiplex conductors. Outside of potential wind load or a tree falling on it there are a lot of things physically strong enough to secure this to.

 

[retirede]

In Washington we have very specific requirements that have been in the state electrical code since the 1960s, with just a few modifications since then. The drawing does not specify the size of the mast, couplings are allowed as long as there are two means of support above the coupling. Most masts are 2" RMC. The drawings says service but it also applies to feeders. I would say use this drawing for your installation and tell the AHJ those folks in the Evergreen State have been doing this way since the 1960s

That has to account for worse-case scenario since specifics of the run aren’t specified. It is likely way overkill for smaller wire sizes and/or shorter spans.

 

[hbiss]

I don't see any mention of a service in the OP.

No, but why reinvent the wheel.

-Hal

 

[tortuga]

Me too, though for only a 25 foot span and that light of conductors I'd bet even 3/4" likely holds up pretty well in most cases, just don't stretch it very tight.

Ill third with fellon and kwired, I'd probably go with 1-1/4 In IMC, it that seems flimsy I skip over 1-1/2 and just use 2" IMC.
That's after I can't talk the customer into doing an underground.

 

[topgone]

This is not a service drop so the POCO and their rules are not involved. The question is how do you size the mast for feeder conductors which is a great question because the NEC seems to be silent on the subject.

Try NESC!