Wire Size and Length

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Old 11-11-06, 10:12 PM
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Wire Size and Length

I need to run a circuit for an electric heater. The unit requires a 60amp service, so I will be using NM size 6/3 with ground (no conduit) to provide power.

My question has to do with the length of the line - the heating unit is about 100ft. from the main panel. Does this distance affect the size of the conductor I use?

Thanks!
 
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Old 11-12-06, 08:35 AM
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rdn2113,

Just a quick point on semantics. A service comes from the utility and feeds a main panel. A feeder goes from a main panel to another panel (sub panel) and a branch circuit is what you are installing.

Here is a good link to an online voltage drop calculator.

http://www.csgnetwork.com/voltagedropcalc.html

If your heater is 240 volt, I get a 2.4 percent VD which is fine. Five total from the main service to the equipment is considered acceptable.
 
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Old 11-12-06, 08:48 AM
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If it is strt. 240V, you only need a 6/2 with ground.
Also I would recomend a disconnect at/near the unit due to the distance from the panel.
 
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Old 11-12-06, 06:55 PM
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Hi guys,

Sorry about the error in terminology - thanks for the clarification jwhite. You are correct, it is a branch circuit. Thanks for the link on voltage drop calculation - it's very helpful! I've added it to my favorites.

lectriclee - the heater has a blower that requires 120v, so I need the neutral. Darn! I completely agree with your suggestion regarding the disconnect - I'll be sure to install one.

Thanks guys!
 
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Old 11-12-06, 07:11 PM
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Good to hear back. It would be wise.
 
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Old 11-12-06, 07:13 PM
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Do the instructions say it needs 60 amp branch circuit conductors or is the 60 amps coming from the nameplate or the specifications table in the instructions? At anyrate 6 awg NM-b is only rated 55 amps for load purposes but it can have OCPD of 60 amps. This heater is also considered a continuous load so branch circuit conductors must be sized accordingly.

Edit: Unless the unit states that 60 amps is the minimum branch circuit amperage or max breaker size you will need to take 125% of the continuous load amps + the amperage of the blower motor to size those conductors.

Regardless 6 awg NM-B is not rated to carry 60 amps you will need #4 minimum but this depends on how the manufactuerer is using the 60 amps.

Roger
 

Last edited by Roger; 11-12-06 at 08:25 PM.
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Old 11-12-06, 07:46 PM
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Hi Roger - yes, the instructions call for an OCPD of 60amps. The actual load is less than 55amps.

Thanks!
 
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Old 11-12-06, 08:13 PM
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Then sounds like you are good to go. I just wanted to be sure we weren't overlooking the correct conductor size and OCPD. for instance if that 60 amps had been the units heating element load then you would have needed 60 amp + the amperage of the blower motor. So lets say the blower was 3 amps you would have a continuous load of 60 amps + 3 amps..... 125% of 60 amps + the 3 amps of the blower motor would give you conductors needing 78.75 amps. So a #3 awg copper conductor in NM-B style !! Quite a difference.

Roger
 
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